Grape Leafroll Disease
Grape leafroll disease is more prevalent in vineyards than home plantings, but can have devastating effects, including the loss of 30 to 50 percent of yields. On red grape varieties, the leaves turn red. On white varieties, the leaves turn yellow. Both varieties exhibit cupping or downward turning of the leaves. The veins remain green. No chemical controls exist for leafroll disease, although treating aphids and scales through the use of pesticides may limit the spread of the disease. Plant certified disease-free plants and remove and destroy any infected plants.
Phosphorus and potassium soil deficiencies may mimic grape leafroll disease. Send a soil sample to a university extension office to evaluate the nutrient levels in the soil. Ask the extension office for fertilizer advice specific to grapes. Amending the soil with a balanced fertilizer may be all that's needed to resolve the issue.
Glyphosate or phenoxy herbicides, such as 2,4-D, can cause leaf distortion and even plant death. The herbicides may drift from surrounding areas or may be applied by using a contaminated sprayer on the grapevines. Water the plants well to dilute the effects. Unless the damage is severe, plants usually rebound within a few months.
Grapevines given proper care rarely suffer problems in a home garden. Plant them in full sun, allowing at least six to eight feet for each vine. Adequate air circulation keeps many diseases in check. Don't overfertilize the grapevines, which can cause weak, vulnerable stems. Remove dead or diseased foliage, stems and fruit promptly.