Diseases and Insects

Insect and disease problems can be reduced by planting vines in a sunny location with good air drainage. Troubles can still occur even when following proper growing practices in an ideal location. Weather conditions, winter hardiness of the cultivar, infection from the previous year, history of pesticide use, surrounding vegetation, and insect life cycles all influence a cultivar's susceptibility to insects and diseases for a particular year.

Below are the insects and diseases that you are most likely to encounter:

Powdery mildew. This fungus can infect all green tissues of the grapevine. It appears as a white or grayish white powdery covering on the upper and lower surfaces of the leaves and fruit. Leaves infected while they expand become distorted and stunted. When green shoots are infected, the fungus appears dark brown to black and remains as brown patches on the surface of dormant canes. Cluster infection before or shortly after bloom can result in poor fruit set and considerable crop loss.

Fruits are susceptible to powdery mildew as well. If grapes are infected when they are pea size or larger, their skin stops growing but the pulp continues to expand and the berry splits. When attacked as they begin to ripen, purple or red cultivars fail to color properly and look blotchy at harvest. High relative humidity promotes infection. Infected shoots should be pruned and destroyed. Good winter pruning increases air circulation and reduces the chances of heavy infection.

Susceptible cultivars include Aurore, Chancellor, Chardonnay, Delaware, Einset, Niagara, Riesling, and Cabernet Sauvignon. Less susceptible cultivars include Canadice, Cayuga, Ives, and Steuben.

Black rot. This fungal disease can cause complete crop loss in warm, humid climates. All green tissues can be infected. Leaves are susceptible for about one week after they unfold. When infected, they develop brown circular lesions, and within a few days black spherical spore-producing bodies form within the lesions. Leaf stem infection causes the leaves to wilt. Shoots display large, black elliptical lesions, weakening them and making them easily broken by wind. Berries are susceptible from bloom until they begin to ripen, and fruit infection can result in substantial loss. An infected berry first appears light brown, and then black spore-producing bodies develop on its surface. Later, the berries shrivel and turn hard and black to become so-called mummies.

Warm, humid, or wet weather encourages the spread of black rot. Preventive measures include proper site selection and row orientation to maximize good air drainage. Proper pruning, to open up the canopy to improve air circulation and spray coverage, and spring cultivation of mummies are also beneficial. American cultivars and their hybrids vary widely in their susceptibility to disease. Susceptible cultivars include Concord, Dutchess, Niagara, Riesling, Seyval, Aurore, Catawba, Canadice, and Cabernet Sauvignon. Less susceptible cultivars include Cayuga, Chancellor, Elvira, Foch, Fredonia, Ives, and Remaily.

Downy mildew. This fungus can infect all green, actively growing parts of the vine. Leaves develop yellowish green lesions on their upper surfaces 7 to 12 days after infection. As lesions expand, the affected areas become brown, withered, or mottled. A white "downy" fungus grows on the lower leaf surface within the borders of the lesion. Severely infected leaves 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. When young shoots, leaf stems, tendrils, or cluster stems are infected, they frequently become distorted, thickened, or curled with a white downy appearance. Eventually, severely infected portions wither, turn brown, and die. Infected green grapes turn light brown to purple, shrivel, and detach easily. White cottony spores are abundant on these berries during humid weather.

In general, Catawba, Chancellor, Chardonnay, Delaware, Fredonia, Ives, Niagara, White Riesling, and Rougeon are susceptible to downy mildew. Canadice, Cascade, Concord, Himrod, Remaily, and Steuben are less sus ceptible. Watch for this disease in warm, humid weather, and follow the cultural practices recommended for controlling black rot.

Botrytis. This fungus causes blight infection and bunch rot of grapes. Blight infection begins on the leaves as a dull green spot (commonly surrounding a vein) that rapidly becomes a brown, withered lesion. The fungus also causes blossom blight or shoot blight, resulting in significant crop losses if not controlled. It can grow on dead blossom parts in the cluster and then, before grapes begin to ripen, move from berry to berry within the bunch, initiating the early development of sour rot. Botrytis occurs most commonly on ripening berries, where infection and rot spread rapidly throughout the clusters. The berries of white cultivars become brown and shriveled, and those of purple cultivars develop a reddish color. Under proper weather conditions, the fungus produces a fluffy, grayish brown growth.

The fungus readily colonizes tissue injured by hail, wind, birds, or insects. Fog or dew and temperatures of about 60 to 80 degrees F are ideal for spore production. Rainfall is not required for disease development. Again, good cultural practices assist in controlling this disease. Aurore, Chardonnay, Elvira, and Riesling are susceptible cultivars.

Japanese beetles. Most gardeners will experience Japanese beetle damage on grapevines nearly every year. Fortunately, vines can tolerate a large amount of feeding before fruit quality or yields are reduced. But if populations are high enough, beetles can defoliate a grapevine in just a few days. Check vines daily once beetles appear. Most gardeners either pick the beetles off the vines and kill them or treat the vines with an insecticide.

Grape berry moth. Not frequently a problem, these small, inconspicuous brown moths lay eggs singly on buds, stems, or newly formed berries in early spring. Later, the moth usually deposits eggs directly on the grapes. The newly hatched larvae feed on tender stems, blossom buds, and berries, often inside protective silk webbings that can surround the entire cluster. When grapes are about 1/8 inch in diameter, the larvae burrow into them, creating sites for infection by fungi and attack by fruit flies. Infestations vary greatly from year to year and even within a vineyard.

With light infestations, remove injured berries by hand and destroy them (do not discard them on the ground because insects may continue to develop). You can control the moths somewhat by gathering the infected leaves in the fall and destroying them.

Grape cane girdler. Adults are small, shiny black weevils. In May the female hollows out a small cavity in a shoot and deposits a single egg into it. She then girdles the cane just below and several inches above the egg cavity. The shoot either breaks off at the girdled point or dies back to the first node below the egg cavity and drops to the ground. The girdling by the female causes the terminal growth of new shoots to bend over above the upper girdle and blacken, shrivel, and drop to the ground. Later the whole infested shoot dies back to the lower girdle and falls from the vine. Vines "pruned" by the grape cane girdler have a ragged appearance, suggesting serious injury to the plant. But the actual damage is usually minor. Girdling of the terminal growth has little or no effect on the crop unless fruit-producing nodes are close to the attacked shoot tips

Grape leafhoppers. Populations of these insects fluctuate widely from place to place and year to year. Adults begin feeding in May on grapes and other plants such as strawberries, raspberries, and several woody plants. In June, they lay eggs singly, just beneath the underside of the leaf, producing a slight blister. Nymphs appear in late June and reach the adult stage by late July. Adults and immature leafhoppers feed on the undersides of leaves, sucking out the leaves' juices. Feeding is limited initially to the lower leaves. The tissue surrounding the feeding puncture turns pale white and eventually dies. Plant injury shows up first along the veins, but later the whole leaf is affected. Heavy feeding by leafhoppers causes premature leaf drop and lowered sugar content, increased acidity, and poor fruit color. Ripening fruit is often smutted or stained by the sticky excrement of the leafhoppers.

Cold, wet weather in the spring and fall decreases leafhopper populations, as do wet winters. Fall cultivation and cleaning up adjacent weedy land eliminate favorable overwintering sites in and near a vineyard.

Pick grapes only after they are fully ripe, about a week after they reach full size and color.

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