Phomopsis Cane, Leaf Spot, and Fruit Rot
Phomopsis cane, leaf spot, and fruit rot is one of two distinct diseases that used to be referred to as "dead arm" and is widely distributed in vineyards. The disease can weaken vines, reduce yields, and lower fruit quality.
This disease was often the first disease of the growing season to appear in the vineyard. Infections on new shoots first appear as reddish spots about 1/16 inch in diameter. These are most common on the first 8 inches of new shoots and can be seen when the shoots are about 18 inches long. Infected portions of the leaf turn yellow then brown. When infections on shoots are numerous, they often run together and form dark blotches that crack. Cluster stems can blight and become brittle if infections are high. These clusters usually break and the fruit is lost. This fungus also causes a fruit rot. Infected fruit will turn brown, shrivel, and eventually drop. In winter, cane infections can be observed.
The disease is caused by the fungus Phomopsis viticola. The fungus overwinters in bark and leaf petioles. In the spring, especially under wet conditions, spores produced by the fungus exude from infected tissue and are splashed onto shoot tips. Only very young tissues are infected. The fungus becomes inactive in summer, but by fall it resumes activity. Infection in the vineyard is localized because disease is spread mostly within the vine rather than from vine to vine. If the disease is not controlled, it will become more severe in the vineyard with each year.
Phomopsis cane and leaf spot can be controlled by a combination of sanitation and fungicide application. At pruning, remove dead and diseased wood. Destroy prunings and debris by burning, burying, or plowing them into the soil. The cane and leaf infections can be prevented by one or two early season fungicide sprays. The number of new shoot infections during the previous 2 years and the frequency of prolonged rainy periods during the current year are indicators for performing zero, one, or two fungicide applications. Fruit and cluster stem infections occur from bloom until the fruit are pea sized. Regular fungicide applications are necessary to prevent disease.