The common and spotted asparagus beetle adults lay eggs perpendicularly on the tips of asparagus spears, making them unmarketable (Figures 17, 18, and 19). Harvesting on a timely basis and not letting spears become overmature are the best prevention measures. The common asparagus beetle can severely damage asparagus in the fern stage. It chews and strips off the green material on the fern stalks, causing dieback and reducing the amount of photosynthesis taking place to provide carbohydrate storage in the roots for next year's crop. Spotted asparagus beetle larvae feed mainly on the berries of the female plants. Growing male hybrids eliminates the problem.
The aphids are green and smaller than the common green peach aphid. They start feeding on mature ferns after harvest. As they feed, ferns will take on a dwarf, dense bonsai appearance (Figure 20). Needles on the bottom fern branchlets will turn brown and fall off. Often, as the aphid population increases, many ladybug larvae will be seen feeding on the aphids. Unfortunately, this biological predator does not eliminate all the aphids. Other types of aphids, when present in large populations, can also damage tender asparagus fern and reduce yields in succeeding years.
Chemical control is needed because as the aphids feed they inject a toxin that causes the dwarf appearance of the fern and forces all buds on the crown to release at once, sending up new spears. Releasing all of the buds at once requires a tremendous energy expenditure from the crown that prevents the plant from storing food materials in the crown for next year's crop. This results in a crown that is depleted of all nutrients and is usually dead by the following spring. Also, weakened plants are more susceptible to Fusarium root and crown rot.
White cutworms and the dark-sided cutworms can damage asparagus spears (Figure 21). White cutworm larvae feed on the growing tip of the spear, but also feed readily on the butts of previously harvested spears. Dark-sided cutworm larvae feed on the sides of the spears above or just below the soil surface, causing crooked and twisted spear growth. Check fields for larvae in the fall and note damage during harvest. Fall applications of registered granular insecticides will control cutworm species that overwinter as larvae. Spring foliar applications of registered insecticides will control species that overwinter as eggs or larvae. Snap spears as close to the ground as possible to discourage cutworm feeding on harvested spear butts (37).
Tarnished and alfalfa plant bugs can cause severe "tip dieback" injury to asparagus. Adults and nymphs feed on the new growth of asparagus ferns by inserting their sucking mouthparts into the plant tissue, injecting salivary enzymes, and sucking plant juices. The salivary enzymes are toxic to asparagus and cause a collapse of the conductive tissue and death of the plant above the point of injury (tip dieback). Damage is most common during summer flushes of new fern growth. There are no apparent long-term toxic effects of plant bug feeding, but foliage loss can be severe (37).
This insect pest formerly was thought to be a minor problem in asparagus. Ferro (11) has demonstrated that the miner can significantly increase the incidence of asparagus stem, crown, and root rot caused by Fusarium. As the larvae mine, they move the disease to the upper portions of the stalk. Pupae overwinter just under the stalk surface and are contaminated with fungal spores. Therefore, adults are sources of inoculum the next year. Insecticide sprays significantly reduce miner activity and incidence of Fusarium disease.
Asparagus rust is a fungus disease that attacks fern growth, reducing the amount of photosynthesis and food translocated to the crown for next year's crop. The 'Washington' types available today are susceptible to rust. The high level of rust resistance present in the original 'Washington' types has been lost over the years. The New Jersey hybrids have tolerance to rust, but not resistance.
On the fern, rust symptoms appear first as brick-red pustules (uredia) that develop from late spring through the middle of August. These uredia produce spores (urediospores) that are blown to neighboring plants where they initiate new infections. Infection by urediospores is favored by periods of rainfall or heavy dew. In mid-August, the pustules change color from red to black and are then called "telia" (Figure 22). The spores produced in these telia (teliospores) cannot infect plants in the same season, but are able to survive during the winter, attached to fern debris, and provide inoculum for the following spring.
To remove the source of inoculum, the old ferns can be burned in the early spring before spear production starts. However, any spores present on spears are physically removed during harvest. Infections on the ferns can come later from spores blowing into the field.
A fungus called purple spot (Stemphyllium vesicarium) can sometimes cause small purple spots, often in great numbers, on spears ready to be harvested. This fungus overwinters on plant debris and is a problem in reduced tillage systems. Wounds (such as those caused by blowing sand) are necessary before the fungus can infect. If spores of the fungus are present, the spears have been wounded, and a dew or rain occurs, purple spot may be found on spears at harvest. In the fern, this fungus may cause large, elongate tan lesions with dark borders (37).
Cercospora blight is a very destructive fungus disease that can devastate asparagus in the south-central and southeastern U.S., especially where rainfall and humidity levels favor infection. The fungus overwinters on fern residues left on the soil, and spores produced on this debris infect ferns after harvest when conditions are favorable for infection. Favorable conditions include humidity of 95% or higher and average temperatures of 77 to 86 degrees F. Symptoms first appear on lower portions of the ferns after row closure and periods of high humidity. Symptoms are small, oval, gray to tan lesions (spots) with reddish brown borders on the needles and small branchlets (Figure 23). Spores of the fungus are produced on the lesions and are dispersed by wind and rain (10).
