Diseases represent a major threat to the commercial production of grapes in the Midwest. Climatic conditions are conducive to the development of several major grape diseases, including black rot, downy mildew, and powdery mildew. Each of these diseases has the potential to destroy the entire crop under the proper environmental conditions. In addition, there are several other diseases (Phomopsis cane and leaf spot, Botrytis gray mold, Eutypa dieback and crown gall) that can also result in economic loss. It is important to note that most of these diseases can occur simultaneously within the same vineyard during the growing season.
The development and implementation of Integrated Pest Management (IPM) programs for grapes has great potential for improving our current pest control strategies and reducing our use of pesticides in general. Much of the potential for reducing pesticide use will be in the area of insect control. Many of the IPM methods for monitoring and controlling insects give the grower more flexibility in the decision-making process as to whether insecticides are needed, which insecticides to apply, and when to apply them.
Our currently available disease-management programs and recommendations have much less flexibility, and the level to which we will be able to reduce fungicide use is largely limited by the degree of susceptibility of the cultivars being grown and environmental conditions during the growing season (the most important of which is wet rainy weather). The introduction of new fungicide chemistry, such as the sterol inhibitors or SIs (Bayleton, Rubigan, and Nova), and the strobilurin fungicides (Abound, Sovran, and Pristine), as well as new information related to the disease cycles of the various pathogens are providing opportunities for new disease control strategies that can be implemented in IPM programs.
Developing a disease-management program that successfully controls all of the important grape diseases simultaneously presents a unique challenge. In order to accomplish this, all available control methods must be integrated into one overall disease-management program. The disease management program should emphasize the integrated use of disease resistance, various cultural practices, knowledge of disease biology, and the use of approved fungicides or biological control agents or products when necessary.
It is important for growers to be able to recognize the major grape diseases. Proper disease identification is critical to making the correct disease-management decisions. In addition, growers should develop a basic understanding of pathogen biology and disease cycles for the major grape diseases. The more one knows about the disease, the better equipped one is to make sound and effective management decisions. Color photographs of disease symptoms on grapes, as well as in-depth information on pathogen biology and disease development, can also be found in these publications:
Compendium of Grape Diseases—Published by the American Phytopathological Society, 3340 Pilot Knob Rd., St. Paul, MN 55121. Phone: 612-454-7250, 1-800-328-7560. This is the most comprehensive book on grape diseases available. All commercial growers should have a copy.
Midwest Small Fruit Pest Management Handbook. Bulletin 861, Ohio State University Extension. Can be obtained from Ohio State University Extension, Media Distribution, 2021 Coffey Road, Columbus, OH 43210-1044. Phone 614-292-1607.
A description of symptoms and disease cycles for the most common grape diseases in the Midwest is presented here.
Black rot is caused by the fungus Guidnardia bidwellii. The fungus overwinters in mummified fruit on the vine or on the ground. Spring rains trigger the release of airborne ascospores and/or rain splashed conidia from the mummies. Primary infections occur on green tissues if temperatures and duration of leaf wetness are conducive (Table 13). Recent research indicates that the majority of ascospores from mummies on the ground are discharged within a time period from 1-inch shoot growth to two to three weeks after bloom. If mummies are allowed to hang on the vines, they can discharge ascospores and conidia throughout the growing season.
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| FIGURE 33. Black rot disease cycle. Used with permission of the New York State Agricultural Experiment Station, Cornell University. Figure taken from Grape IPM Disease Identification Sheet No. 4. |
| Table 13. Grape Black Rot. Leaf Wetness Duration-Temperature Combinations Necessary for Grape Foliar Infection by Black Rot. | |
| Temperature °F | Minimum Leaf Wetness Duration (Hr) for Light Infection |
|---|---|
| 50 | 24 |
| 55 | 12 |
| 60 | 9 |
| 65 | 8 |
| 70 | 7 |
| 75 | 7 |
| 80 | 6 |
| 85 | 9 |
| 90 | 12 |
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| FIGURE 34. Black rot lesions on grape leaf. |
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| FIGURE 35. Close-up of black rot leaf lesions showing fungal fruiting bodies (pycnidia). |
In conventional production systems, black rot is controlled primarily through the use of effective fungicides combined with various cultural practices. Black rot may be particularly important in organic production systems because the organically approved fungicides (copper and sulfur) are not very effective for black rot control. Growers should develop a thorough understanding of the black rot disease cycle and the cultural practices used to control it.
Lesions on canes from the previous season can also produce conidia for a period of at least one month starting at budbreak. Cane lesions are probably most important in mechanically pruned or hedged vineyards that have an abundance of canes in the canopy. All green tissues of the vine are susceptible to infection. Leaves are susceptible for about one week after they reach full size.
Brown circular lesions develop on infected leaves about nine to 11 days after infection (Figure 34). Within a few days, black spherical fruiting bodies (pycnidia) form within the lesions (Figure 35). The pycnidia are often arranged in a ring pattern just inside the margin of the lesion. Each one of these pycnidia can produce a second type of spore (conidium). These conidia are spread by rain splash and can cause secondary infections of leaves throughout the growing season and on fruit through about two to four weeks after bloom.
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| FIGURE 36. Black rot symptoms on berries. |
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| FIGURE 37. Close-up of black rot mummy. |
It is important to emphasize that a single ascospore can cause a primary infection (leaf lesion). Within each leaf lesion (primary infection), many pycnidia form. Each pycnidium can produce hundreds of thousands of conidia, each of which can cause another infection (secondary infection) later in the season. Therefore, it is extremely important to control the early season primary infections caused by ascospores. Infection by one ascospore can result in the development of millions of secondary conidia in the vineyard.
Lesions may also develop on young shoots, cluster stems (rachis), and tendrils. These lesions are usually purple to black, oval-shaped, and sunken. Pycnidia also form on these lesions.
The fruit infection phase of the disease can result in serious economic loss (Figure 36). Berries are susceptible to the infection from bloom until shortly after bloom. Older literature reports that berries become resistant when they reach 5% to 8% sugar. Research in New York indicates that berries become resistant to black rot much earlier (three to four weeks after bloom). Therefore, the most critical time to control black rot fruit infections with fungicide is from immediately prior to bloom through three to four weeks after bloom.
An infected berry first appears light brown in color. Soon the entire berry turns dark brown, and then black pycnidia develop on its surface. Infected berries eventually turn into shriveled, hard, black mummies (Figure 37). These mummies also serve as a source of secondary inoculum later in the growing season and are the primary means by which the fungus overwinters.
Sanitation is critical to successful black rot control. Mummies are the most important overwintering source of the black rot fungus. If all mummies and infected canes are removed from the vineyard, there is no source of primary inoculum in the spring and, thus, the disease is controlled. Any practice that removes mummies and other infected material from the vineyard will be beneficial to the disease-management program.
If all mummies cannot be removed from the vineyard, it is extremely important that they are not left hanging in the trellis. As mentioned previously, mummies on the ground appear to discharge their ascospores early in the season, while those hanging in the trellis may discharge ascospores and conidia throughout the growing season. The most critical period to control black rot with fungicide is from immediate prebloom through three to four weeks after bloom.
Powdery mildew is caused by the fungus Uncinula necator. If not controlled on susceptible cultivars, the disease can reduce vine growth, yield, quality, and winter hardiness. Cultivars of Vitis vinifera and its hybrids (French hybrids) are generally much more susceptible to powdery mildew than are native American cultivars such as Concord (see Table 14 on page 84-85). On susceptible cultivars, the use of fungicides to control powdery mildew is an important part of the disease-management program. Failure to provide adequate control of powdery mildew early in the growing season can result in increased levels of other fruit rots such as Botrytis bunch rot and sour rot.
The fungus can infect all green tissues of the grapevine. Disease losses due to fruit infection can be severe and can result in complete loss of the crop (Figure 39). It was previously thought that the fungus overwintered inside dormant buds of the grapevine. Research in New York has shown that almost all overwintering inoculum comes from cleistothecia, which are fungal fruiting bodies that overwinter primarily in bark crevices on the grapevine. In the spring, airborne spores (ascospores) released from the cleistothecia are the primary inoculum for powdery mildew infections.
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| FIGURE 38. Powdery mildew disease cycle. Photo used with permission of the New York State Agricultural Experiment Station, Cornell University. Figure taken from Grape IPM Disease Identification Sheet No. 2. |
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| FIGURE 39. Powdery mildew symptoms on grape berries (L) and Rachis (cluster stem) (R). | ||
NOTE: Ascospore discharge from cleistothecia is initiated if 0.10 inch of rain occurs at an average temperature of 50°F. Most mature ascospores are discharged within four to eight hours. These conditions can occur very early in the growing season. Thus, on highly susceptible cultivars, control needs to be initiated early in the growing season.
