Fruit showing signs of downy mildew have been arriving in the Plant Disease Clinic. Infected berries are usually brown and wrinkled and are covered with the whitish fungus. The undersides of infected leaves also show fluffy, white to grayish fungal growth. Pale-yellow spots are evident on the upper surface of leaves.
The fungus Plasmopara viticola causes downy mildew. The disease is favored by factors that increase the moisture content of soil, air, and the plant. Rain is the principal factor in promoting disease. The optimum temperature for disease development is 64 F to 76 F. On fruit, most infection occurs during two periods in the growing season. The first is when the berries are about the size of small peas. Little infection occurs during hot summer months. As nights become cooler in late summer, a second infection period may develop.
Cultural practices that improve air circulation are recommended to reduce the threat of disease. Plant grapes in sunny areas with good air circulation. Removing weeds and orienting rows to maximize air movement will also speed drying of leaves. Preventative fungicide programs from bloom through harvest can be use to control downy mildew.
Photos of disease symptoms can be found in the publication IDEA 2 "Small Fruits - Insect and Disease Management for Backyard Fruit Growers in the Midwest". This publication is available at your local Extension office or from ISU Publications Distribution Office. Extension Distribution Center, 119 Printing and Publications Building, Iowa State University, Ames, Iowa 50011-3171, Telephone: (515) 294-5247, Fax: (515) 294-2945.
This article originally appeared in the July 23, 1999 issue, p. 99.