Powdery Mildew Two types of powdery mildew, American and European, attack Ribes plants. We are concerned only with the American type. Mildew is most important as a disease of gooseberries, but does occur in a mild form on currants. White, powdery patches of the fungus appear first on the lower parts of the bush, attacking the leaves, shoots, and berries. As the infection progresses, the entire surface of these parts becomes covered with the growth.
Older infections form a thin, feltlike coating, tan to reddish-brown in color.
Black dots called perithecia, which contain spores of the fungus, appear in the mycelial mats covering the affected areas. Heavy mildew deposits will cause stunting and premature drying of the foliage, affecting fruit production and weakening the plants. The infection produces a dark-colored dieback of the tips of the branches and a gray mold rot of the berries. May occur during wet, humid weather where plantings are in low areas with poor air circulation. The uredial stage appears in the spring as small yellow spots on the underside of the leaf.