Ascospores are carried by wind. They germinate on any green surface on the developing vine, resulting in primary infections. The fungus grows and another type of spore (conidium) is formed over the infected area after six to eight days. The conidia and fungal mycelia give a powdery or dusty appearance to infected plant parts (Figure 40). Severely affected leaves may curl upward during hot dry weather. Young expanding leaves that are infected may become distorted and stunted (Figure 41). The conidia serve as secondary inoculum for new infections throughout the remainder of the growing season.
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| FIGURE 40. Powdery mildew. Primary infections of powdery mildew on grape leaf (Top) and powdery mildew covering grape leaf surface (Bottom). | FIGURE 41. Powdery mildew infections on young grape leaves can result in distortion of the leaves. |
It is important to note that a primary infection caused by one ascospore will result in the production of hundreds of thousands of conidia, each of which is capable of causing secondary infections. Therefore, early season control of primary infections caused by ascospores is necessary. If primary infections are controlled until all the ascospores have been discharged, the amount of inoculum available for causing late-season (secondary) infections is greatly reduced. On young shoots, infections are more likely to be limited, and they appear as dark-brown to black patches that remain as dark patches on the surface of dormant canes.
Most economic losses from powdery mildew result from fruit infection. Infected berries often are misshapen or have rusty spots on the surface. Severely affected fruit often split open. When berries of purple or red cultivars are infected as they begin to ripen, they fail to color properly and have a blotchy appearance at harvest. Research in New York has shown that berries are susceptible to infection from bloom through a few weeks after bloom. Berries of Concord grapes are quite resistant within two to three weeks after bloom. Therefore, the most critical time to control fruit infection with fungicide is from immediately prior to bloom through two to four weeks after bloom. Even though the berries become resistant with age, cluster stems (rachis) and leaves remain susceptible throughout the season. Therefore, a full-season fungicide program is generally required for powdery mildew control on susceptible cultivars.
Although infection can occur at temperatures from 59° to 90°F, temperatures of 68° to 77°F are optimal for infection and disease development. Temperatures above 95°F inhibit germination of conidia and above 104°F they are killed. High relative humidity is conducive to production of conidia. Atmospheric moisture in the 40% to 100% relative humidity range is sufficient for germination of conidia and infection. Free moisture, especially rainfall, is detrimental to the survival of conidia. This is in contrast to most other grape diseases, such as black rot and downy mildew, that require free water on the plant surface before the fungal spores can germinate and infect. Low, diffuse light seems to favor powdery mildew development. Under optimal conditions, the time from infection to production of conidia is only about seven days.
It is important to remember that powdery mildew can be a serious problem during growing seasons when it is too dry for most other diseases, such as black rot or downy mildew, to develop. Thick canopies that retain high levels of relative humidity are highly conducive to infections in the center of the row canopy.
Cleistothecia are formed on the surface of infected plant parts in late fall. Many of them are washed into bark crevices on the vine trunk where they overwinter to initiate primary infections during the next growing season.
For many years, the Eastern grape industry recognized a disease called dead-arm, which was thought to be caused by the fungus Phomopsis viticola. In 1976, researchers demonstrated that the dead-arm disease was actually two different diseases that often occur simultaneously. Phomopsis cane and leaf spot is the name for the cane-and-leaf-spotting phase of what was once known as dead-arm. Eutypa dieback is the name for the canker-and-shoot-dieback phase of what was also once known as dead-arm. The name dead-arm has been dropped. Growers should remember that Phomopsis cane and leaf spot and Eutypa dieback are distinctly different diseases and their control recommendations vary greatly.
Disease incidence of Phomopsis cane and leaf spot appears to be increasing in many vineyards throughout the Midwest. Crop losses up to 30% have been reported in some Ohio vineyards in growing seasons with weather conducive to disease development. The most commonly observed symptoms are on shoots where infections give rise to black spots or elliptical lesions that are most numerous on the first three to four basal internodes. Although this phase of the disease can appear quite severe, crop loss due to shoot infections has not been demonstrated. Heavily infected shoots are more prone to wind damage.
Although shoot infections may not result in direct crop loss, lesions on shoots serve as an extremely important source of inoculum for cluster stem (rachis) and fruit infections in the spring. Rachis and fruit infection is the phase of the disease that causes most economic loss.
Phomopsis cane and leaf spot is caused by the fungus Phomopsis viticola. The fungus overwinters in lesions or spots on one- to three-year-old wood infected during previous seasons. It requires cool weather and rainfall for spore (conidia) release and infection. Conidia are released from pycnidia (fungal fruiting bodies) in early spring and are spread by rain to developing shoots and leaves. Shoot and leaf infection (Figures 43 and 44) is most likely during the period from bud break until shoots are 6 to 8 inches in length. Lesions appear three to four weeks after infection.
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| FIGURE 42. Phomopsis cane and leaf spot disease cycle. Used with permission of the New York State Agricultural Experiment Station, Cornell University. Figure taken from Grape IPM Disease Identification Sheet No. 6. |
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| FIGURE 43. Phomopsis cane and leaf spot symptoms on internodes and rachis. | FIGURE 44. Phomopsis symptoms on young grape leaf (Top) and old grape leaf (Bottom). |
The critical period for fruit and rachis infection (Figure 45) is also early in the season. The rachis and young fruits are susceptible to infection throughout the growing season; however, most infections appear to occur early in the growing season. The fungus does not appear to be active during warm summer months, and most or all of its primary inoculum is probably released and expended early in the growing season. Thus, the critical period to provide fungicide protection for fruit and rachis infection is probably from when the clusters are first exposed until two to four weeks after bloom.
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| FIGURE 45. Phomopsis rachis infection (L) and fruit rot (R) on grape. | ||
The tiny green fruits that are infected during this critical period may appear to remain normal. The fungus remains inactive in these fruits as a latent infection. Not until the fruit starts to ripen near harvest does the fungus become active and cause the fruit to rot. Therefore, fruit rot that appears at harvest is probably due to infections that occurred during or shortly after bloom.
Fruit rot first appears close to harvest as a light-brown color. Black, spore-producing structures of the fungus (pycnidia) then break through the berry skin, and the berry soon shrivels.
At this advanced stage, Phomopsis fruit rot can be easily mistaken for black rot. Growers should remember that the black rot fungus does not infect berries late in the growing season, and black rot symptoms develop long before harvest. Berries become resistant to black rot infection by three to four weeks after bloom. Fruit rot symptoms caused by Phomopsis generally do not appear until harvest. Although the fungus does not appear to be active during the warm summer months, it can become active during cool, wet weather later in the growing season.
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| FIGURE 46. Downy mildew fruit infection. |
Downy mildew is a major disease of grapes throughout the eastern United States. The fungus causes direct yield losses by rotting inflorescences, clusters (Figure 46), and shoots. Indirect losses can result from premature defoliation. Premature defoliation is a serious problem, because it predisposes the vine to winter injury. It may take a vineyard several years to fully recover after severe winter injury. In general, vinifera (Vitis vinifera) cultivars are much more susceptible than American types; the French hybrids are somewhat intermediate in susceptibility (see Table 14 on page 84-85).
Downy mildew is caused by the fungus Plasmopora viticola. The causal fungus overwinters as tiny oospores in leaf debris on the vineyard floor. In the spring, the oospores serve as primary inoculum and germinate in water to form sporangia. The sporangia liberate small swimming spores, called zoospores, when free water is present. The zoospores are disseminated by rain splash to grape tissues where they swim to the vicinity of stomata and encyst. Stomata are tiny pores through which the plants exchange air, and transpiration occurs. Stomata are concentrated on the underside of the leaves. Encysted zoospores infect grape tissues by forming germ tubes that enter stomata and from there invade inner tissues of the plant. The fungus can infect all green, actively growing parts of the vine that have mature, functional stomata.
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| FIGURE 47. Downy mildew disease cycle. Used with permission of the New York State Agricultural Experiment Station, Cornell University. Figure taken from Grape IPM Disease Identification Sheet No. 5. |
Infected leaves develop yellowish-green lesions on their upper surfaces (Figure 48) seven to 12 days after infection. As lesions expand, the affected areas turn brown, necrotic, or mottled (Figure 49). At night, during periods of high humidity and temperatures above 55°F, the fungus sporulates by forming sporangia on numerous branched structures, called sporangiophores, that protrude out through stomata on the undersides of the leaf. Sporulation only occurs on plant surfaces that contain stomata, such as the underside of leaves, and it gives the surface of the lesion its white, downy appearance, which is characteristic of the disease (Figure 50). The sporulation (downy growth) generally occurs directly below the yellowish-green spots that develop on the upper surface of the leaf.
