The Plant Disease Clinic has been inundated with sick grapes this summer. The culprit is the very wet spring and early summer we experienced, which created ideal conditions for infection by two fungal diseases, anthracnose and black rot.
Anthracnose. Many grape disease guidebooks don't even mention anthracnose, but it's the number one disease problem on backyard grape plantings in Iowa. The shoots, berries, and leaves are all attacked, but symptoms on shoots and berries are easiest to recognize. If you've ever seen anthracnose on raspberry canes, you have a pretty good idea of what anthracnose looks like on grape shoots (but keep in mind that the two anthracnose diseases are caused by different fungi): sunken spots with dark brown to black margins and light gray centers, especially on newer growth. When the disease is severe, anthracnose can kill the tips of many new shoots by girdling them. Leaf spots also whitish in the centers at first, but the centers soon drop out to create a "shot-hole" effect. Young leaves are most susceptible; anthracnose often results in twisting and deformation because it damages part but not all of the expanding leaf blade. Berry spots are the most distinctive - round, purple spots that later turn ashy gray in the centers with dark brown to black margins. Most or all of the berries in a cluster can show symptoms. Berry infection often cracks the fruit skin, which leads to decay of the berry.
If your planting has had problems with anthracnose in the past, a delayed dormant spray (just before the buds break) of liquid lime sulfur will help to reduce the threat for the upcoming season. In addition, you may wish to consider a program of preventive fungicide sprays (mancozeb or captan) at 2-week intervals from bud break until the fruit begin to turn color. Prune out damaged shoots and clusters and remove the prunings from the vineyard, and rake the ground under the plants in fall in order to remove all fallen berries.
Black rot. Black rot is a major problem for commercial grape growers in many areas of the world, including the moister regions of North America. In Iowa, two types of symptoms are most common: leaf spots and fruit rot. The leaf spots are distinct, rounded to angular, up to 1/4" across, and light tan in the centers with darker tan margins. If you look closely at these spots, you can often see clusters of tiny, black dots in them; these are the fruiting bodies of the black rot fungus. The fruit rot advances very rapidly. A reddish brown spot on a berry spreads over the entire berry in a day or two. Shortly thereafter, the berry cracks, shrivels, and wrinkles until it becomes a hard, blue-black mummy.
Control of black rot resembles that on anthracnose. The first step is cleanup; rake out all fallen mummies and remove any others still hangng on the vines after the leaves have fallen. A preventive program of fungicide sprays, beginning shortly after bud break and continuing until the fruit begin to develop color, can help to suppress black rot in plantings where it has been damaging in past years.
This article originally appeared in the August 11, 1995 issue, p. 120.