See basic cultural guidelines for the control of plant diseases. Table 2.4 lists pesticides available on various fruit crops for the control of diseases.
This is a fungus infection appearing first as numerous dark-brown to black dots scattered at random over one or both surfaces of the leaf. The infection may appear at any time during the growing season. The spots enlarge, become more angular in outline, and sometimes have a purplish margin. Affected leaves soon turn yellow and then drop. This weakens the plant, reduces vigor and productivity, and results in smaller fruit of lower quality.
This disease quite commonly is called septoria leaf spot, the name of the parasitic stage of the fungus causing the infection. This leaf spot can be distinguished from that caused by anthracnose by certain characteristics. The spots typically appear on the foliage in June, at which time they resemble anthracnose. Spots enlarge and the central area becomes light in color with a brown border. Tiny, black specks soon appear scattered over the surface of each spot. These specks are the bodies of the fungus, which contain the spores. They do not appear on anthracnose leaf spots. The diseased leaves, especially on currants, turn yellow and drop.
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 it 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 a whitish growth. Older infections form a thin, felt-like coating, which is tan to reddish brown in color. Black dots called perithecia, which contain spores of the fungus, appear in the fungal mats covering the affected areas. Heavy mildew deposits will cause stunting and premature drying of the foliage, affecting fruit production and weakening the plants.
Diseases of Minor Importance
Caneblight or Wilt
This is a fungal organism that causes a sudden wilting and dying of scattered canes or whole bushes. It is most evident just before fruit ripens.
This fungal infection produces a dark-colored dieback of the tips of the branches and a gray mold rot of the berries. Infection occurs during wet, humid weather in plantings in low areas with poor air circulation.
White Pine Blister Rust
In the spring, small, yellow spots can be seen on the underside of leaves. By late summer, a yellow to brown, threadlike growth develops on or near these infection spots. These growths contain another type of spore, which germinate and infect the white pine in the fall. European black currants and wild gooseberries are the main hosts of blister rust when white pines are growing in the vicinity.
Cluster Cup Rust
This rust disease can produce striking symptoms on species of wild gooseberries or in neglected home garden plantings but causes slight damage. The rust affects leaves, stems, and fruit but is commonly found on the leaves and leaf petioles. The leaf is thickened where the cluster cup later appears. The spots have a reddish appearance. The sedge plant is the alternate host of this rust.
This viral disease appears as a chlorotic pattern (light and dark areas) on the leaves. The lighter-green areas gradually turn white.