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| FIGURE 48. Downy mildew symptoms on upper leaf surface. | FIGURE 49. Downy mildew-affected areas turn brown, necrotic, or mottled with age later in the growing season. | FIGURE 50. Downy mildew symptoms on lower leaf surface. The patches of downy growth are usually directly beneath the yellowish-green spots observed on the upper leaf surface. |
Sporangia are disseminated by wind or rain splash. On susceptible tissue they liberate zoospores into water films formed by rain or dew. These zoospores initiate secondary infections, which can occur in as little as two hours of wetting at 77°F or up to nine hours at 43°F. Infections are usually visible as lesions in about seven to 12 days, depending on temperature and humidity. The number of secondary infection cycles depends on the frequency of suitable wetting periods that occur during the growing season and the presence of susceptible grape tissue. In general, Catawba, Chancellor, Chardonnay, Delaware, Fredonia, Ives, Niagara, White Riesling, and Rougeon are highly susceptible cultivars (see Table 14 on page 84-85).
Severely infected leaves may curl and drop from the vine. The disease attacks older leaves in late summer and autumn, producing a mosaic of small, angular, yellow to red-brown spots on the upper leaf surface. Lesions commonly form along leaf veins, and the fungus sporulates in these areas on the lower leaf surface. When young shoots, petioles, tendrils, or cluster stems are infected, they frequently become distorted, thickened, or curled. White, downy sporulation can be abundant on the surface of infected areas. Eventually, severely infected portions of the vine wither and die.
Infected green fruit turn light brown to purple, shrivel, and detach easily. White, cottony sporulation is abundant on these berries during humid weather. The fruits remain susceptible as long as stomata on their surfaces are functional. After that, new infections and sporulation do not develop.
Recent research indicates that fruit become resistant to infection by downy mildew about three to four weeks after bloom. Although fruit become resistant shortly after bloom, cluster stems (rachis) and leaves remain susceptible throughout the growing season. Later in the season, some berries that were infected earlier in the growing season may turn dull green to reddish purple, remain firm, and are easily distinguished from non-infected ripening berries in a cluster. Infected berries are easily detached from their pedicels leaving a dry stem scar.
Throughout most of the Midwest, downy mildew symptoms often do not appear until after bloom. This is why we often refer to it as a late-season disease. The role of oospores in causing early season primary infections is not clearly defined. Although we emphasize the use of fungicides for downy mildew control after bloom, early season fungicide applications are very important. Especially on highly susceptible cultivars, the early season fungicide program should contain a fungicide effective against downy mildew. As with black rot and powdery mildew, the period from immediate prebloom through three to four weeks after bloom is critical for controlling fruit or cluster infections by downy mildew.
Botrytis bunch rot (gray mold) is caused by the fungus Botrytis cinerea. The fungus causes blight of leaves, shoots, and blossom clusters and occurs throughout the viticultural world. The fungus causing the disease grows and reproduces on senescent or dead plant tissue. Botrytis bunch rot is especially severe in grape cultivars with tight, closely packed clusters of fruit. Botrytis is also responsible for storage losses of grapes picked for fresh market.
Botrytis infection of leaves begins as a dull, green spot, commonly surrounding a vein, which rapidly becomes a brown necrotic lesion. The fungus may also cause a blossom blight or a shoot blight, which can result in significant crop losses. However, the most common phase of this disease is the infection and rot of ripening berries (Figure 51). Fruit rot can spread rapidly throughout the cluster. Infected berries of white cultivars often become brown and shriveled, and those of purple cultivars develop a reddish color. Under proper weather conditions, the fungus produces a fluffy, gray-brown growth containing spores (Figure 52).
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| FIGURE 51. Botrytis bunch rot of grape. | FIGURE 52. Close-up showing the Botrytis fungus sporulating on infected berries. |
The fungus overwinters on debris in the vineyard floor and on the vine. The fungus produces small, dark, hard, resting structures called sclerotia. Sclerotia are resistant to adverse weather conditions and usually germinate in spring. Germinated sclerotia produce conidia, which spread the disease. Sporulation may also occur on debris left on the vine during the previous growing season, such as cluster stems remaining after mechanical harvest or mummified fruit.
The fungus usually gains a foothold by colonizing dead tissue prior to infection of healthy tissue. Tissue injured by hail, wind, birds, other diseases or insects is readily colonized by Botrytis. Ripe berries that split because of internal pressure or because of early season infection by powdery mildew, are especially susceptible to infection by Botrytis. Botrytis conidia are usually present in the vineyard throughout the growing season. Moisture in the form of fog or dew and temperatures of 59°F to 77°F are ideal for conidia production and infection. Rainfall is not required for disease development, although periods of rainfall are highly conducive to disease development.
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| FIGURE 53. Botrytis bunch rot disease cycle. Used with permission of the New York State Agricultural Experiment Station, Cornell University. Figure taken from Grape IPM Disease Identification Sheet No. 3. |
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| FIGURE 54. Canker on Eutypa-infected vine. |
Eutypa dieback is caused by the fungus Eutypa lata. Eutypa dieback is the name for the canker-and-shoot-dieback phase of what was once known as dead-arm. The name dead-arm should be dropped.
The earliest symptom to develop is a canker that generally forms around pruning wounds in older wood of the main trunk (Figure 54). These cankers usually are difficult to see because they are covered with bark. One indication of a canker is a flattened area on the trunk. Removal of bark over the canker reveals a sharply defined region of darkened or discolored wood bordered by white, healthy wood. Cankers may be up to three-feet long and extend below the soil line (Figure 55). When the trunk is cut in cross-section, the canker appears as darkened or discolored wood extending in a wedge shape to the center of the trunk (Figure 56).
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| FIGURE 55. Eutypa canker extending the entire length of the trunk. | FIGURE 56. Eutypa-infected trunk cut in cross section. |
The most striking and obvious symptoms of Eutypa dieback are the leaf-and-shoot symptoms (Figure 57), which may not develop for two to four years after the vine was first infected. These symptoms are most obvious in spring, when healthy shoots are 12- to 24-inches long. Spring shoot growth on diseased canes is weak and stunted above the cankered area. Leaves are at first smaller than normal, cupped, distorted, and yellow. These leaf and shoot symptoms may not be as obvious later in the season (mid July). Leaf and shoot symptoms are more pronounced each year until the affected portion of the vine finally dies.
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| FIGURE 57. Close up of Eutypa symptoms on grape leaves (L); leaf symptoms on a Eutypa-infected grape vine (R). Note that symptoms may occur on some shoots, while others appear normal. | ||
The fungus survives in infected trunks for long periods, whether as part of the in-place vine or as old, dead grape wood in the vineyard. The fungus is generally present in older wood, such as vine trunks, but generally not in younger wood, such as one- or two-year-old prunings.
The fungus eventually produces reproductive structures (perithecia) on the surface of infected wood. Spores (ascospores) are produced in these structures and discharged into the air. Ascospore discharge is initiated by the presence of free water (rainfall or snow melt). Most spores appear to be released during winter or early spring; few are released during the summer.
Unfortunately, most spores are released at about the same time pruning is being conducted. Air currents can carry the ascospores long distances to fresh wounds on the trunk. Pruning wounds are by far the most important points of infection. The ascospores germinate when they contact the newly cut wood, and a new infection is initiated. Stunted shoots and small, cupped leaves appear two to four years after infection. After approximately five years, the fungus produces perithecia and ascospores in the dead wood on cankers.
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| FIGURE 58. Eutypa dieback disease cycle. Photo used with permission of the New York State Agricultural Experiment Station, Cornell University. Figure taken from Grape IPM Disease Identification Sheet No. 1. |
The primary control method is removal of infected trunks from the vineyard. The vine must be cut off below the cankered or discolored wood. If the canker extends below the soil line, the entire vine must be removed. If the canker does not go below the soil line, the stump can be left and a new trunk formed. The best time to identify and remove infected vines is in early spring (May and June) when leaf and shoot symptoms are most obvious. In addition, large wounds are less susceptible to infection at this time of year, and fewer ascospores are present to cause reinfection. If trunks cannot be removed in the spring, they should be marked for easy identification and removal later in the growing season.
Sanitation is critical. All wood (especially trunks and stumps) from infected plants must be removed from the vineyard and destroyed (either buried or burned) as soon as possible. An old infected stump or trunk lying on the ground may continue to produce spores for several years.
The double trunk system of training, where each trunk is pruned to carry half the number of buds, may help reduce crop loss caused by Eutypa dieback. If a diseased trunk must be removed, the remaining trunk can be pruned to leave the full number of buds until a new second trunk can be established.
Fungicide recommendations currently are not available for control of this disease.
