We have already covered phylloxera. Phylloxera might be the worst scourge ever to hit wine grapes, but it is by no means the only one. Vines are subject to attack from pests that range from deer to birds. We cover here the major fungal vine disease, several bacterial diseases, and common insect pests. Sometimes these appear together as in the cases where insects damage parts of the plant encouraging either fungal or bacterial infection.
There is no cure for some scourges of the vine. Sometimes winegrowers need to remove affected vines, or parts of vines. Other problems respond to chemical spraying. There is an increasing trend toward using natural means of pest control, such as encouraging beneficial insects who prey on the bad guys. Often the best defense against some problems is simply to plant the appropriate grape for the environment. Growers in humid Virginia, as one example, have had good results propagating Viognier. Viognier’s loose bunches allow air circulation and inhibit fungal problems that are less widespread in largely arid California. In southern California, on the other hand, growers have had little success in combating insect-borne Pierce’s disease, regardless of the grape variety they plant.
Fungal Vine Diseases
Powdery Mildew shows itself as powder-like splotches on grapes, stems and leaves. It can kill leaves, leaving the vines defoliated, which interferes with photosynthesis and eventually kills the plant. Copper-based fungicides have been the traditional response.
Downy Mildew appears as greasy yellow or green spots on leaves. Leaf infection by bacteria can result.
Black Rot, as the name implies, consists of dark circular lesions on grape leaves. This has the potential to destroy an entire crop. Removal of affected areas and fungicides are the response.
Bunch Rot gives grape berries a soft and watery appearance as they become covered in fungal growth. Bunch rot is most likely to affect grape varieties with tight clusters of berries.
Gray Rot is caused by an unwanted invasion of the grapes by the Botrytis cinerea fungus. The fungus enters the grapes and shrivels them by removing the water inside. Under certain circumstances, the same Botrytis fungus can be responsible for what is called Noble Rot, a desirable effect of the fungus in which it adds a distinctive flavor and concentrates the grape sugars and flavors, forming the basis for rare and super-expensive sweet wines.
Anthracnose (bird’s eye rot) is caused by the Elsinoe ampelina fungus and consists of small round spots that progress to small round shotgun pellet type holes. It affects all parts of the vine and is exacerbated by moisture contact. The copper-based Bordeaux mixture is a possible treatment.
Leaf Spot is caused by the phomopsis viticola fungus. The grape leaves yellow at the edges.
Eutypa Dieback (dead-arm disease) is a fungal infection that enters vines through pruning wounds, especially in wet conditions, and results in the internal rot and death of entire sections of the vine. It is almost impossible to control without chemical fungicides.
Esca (Black Measles or Spanish Measles) consists of a fungal infection on nearly any part of the vine. Esca leaves black stripes, akin to tiger stripes, and often affects relatively young vineyards. Removal and destruction of the infected areas is about the only response.
Bacterial Vine Diseases
Crown Gall is caused by Agrobacterium tumefaciens, and consists of growths on vine roots, trunks and cordons. It usually attacks areas that have been injured, by pruning or even as a result of freezing winter temperatures.
Pierce’s Disease is caused by Xylella fastidiosa bacteria, which is spread by leaf-hopping insects called sharpshooters, which feed on the plant’s xylem. It causes scorching of vine leaves and eventually kills the plant. It is primarily a problem in hotter regions of the US like Texas and Southern California. Chemicals are the usual response.
Japanese Beetles consume grape leaves and buds. These are best controlled using insect-killing soaps or neem oil.
Grape Mealybugs secrete a sugary substance that collects on grape berries and encourages growth of mold. These can be controlled either by spraying or by encouraging populations of natural predators.
The Black Vine Weevil feeds on leaves, buds and flowers and reduces vine vigor. Its larvae live in the soil and feeds on grape roots.
The Grape Cane Girdler encircles and punctures vine canes. Pruning out infested shoots is about the only control method short of spraying.