Wednesday, July 24, 2013

Grape leafroll or curled leaf

White grape with leafroll or curled leaves
Neptune seedless grapes are the largest white grape. These grapes grow over an arbor in the back of our garden. They should ripen in August or early September. 

This relatively new variety produces big, juicy, highly flavorful grapes that resist cracking, even in adverse conditions. In conical clusters, they make beautiful additions to fruit baskets or centerpieces. Disease-resistant and cold-tolerant. Self-pollinating.

Grape leafroll disease

by Sara Ipatenco, Demand Media
Wine grapevines, Vitis vinifera L., are susceptible to many pests and diseases, but grape leafroll disease is one of the most devastating. Healthy grapevine leaves are vividly colored and flat. When grape leafroll disease attacks a healthy grapevine, it leaves behind curled and unsightly leaves. Several factors could be the cause of leafroll disease and must be identified before treatment can begin.

The symptoms of grape leafroll disease vary among different varieties, which makes it difficult to detect and diagnose early enough to save the plants and grapes. Visual symptoms are usually not detectable until late summer or early fall. In red-skinned varieties, leaf tissue turns dark red or purple and the leaves curl downward. In white-skinned varieties, the leaves turn yellow and curl downward. For both varieties, the veins of the leaves remain green. There is no way to cure grape leafroll disease, so management and prevention are essential.

Ten different viruses can be responsible for the development of grape leafroll disease. The viruses, named GLRaV-1 through GLRaV-10, are often spread when grapevines are propagated or grafted. In some instances of grape leafroll disease, a single virus is responsible for the damage. In other cases, a grapevine might be infected with more than one virus, which leads to more extreme and devastating damage.

Two species of mealybugs, longtailed mealybug and citrus mealybug, are known to transmit grape leafroll viruses among grapevines in California vineyards. Scales are another type of pest that can cause grape leafroll disease. As the pests migrate from grapevine to grapevine to feed, they take the virus with them, spreading it to healthy vines.

Additional Causes
The use of certain herbicides can lead to leaf curling, though this is not considered true grape leafroll disease. Nutrient deficiencies can also cause grapevine leaves to curl. If the soil where the grapevines are planted is deficient in potassium and phosphorus, the leaves might begin to curl. Amending the soil can remedy the problem and restore the health of the grapevines.

Management and Prevention
Remove branches and vines that have evidence of grape leafroll disease and watch for the presence of pests to catch the problem early. Prevention is the only way to keep grapevine leafroll disease away because there is no cure once the vines are impacted. Because most instances of grape leafroll disease are caused by propagation and grafting, new cuttings should be tested for viruses before they are introduced to other grapevines. Purchasing certified cuttings ensures that new grapevines are virus free.

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