Anthracnose of grape was first detected in the United States in the mid 1800s. The disease was probably introduced into this country by grape plant material imported from Europe. It quickly established in American vineyards and became a significant disease of grape in rainy, humid, and warm regions of the United States. Anthracnose reduces the quality and quantity of fruit and weakens the vine. Once the disease is established in a vineyard, it can be very destructive.
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| FIGURE 59. Anthracnose symptoms on grape cane. |
All succulent parts of the plant, including fruit stems, leaves, petioles, tendrils, young shoots, and berries, can be attacked, but lesions on shoots and berries are most common and distinctive. Symptoms on young, succulent shoots first appear as numerous small, circular, and reddish spots. Spots then enlarge, become sunken, and produce lesions with gray centers and round or angular edges (Figure 59). Dark reddish-brown to violet-black margins eventually surround the lesions. Lesions may coalesce, causing a blighting or killing of the shoot.
A slightly raised area may form around the edge of the lesion. Infected areas may crack, causing shoots to become brittle. Anthracnose lesions on shoots may be confused with hail injury; however, unlike hail damage, the edges of the wounds caused by the anthracnose fungus are raised and black. In addition, hail damage generally appears on only one side of the shoot, whereas anthracnose is more generally distributed. Anthracnose on petioles appears similar to that on the shoots.
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| FIGURE 60. Anthracnose symptoms on grape leaf. |
Leaf spots are often numerous and develop in a similar manner to those on shoots. Eventually, they become circular with gray centers and brown to black margins with round or angular edges. The necrotic center of the lesion often drops out, creating a shot-hole appearance (Figure 60). Young leaves are more susceptible to infection than older leaves. When veins are affected, especially on young leaves, the lesions prevent normal development, resulting in malformation or complete drying or burning of the leaf. Lesions may cover the entire leaf blade or appear mainly along the veins.
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| FIGURE 61. Anthracnose symptoms on grape berry. Note that the lesion resembles a bird’s eye. |
On berries, small, reddish circular spots initially develop. The spots then enlarge to an average diameter of 1/4 inch and may become slightly sunken. The centers of the spots turn whitish gray and are surrounded by narrow reddish-brown to black margins (Figure 61). This typical symptom on fruit often resembles a bird’s eye, and the disease has been called bird’s eye rot. Acervuli (fungal fruiting structures) eventually develop in the lesions. A pinkish mass of fungal spores (conidia) exudes from these structures during prolonged wet weather.
Anthracnose of grape is caused by the fungus Elsinoe ampelina. The fungus overwinters in the vineyards as sclerotia (fungal survival structures) on infected shoots. In the spring, sclerotia on infected shoots germinate to produce abundant spores (conidia) when they are wet for 24 hours or more and the temperature is above 36°F. Conidia are spread by splashing rain to new growing tissues and are not carried by wind alone.
Another type of spore, called an ascospore, is produced within sexual fruiting bodies and may also form on infected canes and berries left on the ground or in the trellis from the previous year. The importance of ascospores in disease development is not clearly understood.
Conidia are by far the most important source of primary inoculum in the spring. In early spring, when free moisture from rain or dew is present, conidia germinate and infect succulent tissue. Conidia germinate and infect at temperatures ranging from 36°F to 90°F. The higher the temperature, the faster disease develops. Disease symptoms start to develop approximately 13 days after infection occurs at 36°F and at four days after infection occurs at 90°F. Heavy rainfall and warm temperatures are ideal for disease development and spread.
Lesions may extend into the pulp and cause the fruit to crack. Lesions on the rachis and pedicels appear similar to those on shoots. Clusters are susceptible to infection before flowering and until veraison.
Once the disease is established, asexual fruiting bodies called acervuli form on diseased areas. These acervuli produce conidia during periods of wet weather. These conidia are the secondary source of inoculum and are responsible for continued spread of the fungus and the disease throughout the growing season.
Crown gall is caused by the bacterium Agrobacterium tumefaciens. The disease infects more than 2,000 species of plants. Crown gall of grape is a major problem in cold climate regions. Wounds are necessary for infection to occur. Observations suggest that freeze injury wounds are highly conducive to infection. The disease is particularly severe following winters that result in freeze injury on cold-sensitive cultivars, such as those of Vitis vinifera.
Crown gall is characterized by galls or overgrowths that usually form at the base of the trunk. Galls form as high as 3-feet or more up the trunk (aerial galls). Galls generally do not form on roots. The disease affects all grape cultivars. Vines with galls at their crowns or on their major roots grow poorly and have reduced yields. Severe economic losses result in vineyards where a high percentage of vines become galled within a few years of planting.
 |
| FIGURE 62. Crown gall symptoms on grape trunk. |
The disease first appears as small overgrowths or galls on the trunk, particularly near the soil line. Early in their development, the galls are more or less spherical, white or flesh-colored, and soft. Because they originate in a wound, the galls at first cannot be distinguished from callus. However, they usually develop more rapidly than callus tissue. As galls age, they become dark brown, knotty, and rough (Figure 62).
When galls are numerous on the lower trunks or major roots, they disrupt the translocation of water and nutrients, which leads to poor growth, gradual dieback, and sometimes death of the vine. In some cases, infected vines appear stunted and as if they are suffering from nutrient deficiency.
The causal organism, a bacterium, is soil borne and persists for long periods in plant debris in the soil. Fresh wounds are required to infect and initiate gall formation. Wounds that commonly serve as infection sites are those made during pruning, machinery operations, freezing injury, or any other practice that injures the vine.
In addition to the primary galls, secondary galls may also form around other wounds and on other portions of the plant, even in the absence of the bacterium. Crown gall bacteria also survive systemically within grapevines and probably are most commonly introduced into the vineyard on or in planting material.
Examine new plants before planting and discard any that have galls. Wounding by freeze injury appears to be important in the development of crown gall. If winter injury is controlled, crown gall may not be an important problem. Prevent winter injury to vines. Practices, such as hilling or burying vines of cold-sensitive cultivars, is beneficial.
Proper pruning practices and proper crop loads for maximum vine vigor will result in stronger plants that are less susceptible to winter injury. Controlling other diseases, such as downy and powdery mildew, is also important in preventing winter injury and crown gall.
The double-trunk system of training, in which each trunk is pruned to carry half the number of buds, may help reduce crop loss caused by crown gall. If a diseased trunk must be removed, the remaining trunk can be pruned, leaving the full number of buds until a second trunk is established. Galls on arms or the upper parts of the trunk can be removed by pruning.
There are no current chemical or biological control recommendations for crown gall on grapes.
In an integrated disease-management program where emphasis is placed on reducing overall fungicide use, it is essential that any available disease resistance be identified and used. If resistance is not available, we should at least identify and avoid those cultivars that are highly susceptible to important diseases.
A few grape cultivars have high levels of resistance to most diseases. Norton (Cynthiana) is a cultivar that has great potential for organic production in the southern portion of the Midwest. It has good resistance to most diseases, and in several commercial plantings, growers rarely apply fungicides. It is hoped that new cultivars with improved levels of disease resistance will be introduced in the near future.
Unfortunately, resistance to most of the major diseases is not available in most commercially grown grape cultivars in the Midwest. Thus, the disease-management program must rely heavily on the use of cultural practices and efficient use of approved fungicides or biocontrol agents or products. Whereas resistance is generally not available for most diseases, some grape cultivars are known to be much more susceptible to certain diseases than others (Table 14). For example, the cultivar Chancellor is highly susceptible to downy mildew, whereas downy mildew is seldom a serious problem on Concord and several other cultivars.
Growers should consider disease susceptibility before establishing the vineyard. Segregating highly susceptible cultivars into blocks that can be easily treated separately allows growers to apply more fungicide or other control agents when needed for highly susceptible cultivars while reducing their use on less susceptible cultivars. In addition, cultivars differ greatly in their sensitivity to copper and sulfur fungicides. When planting the vineyard, it is important to isolate blocks of sensitive cultivars from those that will be sprayed with these materials.
