Integrated Management of Grape Diseases

Use of Resistant Cultivars for Grape Disease Management

In an integrated disease management program where emphasis is placed on reducing overall fungicide use, it is essential that any available disease resistance be identified and used. If resistance is not available, we should at least identify and avoid those cultivars that are highly susceptible to important diseases. There are a few grape varieties with high levels of resistance to most diseases. Norton (Cynthiana) is a variety that has great potential for organic production in the southern portion of the Midwest. It has good resistance to most diseases and in several commercial plantings, growers rarely apply fungicides. Hopefully, new varieties with improved levels of disease resistance will be introduced in the near future. Unfortunately, resistance to most of the major diseases is not available in most commercially grown grape cultivars in the Midwest. Thus, the disease management program must rely heavily on the use of cultural practices and efficient use of organically approved fungicides or biocontrol agents or products. Whereas resistance is generally not available for most diseases, some grape cultivars are known to be much more susceptible to certain diseases than others (Table 2). For example, the cultivar Chancellor is highly susceptible to downy mildew, whereas downy mildew is seldom a serious problem on Concord and several other cultivars. Growers should consider disease susceptibility before establishing the vineyard. Segregating highly-susceptible cultivars into blocks that can be easily treated separately allows growers to apply more fungicide or other control agents when needed to highly susceptible cultivars while reducing their use on less susceptible cultivars. In addition, varieties differ greatly in their sensitivity to copper and sulfur fungicides. When planting the vineyard, it is important to isolate blocks of sensitive varieties from those that will be sprayed with these materials.

In many situations, growers do not have the flexibility of avoiding highly susceptible cultivars. The demand for a specific wine, juice grape, or table grape cultivar generally dictates which cultivars are planted. Whereas this situation usually cannot be avoided, growers need to recognize that when highly- susceptible cultivars are planted, their disease management options are greatly restricted.

Table 2. Relative disease susceptibility and Sulfur, and Copper sensitivity among grape cultivars.

The relative ratings in this chart apply to an average growing season under conditions usually favorable

for disease development. Any given cultivar may be more severely affected.