Brambles - Production Management and Marketing

Bulletin 782-99

Chapter 5 Integrated Management of Bramble Diseases

Fungicides for Bramble Disease Control

Fungicides can play an important role in the bramble disease-management program. However, in order to obtain maximum benefits with minimal use, fungicides must be integrated with the use of the previously described cultural practices and resistant or less susceptible varieties. We must remember that several important bramble diseases cannot be controlled with fungicides. These include Verticillium wilt, several rust diseases, and all of the virus diseases. On the other hand, fungicides can be a very effective component in control programs for Botrytis fruit rot (gray mold), powdery mildew, Septoria leaf spot, raspberry leaf spot, anthracnose, cane blight, and Phytophthora root rot.

Although fungicides are an important disease-management tool, it should be noted that very few fungicides are currently labeled for use on brambles in the United States. The lack of currently registered fungicides combined with the fact that several important diseases cannot be controlled with fungicides makes the diligent use of cultural practices within the disease-management program extremely important.

The comments presented here on fungicides are intended to provide some general information about the currently registered fungicides. For the most current information on fungicide recommendations on brambles, growers should obtain the most recent copy of Ohio State University Extension Bulletin 506-B2, Ohio Commercial Small Fruit and Grape Spray Guide. This publication is revised annually. See page 55 for ordering information.

Note: It is always the growers' responsibility to read and follow all label instructions. In today's modern agriculture, new regulations (such as the Food Quality Protection Act) and recommendations can change rapidly; therefore, the information that is presented here could change before you read it.

Benlate 50-WP
Benlate is labeled for control of Botrytis fruit rot and powdery mildew. Although not mentioned on the label, Benlate should also provide some level of control for anthracnose, cane blight, Septoria leaf spot, and raspberry leaf spot. Benlate is about the only broad-spectrum fungicide currently labeled on brambles that can be applied during the growing season. The label states that it can be applied at 3/4 lb. per acre at early bloom (5 to 10%) and at full bloom. Three additional applications at 14-day intervals can be made as needed. Do not apply Benlate within three days of harvest. Thus, a maximum of five applications of Benlate can be made on brambles per season.

A major problem with the use of Benlate is the development of fungicide resistance. Many fungi have developed resistance to Benlate after repeated exposure, including those that cause gray mold (Botrytis fruit rot) and powdery mildew. To maintain its useful life, Benlate should be used no more than necessary (ideally, no more than once or twice a year). Resistance does not "go away" after Benlate use is discontinued; that is, once the fungicide quits working in a particular planting, it's lost to that planting for good. See comments on fungicide resistance management on page 79.

Captan 50-WP and 80-WP
In about 1989, Captan lost its federal registration for use on raspberries and blackberries. The loss of registration was due to the cost of re-registration of Captan on a relatively small crop such as brambles; therefore, the company voluntarily removed brambles from the Captan label. It is important to note that the loss of registration was not due to a toxicological problem.

In 1994, Ohio received a 24(C) registration (special local need) for the use of Captan on raspberries and blackberries. Captan will be of most benefit in situations where the cane-infecting fungi (anthracnose, cane blight, and spur blight) become established in the planting and require additional fungicide applications to achieve satisfactory control. Captan should also have activity against the leaf-spot diseases (septoria leaf spot and raspberry leaf spot) of brambles. Captan will also be beneficial as a broad spectrum protectant fungicide for tank mixing with Benlate, Ronilan, and Rovral in order to prevent or delay fungicide resistance development by Botrytis cinerea to these fungicides.

Note: We have been working through the IR-4 program to obtain federal registration for Captan on brambles. By the time you read these recommendations, Captan may have a federal label. Contact Mike Ellis (330-263-3849) if you have questions about the current status of Captan on brambles.

The 24(C) labels state that for the control of anthracnose, Botrytis, and spur blight - Apply four pounds of Captan 50-WP (2-1/2 lb. of Captan 80-WP) per acre when blossoms are in bud (young canes are 8-10 inches long). Make a second application two weeks later. Apply a fall spray after old canes are removed.

For the control of fruit rot - Apply four pounds of Captan 50-WP (2-1/2 lb of Captan 80-WP) per acre at early bloom (5 to 10% bloom) and again at full bloom. Additional applications can be made at 10- to 14-day intervals as needed. Do not apply within three days of harvest.

Please note that the 24(C) label must be in the possession of the user at the time of pesticide application. A copy of the label can be obtained from:

Mike Ellis, Department of Plant Pathology, The Ohio State University, Ohio Agricultural Research and Development Center, 1680 Madison Avenue, Wooster, OH 44691. Phone: 330-263-3849.