| Table 14. Relative Disease Susceptability and Sulfur and Copper Sensitivity Among Grape Cultivars. | ||||||||||
| Cultivar | BR | DM | PM | Bot | Phom | Eu | CG | ALS | Susceptible or Sensitive to | |
|---|---|---|---|---|---|---|---|---|---|---|
| S1 | C2 | |||||||||
| Aurore | +++ | ++ | ++ | +++ | + | +++ | ++ | +++ | No | ++ |
| Baco noir | +++ | + | ++ | ++ | + | ++ | +++ | ++ | No | ? |
| Cabernet Fanc | +++ | +++ | +++ | + | ? | ? | +++ | ? | No | ? |
| Cabernet Sauvignon | +++ | +++ | +++ | + | +++ | +++ | +++ | ? | No | + |
| Candice | +++ | ++ | + | ++ | ? | ? | ++ | ++ | ? | ? |
| Cascade | + | + | ++ | + | ++ | ++ | + | ? | No | ? |
| Catawba | +++ | +++ | ++ | + | +++ | + | + | + | No | ++ |
| Cayuga White | + | ++ | + | + | + | + | ++ | ++ | No | + |
| Chambourcin | +++ | ++ | + | ++ | ? | ? | ++ | ? | Yes | ? |
| Chancellor | + | +++ | +++ | + | +++ | + | +++ | +++ | Yes | +++ |
| ++ | ++ | ++ | ++ | ? | ? | ++ | ++ | No | ? | |
| Chardonnay | ++ | +++ | +++ | +++ | +++ | ++ | +++ | ++ | No | + |
| Chelois | + | + | +++ | +++ | +++ | +++ | ++ | +++ | No | + |
| Concord | +++ | + | ++ | + | +++ | +++ | + | + | Yes | + |
| Cynthiana/Norton | + | ++ | + | + | + | ? | + | ? | Yes | ? |
| DeChaunac | + | ++ | ++ | + | +++ | +++ | ++ | +++ | Yes | + |
| Delaware | ++ | +++3 | ++ | + | +++ | + | + | + | No | + |
| Dutchess | +++ | ++ | ++ | + | ++ | + | ++ | + | No | ? |
| Elvira | + | ++ | ++ | +++ | + | + | ++ | ++ | No | ++ |
| Einset Seedless | +++ | ++ | +++ | + | ? | ? | + | ? | ? | ? |
| Fredonia | ++ | +++ | ++ | + | ++ | ? | + | + | No | ? |
| Frontenac | ++ | + | ++ | ++ | + | ? | ? | ? | No | ? |
| Gewurtztraminer | +++ | +++ | +++ | +++ | ? | ? | +++ | + | No | + |
| Himrod | ++ | + | ++ | + | ? | ? | ? | + | No | ? |
| Ives | + | +++ | + | + | ? | ++ | + | + | Yes | ? |
| Jupiter | ++ | + | +++ | + | + | ? | ? | ? | ? | ? |
| LaCrosse | +++ | ++ | ++ | +++ | ++ | ? | ? | ? | ? | ? |
| Leon Millot | + | ++ | +++ | + | + | + | ? | ? | Yes | ? |
| Limberger | +++ | +++ | +++ | + | ? | +++ | +++ | ? | No | ? |
| Merechal Foch | ++ | + | ++ | + | ? | +++ | ? | + | Yes | ? |
| Marquis | + | +++ | + | + | +++ | ? | ? | ? | ? | ? |
| Mars | + | + | + | + | + | ? | + | ? | ? | ? |
| Melody | +++ | ++ | + | + | ? | ? | ? | ? | No | ? |
| Merlot | ++ | +++ | +++ | ++ | + | +++ | +++ | ? | No | ++ |
| Moore's Diamond | +++ | + | +++ | ++ | ? | ++ | ? | ? | No | ? |
| Muscat Ottonel | +++ | +++ | +++ | ++ | ? | +++ | +++ | ? | No | ? |
| Niagara | +++ | +++ | ++ | + | +++ | + | ++ | + | No | + |
| Pinot gris | +++ | +++ | +++ | ++ | ? | +++ | +++ | ? | No | ? |
| Pinot Meunier | +++ | +++ | +++ | +++ | ? | +++ | +++ | ? | No | ? |
| Pinot blanc | +++ | +++ | +++ | ++ | ? | ? | +++ | ? | No | + |
| Pinot noir | +++ | +++ | +++ | +++ | ? | ? | +++ | + | No | + |
| Reliance | +++ | +++ | ++ | + | ++ | ? | ? | ? | No | ? |
| Riesling | +++ | +++ | +++ | +++ | ++ | ++ | +++ | + | No | + |
| Rosette | ++ | ++ | +++ | + | ++ | ++ | ++ | ++ | No | +++ |
| Rougeon | ++ | +++ | +++ | ++ | +++ | + | ++ | +++ | Yes | +++ |
| Saint Croix | ? | ++ | ++ | ++ | ? | ? | ? | ? | ? | ? |
| Sauvignon blanc | +++ | +++ | +++ | +++ | ? | ? | +++ | ? | No | + |
| Seyval | ++ | ++ | +++ | +++ | ++ | + | ++ | ++ | No | + |
| Steuben | ++ | + | + | + | ? | ? | + | ++ | No | ? |
| Traminette | + | ++ | + | + | ? | ? | ++ | ? | ? | ? |
| Vanessa | +++ | ++ | ++ | + | + | ? | + | ? | ? | ? |
| Ventura | ++ | ++ | ++ | + | + | ? | + | +++ | No | ? |
| Vidal blanc | + | ++ | +++ | + | + | + | ++ | + | No | ? |
| Vignoles | + | ++ | +++ | +++ | ++ | ++ | ++ | ++ | No | ? |
| Villard noir | ? | + | +++ | + | ? | ? | ? | ? | ? | ? |
| Key to susceptibility or sensitivity: BR = black rot; DM = downy mildew; PM = powdery mildew; Bot = Botrytis; Phom = Phomopsis; Eu = Eutypa; CG = crown gall; ALS = angular leaf scorch; S = sulfur; C = copper. | ||||||||||
| Key to ratings: + = slightly susceptible or sensitive; ++ = moderately susceptible or sensitive; +++ = highly susceptible or sensitive; No = not sensitive; Yes = sensitive; ? = relative susceptibility or sensitivity not established. | ||||||||||
| 1 Slight to moderate sulfur injury may occur even on tolerant cultivars when temperatures are 85ºF or higher during or immediately following the application. | ||||||||||
| 2 Copper applied under cool, slow-drying conditions is likely to cause injury. | ||||||||||
| 3 Berries not susceptible. | ||||||||||
The use of any practice that reduces or eliminates pathogen populations or creates an environment within the planting that is less conducive to disease development should be used. Certain diseases, such as viruses, Eutypa dieback, and crown gall, cannot be directly controlled with pesticides at the present time. Therefore, cultural practices are the major means for their control. When fungicides or other control agents are required, any practice that opens the plant canopy, such as shoot thinning, leaf removal, berry and cluster thinning, and pruning and shoot positioning, can greatly increase the efficacy of the fungicide program by allowing better spray penetration and coverage. These practices also have a direct effect on vine microclimate.
Vine microclimate refers to the climate within the leaf canopy of the vineyard. In relation to disease management, the most important elements of the vine microclimate are relative humidity, ventilation, the temperature of the air and of vine tissues, and the intensity and quality of light. In general, factors that increase relative humidity also increase fungal diseases. Factors that increase ventilation (air movement) of the vine canopy generally reduce disease incidence and severity by lowering the humidity, shortening periods of leaf and fruit wetness, and aiding spray penetration and coverage. Cultural practices should be carefully considered and implemented into the disease management program whenever possible. Here are some cultural practices to consider.
Always start the planting with healthy virus-indexed nursery stock from a reputable nursery. The importance of establishing plantings with virus-indexed nursery stock cannot be overemphasized, since the selection of planting stock and planting site are the only actions a grower can take to prevent or delay the introduction of most virus diseases. Plants obtained from an unknown source or a neighbor may be contaminated with a number of major diseases that reputable nurseries work hard to avoid.
Site selection can have a direct effect on vine microclimate. A site that provides for maximum air drainage, which promotes faster drying of foliage, can substantially reduce the risk of black rot and downy mildew. In the northern hemisphere, north-facing slopes receive less light than south-facing slopes. Therefore, vineyards on north-facing slopes may dry more slowly and be at a higher risk for disease development.
Avoid planting the vineyard adjacent to woods that will prevent sunlight from reaching the vines during any part of the day. Woods act as a windbreak that may be beneficial in preventing shoot breakage in high winds, but woods may also reduce air movement (ventilation) in the vineyard which results in prolonged wetting periods. Close proximity to woods can also increase the risk of introducing certain diseases and insect pests into the vineyard.
Planting rows in a north-south row orientation should be the grower’s first choice for maximum light penetration. However, rows planted in the direction of prevailing winds will promote better air movement, which results in faster drying of foliage and fruit. Rows should never be planted parallel to a steep slope where erosion could be more of a problem than pests.
Good soil drainage is also extremely important. Avoid sites that are consistently wet during the growing season. These soils may have an impervious subsoil or other drainage problems. Such sites will usually result in unsatisfactory vine growth and yields, in addition to providing a humid microclimate that is conducive to disease development. In some situations poor drainage can be corrected by tiling prior to planting.
If nematodes have been a problem in previous crops or nematodes are suspected to be a problem on the site, a soil analysis to determine the presence of harmful nematodes should be conducted. Nematodes are most likely to be a problem on lighter (sandy) soils. Nematode sampling kits and instructions for taking samples can be obtained through your county Extension office.