Development of the disease depends on rainfall and humidity levels during the summer months. Browning progresses upward from the lower ferns as conditions favor spread of the disease. Entire ferns may be blighted by late summer in a wet year. The disease results in reduced photosynthesis of affected ferns. Yield loss the following spring is a result of reduced crown vigor caused by the early defoliation. Repeated yearly defoliation by Cercospora blight weakens crowns and reduces stand productivity and longevity. First-year asparagus is rarely affected by Cercospora blight (10).
An integrated approach is recommended for management of Cercospora blight. Implementation of several cultural practices will help provide partial control. Utilization of a wider row spacing of six feet rather than the usual five will increase air movement to dry foliage and delay canopy closure. Rows should also be planted in a north-south direction to take advantage of prevailing southerly winds in drying foliage (10). No known asparagus variety is resistant to Cercospora blight.
Research in Oklahoma has shown that burning, but not tillage, of infested residue in the spring delays blight appearance by about one week. However, asparagus residue left on the soil is beneficial for reducing soil erosion, abrasion of emerging spears by wind-blown sand, and for maintaining organic matter content of soil (10).
Foliar applications of a fungicide recommended for control of Cercospora blight provide excellent control. In comparison to no disease control, yield increases of 2,000 lbs per acre have been achieved in research plots in Oklahoma in the harvest season following treatment the previous year with three fungicide sprays. Three applications should be made at 3-week intervals beginning prior to row closure when the ferns are approximately 4 feet in height. Sprays should be applied in a sufficient volume of water with high pressure (300-400 psi) to provide thorough coverage of the foliage. Drop nozzles are suggested for increasing spray penetration into the canopy if using a boom sprayer (10). Otherwise, use an air-blast sprayer that can spray into a block of rows from both sides, ensuring good spray coverage by using drive rows.
Asparagus beds over the past years have shown rapid yield decline in just 10 to 12 years after planting. The first signs of decline are premature yellowing of the ferns in midsummer. The cause of the yellowing is Fusarium crown rot, a fungus disease that eventually invades the crowns, stem, and roots. Two Fusarium species are primarily responsible: F. moniliforme and F. oxysporum. Infected areas of the crown turn brown because the cells that transport water and nutrients become clogged (Figure 24). In the severe stage, portions of the crown begin to dry up until the whole crown is dead. This leads to large skips in the field and dramatically lowers yields.
Asparagus decline is actually a syndrome and is caused by many factors. Research has shown that the various species of the Fusarium fungus are present in most soils, even those that have not had asparagus plantings. Fusarium moniliforme can be transported very easily by wind and insects. Because Fusarium infection occurs so readily, any stress factor that inhibits or reduces plant vigor allows the infection to become established and eventually overcomes the plant. Therefore, it is essential for growers to supply the proper soil pH and fertility, use tolerant varieties, avoid excessive harvesting, and control insects, diseases, and weeds. Failure in any of these areas becomes the weak link of the chain and can lead to asparagus decline.
For current control information on asparagus insects and diseases, refer to Bulletin 672, Ohio Vegetable Production Guide, available from the local county Extension office. In other states, contact the local county Extension office.
Frost Damage. Freezing temperatures (below 32 degrees F) can damage spears that are exposed above the soil surface. Cells in the spear are punctured and broken by ice crystals that form on the inside, causing spears to become limp upon thawing. Freeze-damaged spears have a darker green, water-soaked appearance (Figure 25) and should be discarded. If frost is imminent, pick everything that is sticking up in the field, or mow the entire field to ground level after the freeze. Growers cannot extend the harvest season to compensate for the drop in yields due to frost without stressing the plant's stored food reserves.
Wind Damage and Sandblasting. Drying winds and blowing sands can dry out the cells on the side of the spear facing the wind. The spear growth will then bend into the wind. This is sometimes a problem in areas that have sandy soil and usually lasts for one or two harvests throughout the season (Figure 26).
Hollow Stem. Bodnar (5) reports that warm, wet growing conditions favor the development of a hollow cavity in the middle of the spear. This problem is compounded where excessive nitrogen has been applied, causing rapid plant growth.
Spear Dehydration. Occasionally, new spears emerging after recently established fern growth can dehydrate shortly after emerging. This is more prevalent in sandy soils which tend to dry out quickly during periods of drought in summer. These newly emerged, succulent spears start to shrivel at the tips and turn black at about 4 to 12 inches in length. These new spears do not have a tough cuticle formed, and are very vulnerable to moisture loss. The mature fern then withdraws moisture out of the new spears, causing them to dehydrate.