Liquid Lime Sulfur
Lime sulfur is recommended for use on brambles as a delayed-dormant application in early spring (when buds show 1/4-inch green). It is used at the rate of 10-20 gallons per acre. If applied at this rate later in the season (after 1/4-inch green), it can cause severe damage to leaves and young canes. Lime sulfur is recommended for control of the cane-infecting fungi (anthracnose, cane blight, and spur blight). The delayed dormant application in spring is intended to eliminate or reduce the overwintering inoculum for these diseases on canes. Where cane diseases are a problem, this spray is very important. Where good sanitation is used (old fruited and infected canes are removed from the field) and cane diseases are not a problem, this spray may not be necessary, or at least it would probably be safe to use the lower rate, especially on red raspberries.

Lime sulfur has a bad smell (rotten eggs) so there can be a problem spraying it around your neighbors. Some growers have received complaints from neighbors after applying lime sulfur. In addition, lime sulfur is very caustic. It is harmful to machine parts, paint (especially on cars), and sprayers. Special care should be taken to avoid drift to nontarget objects, and proper protective clothing should be worn by the applicator.

Sulfur
Sulfur is available as a wettable powder or in flowable formulations. Sulfur is registered for control of powdery mildew. Sulfur has little or no activity against the other bramble diseases caused by fungi. Because powdery mildew is generally not a serious problem in the Midwest, sulfur is generally of little importance within the bramble disease management program.

Ronilan 50-WP and Rovral 50-WP and 4F
These fungicides are closely related (dicarboximide fungicides) and both have excellent activity against Botrytis fruit rot (gray mold), but have little or no activity against the other fungal pathogens on brambles. Like Benlate, both fungicides are at risk for resistance development by the fungus that causes gray mold fruit rot. If the fungus develops resistance to either Rovral or Ronilan, it is automatically resistant to the other. Therefore, these materials should not be used more often than necessary; the less they're used, the longer they'll last. Ideally, we should limit their use to no more than two or three applications per season.

Ronilan
The label states that it is for use on raspberries. No other brambles are mentioned on the label; therefore, it should only be used on raspberries and will provide excellent control of Botrytis fruit rot (gray mold). The label states that the first Ronilan application should be made no later than 10% primary bloom. Timing of subsequent applications varies from 7 to 14 days based on weather conditions and resultant disease pressure. Ronilan can be applied at rates ranging from 1 to 2 lbs. product per acre. As with all fungicides, use rate and spray intervals should be determined by disease pressure (wet weather and amount of fungus inoculum available). No more than 8 lbs. of Ronilan may be applied per acre in one crop season. Ronilan may not be applied within nine days of harvest.

Rovral
Of the fungicides used for Botrytis control, Rovral is currently the only one labeled on raspberries that can be applied up to and including the day of harvest (0 day preharvest interval [PHI]). It should provide excellent control of Botrytis fruit rot (gray mold). In addition, the label states it can be used on "Caneberries;" therefore, it can be used on all brambles. Rovral may be applied to caneberries at the rate of 1 to 2 pounds (1 to 2 quarts) per acre. Apply Rovral first at early bloom (5-10% bloom) and make a repeat application again at full bloom. Two additional applications can be applied at 14-day intervals or as required. The final application can be made up to and including the day of harvest.

Ridomil Gold
Ridomil Gold is labeled for control of Phytophthora root and crown rot on raspberries. It has no activity against the other bramble diseases. It is available in an EC (liquid), WSP (wettable powder in a water soluble pouch), or a GR (granular) formulation. Although Ridomil is very effective for control of Phytophthora root rot, it should be emphasized that cultural practices (primarily good soil drainage) are the primary means for controlling this disease. In other words, Ridomil is most effective when used in combination with good cultural practices and/or the avoidance of highly susceptible varieties. It is often ineffective if used on poorly drained sites and on highly susceptible varieties.

Note on Disease Resistance - Phytophthora root rot is most serious on red raspberry and some of its hybrids. The black raspberry varieties Cumberland and Munger are reported to be susceptible, and the varieties Bristol, Dundee, and Jewel appear to be moderately to highly resistant. Among red raspberry varieties, none are immune to the disease, but varieties do differ greatly in their level of susceptibility. Among varieties grown in the Midwest and Northeast, Titan, Ruby, and Hilton are extremely susceptible, and Festival, Heritage, Reveille, and Taylor are moderately to highly susceptible. Latham, Newburgh, Cherokee, Boyne, Killarnoy, and Fall Red are considered fairly resistant. The disease has not been documented on blackberry.