Fertility should be based on soil and foliar analysis. The use of excessive fertilizer, especially nitrogen, should be avoided. Sufficient fertility is essential to produce a crop, but excessive nitrogen can result in dense foliage that increases drying time in the plant canopy.
Good weed control within and between the rows is essential. From a disease control standpoint, weeds in the planting prevent air circulation and result in the fruit and foliage staying wet for longer periods. For this reason, most diseases caused by fungi are generally more serious in plantings with poor weed control than in those with good weed control.
Any cultural practice that alters vegetative growth and canopy density has an effect on vine microclimate. Most cultural practices are chosen primarily to enhance yield or fruit quality rather than to influence the microclimate. However, practices, such as shoot thinning, pruning, and positioning, have a direct impact on vine microclimate. Increasing cluster thinning and decreasing pruning stimulates vegetative growth and hence reduces light exposure and ventilation within the canopy.
Shoot thinning, leaf removal, and summer pruning are frequently done specifically to reduce canopy density, so as to increase fruit exposure to light, improve ventilation, and aid spray coverage. Leaf removal in the fruiting zone of the canopy is important for optimal control of Botrytis bunch rot. This is a common practice in California vineyards and has been shown to be effective in Midwest vineyards as well. Shoot positioning is usually done to ensure canopy separation of divided canopies or to enhance light exposure of the renewal zone of the vine; it also decreases vegetative growth and canopy density and increases light exposure of fruit.
Wounding by freeze injury is important in the development of crown gall. If winter injury is reduced, crown gall may not become an important problem. Practices such as hilling or burying vines of cold-sensitive cultivars are beneficial. Proper pruning practices and proper crop loads for maximum vine vigor will result in stronger plants that are less susceptible to winter injury. Controlling other diseases, such as downy and powdery mildew, is also important in preventing winter injury and crown gall.
Vineyard sanitation is an extremely important part of the disease-management program. Most pathogens overwinter (survive from one season to the next) in old diseased plant material, such as mummified fruit, leaves, and infected canes or trunks, within the vineyard. Removal of old, infected wood, tendrils, and clusters with mummified berries from the vines and wires greatly reduces overwintering inoculum of several diseases.
Wild grapes in nearby woods and fence rows also are sources of disease inoculum and insects. Removal of these wild hosts is beneficial to the disease-management program. This especially applies to abandoned vineyards adjacent to managed sites with respect to contamination from powdery and downy mildews.
Fungicides are an important part of the grape disease-management program. Due to the lack of disease resistance in most of our currently grown cultivars combined with our environmental conditions (abundant moisture) that are highly conducive to disease development, successful commercial grape production in the Midwest is highly unlikely without the use of at least some fungicide.
While fungicides are important, growers need to recognize that they are only one part of the overall integrated disease-management program. The effectiveness of the fungicide program is greatly influenced by use of the various cultural practices described previously and the level of disease susceptibility of the cultivars being grown. For example, given a poorly pruned (dense canopy) vineyard of Chancellor grapes (highly susceptible to downy mildew) planted on a poor site (little air circulation) and with poor weed control, the chance of any reasonable fungicide program providing an acceptable level of downy mildew control is highly unlikely.
To use any fungicide effectively, consider the following points:
If you do not know what disease or diseases are present in the vineyard, you cannot choose the most effective fungicide or fungicides for their control. Correct disease identification is essential for selecting the proper fungicide or fungicide combinations to use in the vineyard.
Fungicides differ greatly in their spectrum of activity (which fungi they can control). Selection of the wrong fungicide for use on a specific disease can result in financial loss and no control. For example, if a grower misidentified downy mildew for powdery mildew and sprayed Nova or Bayleton to control it, neither of these fungicides would have any effect on the downy mildew, although they would provide excellent control of powdery mildew.
For most diseases it takes at least a week from the time the fungus enters the plant until the symptoms appear. In the case of Phomopsis fruit rot, the fungus enters the fruit during bloom, and symptoms do not appear until the fruit begins to ripen (harvest). Depending upon the weather, it may take two weeks for black rot symptoms to appear. Once symptoms appear, it is too late to control the disease; therefore, proper timing of the application is critical. The fungus must be controlled before or shortly after it enters the plant.
If the fungicide is not on or in susceptible plant parts, it cannot control the fungus. Cultural practices that open the plant canopy greatly improve fungicide coverage. Proper calibration and use of the sprayer is also critical to good coverage.
Unfortunately, there are not many options to choose from when one considers our current fungicide-use strategies. The current options are:
This is always an option, but it is not recommended for commercial plantings. This option should not be confused with organic production. Grape growers in organic production systems will most probably use sulfur or copper to some extent for disease control. Sulfur, lime-sulfur, and copper are fungicides. Growers who choose not to use fungicides must rely completely on cultural practices and disease resistance for disease control.
In a protectant program, fungicides are used to form a protective barrier on the plant surface. This chemical barrier prevents the fungus from entering the plant. It works much like paint on a piece of wood to keep out water. Protectant fungicides are not systemic and cannot move into plant tissues. Once the fungus penetrates into the plant, protectant fungicides will not control it. As the protective barrier breaks down or new foliage is produced, additional applications are required to maintain the protective barrier.
Protectant fungicide programs have been and still are very effective; however, they generally result in a fairly intensive use of fungicides. Protectant fungicides are usually applied on a 7- to 10-day schedule early in the growing season and on a 10- to 14-day schedule later in the season. Obviously, maintaining a protective barrier on the plant surface throughout the growing season requires many applications.
The development and introduction of new systemic fungicides allows the use of a post-infection or curative fungicide-use strategy. In a post-infection program, fungicides are applied only after infection periods occur. The systemic properties of the fungicide allow it to move into plant tissues where it stops further development of the fungus after it has penetrated the plant. In the post-infection program, the fungicide is applied after the initiation of an infection period, but before symptoms develop. Thus, the fungicide must be applied within three to four days (72 to 96 hours) after the initiation of an infection period in order to be effective.
The sterol-inhibiting (SI) fungicides (Bayleton and Nova) have excellent post-infection activity against black rot and powdery mildew. Ridomil and Aliette have excellent post-infection activity against downy mildew. In dry growing seasons, with few or no infection periods, a post-infection program should result in reduced fungicide use.
There are several important points to remember about the post-infection program. In order to use a post-infection program, you must:
If these criteria cannot be met, growers should use a protectant fungicide spray program.
Specific fungicide recommendations cannot be made in this publication because of constantly changing regulations and recommendations regarding their agricultural use. For specific fungicide recommendations, consult your local Extension service. Most Midwestern states have a small fruit and grape spray guide that is revised annually. General information about fungicides that were available at the time this bulletin was published is presented here.
Mancozeb, Ferbam, and Ziram are all highly effective against black rot (Table 15 on page 96). Because these fungicides are strictly protectants, they must be applied before the fungus infects or enters the plant. They protect fruit and foliage by preventing spore germination. They will not arrest lesion development after infection has occurred.
Mancozeb provides an excellent foundation for a protectant spray program for grapes in the Midwest. It is a good protectant fungicide that will provide good to excellent control of downy mildew and Phomopsis cane and leaf spot in addition to black rot. The major problem with Mancozeb is a 66-day preharvest interval (PHI) on grapes. It cannot be applied within 66 days of harvest. Mancozeb is available under many trade names and formulations. Some common trade names are Manzate 200, Penncozeb, Dithane M45, Dithane F45, and Dithane Rainshield DF.
Some food processors may not accept Mancozeb-treated fruit or may have special restrictions on its use. This also applies to Captan. Prior to initiating a control program in the spring, growers need to know where they will sell their fruit and if the buyer has any restrictions on pesticide use.
Ziram is similar in efficacy to Ferbam. It is highly effective against black rot and provides moderate control of downy mildew and Phomopsis cane and leaf spot.
Growers of processing grapes who cannot apply Mancozeb past the initiation of bloom could use Ziram during this period. Ziram can be applied up to 21 days before harvest.
Ferbam will provide excellent control of black rot but is not highly effective against the other grape diseases. In addition, there are restrictions on the number of applications that can be used. Always read and understand the label before using or purchasing a pesticide.
Captan and copper fungicides (fixed copper or Bordeaux mixture) are only slightly to moderately effective against black rot and will probably not provide adequate control under heavy disease pressure.
The locally systemic fungicides, Bayleton, Nova, Elite, and Procure, are also highly effective against black rot and will provide some post-infection (curative) activity of the disease if applied at the higher labeled rates within 72 to 96 hours after the initiation of an infection period. Post-infection or curative control must be achieved prior to symptom development on leaves or fruit. Once the symptoms are present, these fungicides will not eradicate or burn out the fungus. Bayleton, Nova, Elite, and Procure also appear to provide good protectant activity against black rot if applied at the lower labeled rates in a protectant program. These fungicides also have excellent activity against powdery mildew as well.
