Phomopsis cane and leaf spot of grape was diagnosed on a grape sample from Polk County after chemical injury was suspected. The disease is caused by the fungus Phomopsis viticola and is especially destructive if the weather following budbreak is wet for several days. It can weaken the vine, reduce yield, lower the quality of fruit for table use, and kill grafted and other nursery stock.
Dark brown to black necrotic spots may occur along primary and secondary leaf veins and on petioles. The necrotic spots may drop out of the leaf, causing a "shot hole" appearance. Infected portions of the leaf may turn yellow then brown. Severely infected leaves and leaves with infected petioles usually drop from the plant. Infected young shoots, cluster stems and petioles have yellow spots with dark centers. As these spots enlarge the infected tissue turns dark brown to black and appear as streaks and blotches.
The fungus also can cause fruit rot making the infected fruit gradually turn brown and shrivel. Some susceptible grapes may become infected through the skin when very young and show blackish flecks in the skin. When grapes are mature the fungus rots the fruit.
Phomopsis can be controlled by a combination of sanitation and fungicide application. To avoid introducing the pathogen into the vineyard, use pathogen-free propagation materials when planting and re-planting. Destroy debris after pruning by shredding it, burning, disking or plowing it into the soil.
Where permitted, erradicant chemical sprays may be applied during late dormancy (two to three weeks before bud swell) to kill fungal structures on the surface of vine parts. Two applications of a protective chemical are generally recommended, the first when the shoots are 1-3 cm long and the second when the shoots average 6-12 cm long. Captan and maneb are affective fungicides. When the temperature is cool and shoot growth is slow one or more additional applications may be necessary. However, best management practice is to select cultivars tolerant or resistant to Phomopsis cane and leaf spot. More information about disease can be found in the Midwest Small Fruit Pest Management Handbook, chapter 5, pages 129-130.
For a comprehensive list of grape cultivars for consideration in Iowa, their characteristics and level of resistance to several diseases see the ISU Extension Viticulture home page.