Comments on Fungicide Use

Choosing Not to Use Fungicides
Growers who choose not to use fungicides within the disease-management program must rely completely on disease resistance and cultural practices for disease control. Most of our currently available cultivars do not have high levels of disease resistance to the major diseases encountered in Ohio; therefore, the use of various cultural practices will be the principal means of disease control. This option may be "risky" during growing seasons that are favorable for disease development (prolonged wet periods) but will probably provide satisfactory disease control during drier growing seasons or in new plantings before diseases are introduced. Growers considering this option may wish to try it on a treat-as-needed basis. In other words, growers may wish to use this option until diseases start to develop, then initiate and maintain a spray program for control of specific diseases.

For certain diseases such as anthracnose, this wait-until-a-problem-develops approach may be satisfactory. For other diseases, such as Botrytis fruit rot (gray mold), by the time you see the disease, it is generally too late to initiate an effective fungicide program. Thus, for diseases such as Botrytis fruit rot that develop quickly and can cause yield losses of 50% or more, this option is risky, especially for large commercial plantings and during wet growing seasons.

Effective Use of Fungicides for Botrytis Fruit Rot Control

Importance of Bloom Sprays
Research has shown that bloom is a critical time for infection by Botrytis. The fungus colonizes flower parts and moves into the young green fruit where it remains dormant until the fruit becomes ripe. As fruit ripens, the fungus becomes active, and fruit rot develops. The direct infection of mature fruits can occur but is generally considered to be of secondary importance, during the preharvest period.

Timing Bloom Applications for Botrytis Control

We must emphasize protective coverage of all flowers during bloom. This is difficult because flowers are continually opening. To be practical, we should try to limit the fungicide program to no more than two to three applications during bloom per variety. It is important to remember that if you have several varieties with a range of maturation dates, bloom may extend over a two- to three-week period. Therefore, it is important to treat each variety separately. The first application should be made around 5-10% bloom with the second application being made at full bloom or slightly after. It is important to remember that the early blossoms are the "primary bloom" and represent a significant part of the crop. Protecting these early blossoms is probably more important than protecting the last blossoms to open.

If wet weather persists prior to and during early bloom, the first application may need to be made as soon as the first flowers are open. In contrast, if weather conditions are extremely dry, this spray can usually be delayed until wet weather is forecast. The gray mold fungus is very active when the weather is foggy or rainy, but it is relatively inactive during dry periods. The optimum temperature for fungus activity and disease development is approximately 60-80°F. Although infection can also occur at lower temperatures, progressively longer periods of wetness are required as temperatures fall below 60°. These principles should be used in determining fungicide use intensity. Thus, shorter intervals between sprays and higher rates should be used when conditions are optimal for disease development, and longer intervals between sprays and lower rates should be used when conditions are less favorable for disease development.

Additional Fungicide Sprays During Harvest for Botrytis Control

On strawberries, it has been observed that the vast majority of Botrytis infection occurs during bloom. In fact, many strawberry growers in Ohio spray for Botrytis only during bloom and have eliminated fungicide applications during harvest. Although the vast majority of Botrytis infections on raspberry occur during bloom, it appears that infection of mature fruit is more common on raspberry than on strawberry. Thus, the approach of spraying fungicide only during bloom on raspberry is probably sufficient during dry growing seasons. However, in wet growing seasons or if wet weather exits during harvest, or if high levels of rot develop on old unpicked fruit in the field, additional fungicide protection during harvest may be required. This is especially true if the berries are going to be stored for more than 24 hours prior to processing. Many growers have observed postharvest development of Botrytis when they try to hold berries for any length of time. Preharvest sprays may be beneficial in reducing the amount of Botrytis development in cold storage. When preharvest sprays are made, it is critical to observe the preharvest interval (PHI) of the fungicide. At present, Rovral is the only fungicide that can be applied up to and including the day of harvest (0 day PHI). It is also important to recognize that for sprays during harvest to be effective, thorough coverage of the berries is critical.

Cultural practices that promote faster drying of fruit and timely harvest of ripe fruit are also extremely important. If fruit are allowed to sit in the field until they are overripe or are damaged during harvest, they will deteriorate and rot, regardless of fungicide use.

Fungicide Applications for Everbearing (Primocane-Fruiting) Raspberries

Cane Diseases
On varieties such as Heritage that are mowed off each year, the incidence of cane diseases, such as anthracnose, cane blight, and spur blight, should be minimal. By removing all diseased canes each year, the overwintering inoculum for these diseases is removed, and the disease cycle is broken. In production systems such as this, the use of lime-sulfur is probably not required. Emphasis for controlling cane diseases needs to be placed on maintaining proper row width and plant spacing within the row in order to open the canopy and promote faster drying in the row.

Most people do not remove cut canes from the field. They usually put them between the rows and run a mower over them. This is fine as long as diseases are not a problem in the planting. If cane diseases are a problem, cut canes should be removed from the planting and destroyed. In addition, canes should be cut off low enough so as not to leave long stubs that can serve as reservoirs for disease. Canes should not be cut off until the plants are completely dormant. Late winter or very early spring is probably the best time to remove canes.