Rubigan is another SI fungicide that is registered for use on grapes and will provide moderate control of black rot if applied in a protectant program. This fungicide is in the same general class of fungicides as Bayleton, Nova, Elite, and Procure; however, it does not provide adequate curative or post-infection control of black rot. Nova, Elite, Procure, or Bayleton are the preferred SI fungicides for black rot control.
Abound, Sovran, Flint, and Pristine are locally systemic fungicides that are all highly effective for control of black rot. They do differ in their efficacy against some of the other important grape diseases.
Note: Flint or Pristine cannot be applied on Concord grapes or phytotoxicity (damage) could occur. Always read the fungicide label carefully.
Sulfur is highly effective against powdery mildew if used in a protectant program with a minimum of seven to 14 days between applications (see Table 15 on page 96). There are many formulations of sulfur (wettable powders, dusts, dry flowables, and flowables). The flowable formulations appear to be most effective and result in much less applicator exposure when preparing sprays.
Note: On sulfur-tolerant cultivars that are susceptible to powdery mildew (Table 14), sulfur will probably be a major component of the fungicide program. On highly susceptible cultivars, spray intervals shorter than 14 days (7 to 10 days) will probably be required with sulfur. Although sulfur is highly effective for powdery mildew control, it has little or no effect on the other grape diseases (Table 15). It is important to remember that sulfur will cause severe injury on some grape cultivars. Sulfur should only be used on cultivars known to be sulfur tolerant (Table 14).
Note: Chancellor, Concord, DeChaunac, Foch, Norton, and Rougeon grapes are highly sensitive to sulfur. Sulfur injury may occur even on sulfur-tolerant cultivars when temperatures of 80 to 85°F or higher are experienced during or immediately after application.
Copper fungicides (fixed coppers or Bordeaux mixture) have been rated moderately effective against powdery mildew; however, care must be taken when using copper due to the danger of foliage injury (phytotoxicity). Grape cultivars differ in their sensitivity to copper fungicides (Table 14). Under heavy disease pressure, copper fungicides may not provide adequate control. Copper is not the preferred fungicide for powdery mildew control. However, if copper is applied for downy mildew control, it will provide some protection against powdery mildew. On less susceptible cultivars, such as Concord, copper fungicides may provide satisfactory control.
Nova, Elite, Procure, and Rubigan are locally systemic and highly effective for control of powdery mildew. They will also provide good to excellent control of black rot, but they will not control downy mildew. Bayleton was highly effective against powdery mildew when it was first introduced; however, due to development of fungicide-resistant strains of the powdery mildew fungus, Bayleton is no longer recommended for powdery mildew control.
Abound, Sovran, and Flint are locally systemic, and all were good to excellent for control of powdery mildew when they were first introduced. Fungicide resistance development in powdery mildew has been observed in strobilurin fungicides. At present, it may be necessary to combine the strobilurin fungicides with a fungicide of different chemistry with activity against powdery mildew in order to achieve acceptable control.
Pristine 38WG Fungicide is a combination of pyraclostrobin (a strobilurin fungicide) and Boscalid (Endura). The addition of Boscalid (Endura) gives Pristine activity against strains of the powdery mildew fungus with resistance to the strobilurins (Abound, Sovran, and Flint).
Note: Flint or Pristine cannot be applied on Concord grapes or phytotoxicity (damage) can occur. Always read the fungicide label carefully.
Endura 70WG Fungicide is new fungicide chemistry and is highly effective for control of powdery mildew and provides good control of Botrytis bunch rot. It is different chemistry from the sterol-inhibiting and strobilurin fungicides; therefore, it is an excellent material to use in rotation with these materials in a fungicide resistance management program.
Quintec 2.08SC is new fungicide chemistry that is very effective for control of powdery mildew, but it has no activity against the other grape diseases. It is a protectant fungicide so it must be applied before infection occurs. It does not have curative activity. It is registered for use at the rate of 3 to 4 fluid ounces per acre on a seven- to 14-day schedule. Because it is new chemistry (not related to other fungicides), it will control strains of the powdery mildew fungus that are resistant to the strobilurin fungicides (Abound, Sovran, Flint, and Cabrio) and the sterol-inhibiting fungicides (Nova, Elite, Procure, and Rubigan). Quintec has a 12-hour re-entry interval and a 14-day preharvest interval.
JMS Stylet-Oil is a highly refined petroleum distillate that is registered for use on grapes in the United States. It has provided excellent powdery mildew control in fungicide tests in Ohio and New York and is currently being used rather extensively by California grape growers for powdery mildew control. It is registered for use at the rate of 1 to 2 gallons oil per 100 gallons water (1% to 2% concentration). The label states on grapes: “Make first application pre-bloom and continue sprays every two to three weeks depending on level of disease pressure. Use higher rates and shorter spray interval when disease conditions are severe.”
Although this fungicide has not been used on grapes extensively in the Midwest or northeastern United States, it appears to have good potential as an alternative fungicide for powdery mildew control on grape.
Note: One potential problem with stylet oil is that it removes the “bloom” or waxy coating from the grape berry. This apparently has no effect on quality of wine or juice grapes, but it does affect the appearance of the berry and probably should not be used for fresh-market table grapes.
Note: DO NOT use CAPTAN or SULFUR within two weeks after applying JMS STYLET- OIL. Mixing Captan or Sulfur with oil could result in severe damage to the vine. In addition, repeated use of oil during the growing season has been shown to be phytotoxic to vines.
Armicarb 100 (potassium bicarbonate) and Nutrol (manopotassium phosphate) have been reported to provide fair control of powdery mildew on grape but provide no control of the other grape diseases. It is assumed that they provide control through limited eradication and antisporulant activity. They do not provide protectant activity.
At present, Mancozeb, Captan, or Ziram are the fungicides recommended for control of this disease (Table 15). They are ranked as moderately to highly effective.
Fungicide test results indicate that the sterol inhibitors are not effective and the strobilurins only provide moderate control. Copper and sulfur fungicides appear to be ineffective.
Note: Especially where Phomopsis is a problem or a concern, Mancozeb, Captan, or Ziram should be included in the early-season fungicide program.
Mancozeb, Captan, and Copper fungicides (fixed coppers and Bordeaux mixture) are highly effective for control of downy mildew (Table 15). Ziram is moderately effective. All of these fungicides are effective only when used in a protectant spray program. They will not provide post-infection or curative activity and will not eradicate or burn out the fungus after symptoms appear.
Of the protectant fungicides currently available, Mancozeb is an excellent choice. Mancozeb is highly effective against downy mildew, black rot, and Phomopsis cane and leaf spot. One problem with Mancozeb is that it cannot be applied within 66 days of harvest. Even with this restriction, Mancozeb is an excellent protectant fungicide for early-season disease control and can also be used on later-maturing cultivars for post-bloom disease control (prior to 66 days of harvest).
Captan is also excellent for downy mildew and Phomopsis cane and leaf spot but is weak for controlling black rot. A good approach to using Mancozeb and Captan for downy mildew control is to use Mancozeb early in the season then switch to Captan within the 66-day preharvest interval for Mancozeb. Currently Captan does not have a preharvest interval -for grapes.
Note: Although Captan has no preharvest interval on grapes, it does have a three-day reentry restriction. The following information is taken from the Captan label: “Do not allow persons to enter treated areas within four days following application unless a long-sleeved shirt and long pants or a coverall that covers all parts of the body except the head, hands, and feet, and chemically resistant gloves are worn. Conspicuously post reentry information at site of application.” Remember, always read the label.
Ziram is similar in efficacy to Ferbam. It provides only moderate control of downy mildew, and excellent control of black rot and Phomopsis cane and leaf spot. Under heavy disease pressure, Ziram may not provide adequate control of downy mildew.
Ridomil Gold MZ and Ridomil Gold/Copper are by far the most efficacious fungicides available for control of downy mildew. Ridomil is locally systemic and has good post-infection or curative activity. If used in post-infection control programs, it should be applied as soon as possible, but within two to three days after the initiation of an infection period. Ridomil should not be applied after symptom development (sporulating lesions). Use of Ridomil in this manner (as an eradicant) will probably lead to a rapid buildup of Ridomil-resistant strains of the downy mildew fungus in your vineyard. If resistance develops in the vineyard, the use of Ridomil as a tool for downy mildew control is lost.
Ridomil also has excellent protectant activity against downy mildew. It should provide at least two weeks of protection, and in some tests in Ohio, it has provided up to three weeks of protection.
