Diseases of Grapevine in Missouri
by Laszlo Kovacs

The conditions of the Missouri summer are highly favorable for downy mildew. Fortunately, most hybrid and native American grape varieties grown in the state offer some genetic resistance to this disease. Vinifera varieties, however, are extremely susceptible to downy mildew, and need to be painstakingly protected. In case symptoms appear in the vineyard, an immediate rescue spray of Ridomil Gold is essential in vinifera varieties, and is also warranted in other cultivars. Although the fruit becomes resistant to downy mildew after veraison, leaves and new succulent tissues remain susceptible all through the season. If the weather stays wet and warm during September and early October, the leaves can get severely infected, and the plants can be prematurely defoliated. This may be a problem because the fallen diseased leaves will help overwinter a large amount of inoculum for the following season, and because the defoliated plants will not be able to accumulate adequates amount of carbohydrate for the winter. For these reasons, it is advisable to control the disease even after harvest.