Botrytis Fruit Rot (Gray Mold)
Unlike most summer-bearing cultivars that bloom for a relatively short period during late spring (two to three weeks), everbearing varieties like Heritage bloom constantly from mid-summer through fall (frost). Thus, blossoms are present throughout the harvest period. Growers who wish to apply fungicides for Botrytis control should consider and follow the same fungicide recommendations as outlined previously for Botrytis control. Special care needs to be taken to observe preharvest intervals.

Fungicides for Controlling Powdery Mildew
Powdery mildew is seldom an economically important problem on brambles in Ohio. If powdery mildew is a significant problem on a particular variety, growers should consider the use of other varieties that are less susceptible.

Benlate and sulfur are both effective against powdery mildew. Benlate is more effective than sulfur and can be applied in three additional applications past full bloom on a 14-day interval. If Benlate has been used extensively in your planting, there is a possibility that the powdery mildew fungus has developed resistance to Benlate. If this is the case, sulfur is the only alternative fungicide at present.

Sulfur should also do a good job of controlling powdery mildew, although frequent applications may be necessary. Sulfur should not be applied if temperatures are going to exceed 85°F. See the label for specific recommendations for sulfur use.

Several new fungicides that have excellent activity against powdery mildew are currently being introduced in the United States. It is possible that some of these new fungicides may be registered on brambles in the near future.

Rust Diseases
There are several important rust diseases that can affect brambles in the Midwest and Northeast. Among the most important are orange rust (which affects only black raspberry and blackberry) and late leaf rust (which affects only red raspberry). At present, there are no fungicides available for control of these diseases.

We have been working for several years through the IR-4 program to get Federal registration for the use of Nova fungicide on raspberries and blackberries. Nova is an ergosterol-inhibiting fungicide that has excellent activity against rusts. Federal registration could be granted anytime; therefore, if you have questions about this fungicide for rust control on brambles, contact Mike Ellis (phone 330-263-3849). By the time you read this, registration may have been received. In addition, several new fungicides with activity against rusts are currently being registered for use on other crops in the United States. It is possible that some of these new fungicides could be registered for use on brambles in the future.

Notes on Fungicide Resistance Management

Benlate, Ronilan, and Rovral all have a high potential for resistance development in strains of the Botrytis fungus. However, resistance appears to develop more rapidly to Benlate than it does to Ronilan and Rovral, and Benlate has been in general use for a far greater period of time. Growers with a long history of Benlate use should be particularly watchful for signs of control failure with this compound and should not rely on it if previous failures cannot be explained by other factors (poor timing, coverage, etc.).

If the fungus becomes resistant to these fungicides, there are no other fungicides that we can use. Therefore, it is important that we take precautions to prevent or at least delay fungicide resistance development.

There are three major strategies (at least in theory) for preventing or delaying resistance development.

1. Reduce Usage of the Fungicide.
   Repeated, season-long usage of a fungicide, especially when used alone, will increase the chances for resistance development. The less a fungicide is used, the less chance there is for resistance development. Minimizing use of a fungicide is by far the best strategy for preventing or delaying the development of resistance.
2. Use Fungicide Combinations.
   Combining a fungicide with another fungicide that has a different mode of action against the fungus should aid in preventing resistance development. Ronilan and Rovral have the same mode of action; therefore, they cannot be used in combination for the purpose of resistance management. Benlate has a very different mode of action than Ronilan or Rovral. A combination of Benlate plus Ronilan or Rovral might, therefore, appear to reduce the risk of resistance development. However, using both materials in every spray defeats our efforts to control fungicide resistance by minimizing their use. Ideally, these fungicides should be combined with a broad spectrum fungicide like Captan.
   
3. Alternate the Use of Different Fungi cides.
   Alternating applications of fungicides with different modes of action has the effect of limiting total seasonal use of both fungicides used in the alternation. This same effect can be obtained by using different fungicides in certain "blocks" of time (for instance, use one fungicide during bloom and a different fungicide in the preharvest period). Therefore, alternating applications of Benlate with either Ronilan or Rovral should be beneficial and is particularly recommended if weather conditions require more than one to two applications of Benlate or two to three applications of Ronilan or Rovral. Alternating applications of Ronilan with Rovral will be of no value to resistance management.

Remember: Fungicide resistance is a serious problem facing modern agriculture. If resistance to these fungicides develops in Botrytis, we will have no chemical protection against Botrytis fruit rot until new fungicides with different modes of action are developed and registered. It is highly unlikely that this will happen within the next few years.