As mentioned previously, Ridomil Gold has a strong potential for fungicide resistance development by the downy mildew fungus. For this reason, the manufacturer (Syngenta) has registered its use only as a Package Mix with a protectant fungicide. The two formulations available for use on grapes are Ridomil Gold MZ (4% Ridomil and 64% Mancozeb) and Ridomil Gold/Copper (5% Ridomil and 60% copper hydroxide). The purpose of the package mix (at least in theory) is to delay the development of strains of the downy mildew fungus with resistance to Ridomil. Both formulations are equally effective for controlling downy mildew. The Ridomil Gold MZ formulation should be used on copper sensitive cultivars.
0Although Ridomil is very effective, the current label use recommendations restrict the timing of its use on grapes. Ridomil Gold MZ cannot be applied within 66 days of harvest, and Ridomil Gold/Copper cannot be applied within 42 days of harvest. Based on these long preharvest intervals, Ridomil will be of limited use for late season downy mildew control in the Midwest.
In seasons when downy mildew is a problem and on highly susceptible cultivars, pre-bloom and post-bloom applications of Ridomil will aid greatly in disease control. However, additional fungicide protection may be required within the 66-day preharvest interval on late-harvested, highly susceptible cultivars. The alternative fungicides for use during this period are Captan, copper fungicides, phosphorus acid fungicides, or the strobilurin fungicides Abound or Pristine.
Strobilurin fungicides are also locally systemic, and some have good to excellent activity against downy mildew. Whereas the strobilurins (Abound, Sovran, Flint, and Pristine) all have good to excellent activity against black rot and powdery mildew, they vary greatly in their efficacy against downy mildew. Abound and Pristine have excellent activity and are the most effective for downy mildew control. Sovran is moderately effective if used at the highest labeled rate, and Flint is registered for “suppression” of downy mildew, not control.
Several products containing phosphorous acid (PA, also called phosphite or phosphonate) are sold as nutritional supplements and plant conditioners. Several of these materials have been registered in the United States as fungicides for control of downy mildew on grape. In multiple New York trials, PA has provided excellent control of downy mildew but has not controlled any other grape disease.
Australian experience suggests that PA provides most control on foliage when it is applied within a few days after the start of an infection period, providing only a few days of additional residual (protective) activity. Experience in New York suggests that spray timing is less critical for control of downy mildew on fruit, perhaps because this highly mobile chemical (which is exempt from residue tolerances) accumulates in these organs. When applied in a seven- to 10-day protectant program, they appear to provide good to excellent control of downy mildew.
Copper fungicides are highly effective against downy mildew and are moderately effective against powdery mildew. Copper fungicides are weak for controlling black rot. A major concern with the use of copper fungicides is the potential they have for phytotoxicity or vine damage. Grape cultivars differ in their sensitivity to copper fungicides (Table 14).
Note: Certain food processors, such as the National Grape Cooperative, will not accept grapes treated with Mancozeb past the initiation of bloom, and the use of Captan is not permitted at any time. If growers cannot use Mancozeb or Captan, alternatives for downy mildew control include Ridomil Gold/Copper, copper fungicides, a phosphorous acid fungicide, or a strobilurin fungicide. Thus, copper may be an important fungicide for producers of processing grapes that have these fungicide use restrictions.
Vangard, Elevate, Endura, Scala, and Rovral all have excellent activity against Botrytis bunch rot on grapes and are the fungicides of choice for control of Botrytis bunch rot. The strobilurins are moderately effective against Botrytis. Botrytis bunch rot is most commonly a problem on tight-clustered French hybrids and Vitis vinifera cultivars.
Proper timing and thorough spray coverage are essential for good control. Make at least two applications:
Field experience suggests that effectiveness of the fungicide is reduced following a heavy prolonged rainfall. If such conditions occur after the last intended spray has been made, an additional application may be necessary. If only one application can be made, wait until the crop average is 5°Brix. Direct the spray toward the fruit; use a minimum of 100 gal/acre of water.
The importance of bloom sprays for control of Botrytis bunch rot is not clear; however, during seasons with wet conditions during bloom, fungicide application during bloom is probably beneficial. Research in New York has shown that the strobilurin fungicides have moderate to good efficacy for Botrytis control. The use of a strobilurin fungicide during the bloom period may be beneficial for Botrytis control, especially on highly susceptible cultivars. In addition, a strobilurin fungicide such as Abound or Pristine during bloom will provide excellent control of black rot, powdery mildew, and downy mildew as well.
Note: Growers in Europe and Canada have experienced loss of disease control due to the development of fungicide resistance when more than three sprays per year of Rovral were applied over a period of three to five years. It is, therefore, strongly recommended that the use of Rovral, Endura, Vangard, Scala, or Elevate be limited to a maximum of two to three applications per year to reduce the probability of developing strains of Botrytis that are resistant to this material. In addition, alternating these fungicides during the growing season or from season to season should be helpful in fungicide-resistance management.
Note: Removal of leaves around clusters on mid- or low-wire cordon-trained vines before bunch closing has been shown to reduce losses caused by Botrytis due to improved air circulation and improved spray penetration and coverage.
On cultivars highly susceptible to downy mildew and powdery mildew, some post harvest application may be required to protect foliage and prevent premature defoliation. This is especially true on early harvested cultivars in southern regions of the Midwest.
The development of strains of the powdery mildew fungus with resistance to the sterol-inhibiting (SI) fungicides (Bayleton, Nova, Procure, and Rubigan) or the strobilurin fungicides (Abound, Sovran, and Flint) is a serious threat to their continued use for powdery mildew control on grapes. There is good evidence that strains of the fungus with resistance to Bayleton and reduced sensitivity to other SI fungicides have developed in several areas.
Other grape diseases (fungi) and fungicides that are at high risk for fungicide resistance development include Botrytis bunch rot (Vangard, Elevate, Rovral, Endura, and Scala) and downy mildew (Ridomil Gold, Abound, Sovran, and Pristine). In order to prevent or delay the development of fungicide resistance, these fungicides should not be used alone for season-long control and should be used as little as possible. This means another fungicide with good activity against the disease should be incorporated into the spray program at some point during the growing season.
A good strategy for resistance management is to use one or two spray blocks of different fungicides. For example, a grower could start the season with two applications of a sterol-inhibiting fungicide, then switch to a strobilurin fungicide for two sprays. Other materials, such as a protectant fungicide, can be used in an alternating program such as this. The important thing is not to use one material season-long. Check with your local Extension service for the most current fungicide recommendations.
| Table 15. Effectiveness of Fungicides for the Control of Grape Diseases. | |||||
| Fungicide Phomopsis | Cane and Leaf Spot | Black Rot | Downy Mildew | Powdery Mildew | Botrytis Rot |
|---|---|---|---|---|---|
| Abound | + | +++ | +++ | +++ | ++ |
| Bayleton | 0 | +++ | 0 | +++ | 0 |
| Captan | +++ | + | +++ | 0 | + |
| Elevate | 0 | 0 | 0 | 0 | +++ |
| Elite | 0 | +++ | 0 | +++ | 0 |
| Endura | 0 | 0 | 0 | +++ | ++ |
| Ferbam | + | +++ | + | 0 | 0 |
| Fixed Copper and Lime | + | + | +++ | ++ | + |
| Flint | + | +++ | + | +++ | ++ |
| JMS Stylet Oil | 0 | 0 | 0 | +++ | ? |
| Mancozeb | +++ | +++ | +++ | 0 | 0 |
| Nova | 0 | +++ | 0 | +++ | 0 |
| Phosphorous acid | 0 | 0 | +++ | 0 | 0 |
| Potassium salts | 0 | 0 | 0 | ++ | 0 |
| Pristine | ++ | +++ | +++ | +++ | ++ |
| Procure | 0 | +++ | 0 | +++ | 0 |
| Quintec | 0 | 0 | 0 | +++ | 0 |
| Ridomil Gold MZ | + | ++ | +++ | 0 | 0 |
| Ridomil Gold Copper | + | + | +++ | 0 | 0 |
| Rovral | 0 | 0 | 0 | 0 | +++ |
| Rubigan | 0 | ++ | 0 | +++ | 0 |
| Scala | 0 | 0 | 0 | 0 | +++ |
| Sovran | + | +++ | ++ | +++ | ++ |
| Sulfur | + | 0 | 0 | +++ | 0 |
| Vangard | 0 | 0 | 0 | 0 | +++ |
| Ziram | ++ | +++ | ++ | 0 | 0 |
| +++ = highly effective, ++ = moderately effective, + = slightly effective, 0 = not effective, ? = activity unknown. | |||||
| Note: These ratings are intended to provide the reader with an idea of relative effectiveness. They are based on published data and/or field observations from various locations. Ratings could change based on varietal susceptibility and environmental conditions for disease development, or changes in fungal sensitivity to specific fungicides. | |||||