Phytophthoric Plant Diseases

plant diseases caused by fungi of the genus Phytophthora. Late blight (downy mildew) of potato is characterized by large, scattered spots on the leaves, primarily on the margins. In humid weather the undersides of the leaves are coated with a white mold—the conidiophores and conidia of the fungus. Diseased leaves die. On the tubers the disease begins with the appearance of lead gray spots, which gradually deepen and become rotten, turning brown in color (rusty in cross section) and hard in consistency. The tubers become infected when conidia from the leaves fall on them during a rain or at harvesting. In storage, infected tubers may become diseased with fusarium rot or wet rot.

Control measures consist in selecting healthy tubers for planting, treating the plants with pesticides, and hilling the plants at the proper time. Also important are the application of increased doses of potassium phosphate fertilizers, the spraying of plants with microdoses of copper or combinations of microfertilizers and fungicides, and the preharvest removal of plant tops. Other measures include harvesting in dry weather and cultivating resistant varieties.

Late blight (downy mildew) of tomato affects adult plants. Brown spots with a white mold are visible on the undersides of the leaves, and brown elongate spots without a mold appear on the stems. The fruits have a brown rot. Control measures consist in the isolation of potato and tomato fields, the application of increased applications of potassium phosphate fertilizers, and the spraying of plants with fungicides. It is also advisable to harvest the fruits early and ripen them artificially and to cultivate resistant varieties.

Southern blight of tomato affects seedlings as well as adult plants. Constrictions appear on the stem, and on the fruits a watery rot appears with concentric zones that range from gray to red-brown in color. When the air is especially humid a white mold appears on the fruits. Control measures include strict adherence to crop rotation, avoiding excessive soil moisture and too high temperature, while cultivating seedlings at a higher temperature, and spraying the plants with fungicides.

Brown gummosis of citrus trees is characterized by rounded, dark brown spots on the leaves, mainly along the central vein. There is a white fungus on the undersides of the leaves, and spots appear on the apexes of shoots and on the stalks of seedlings. The bark dies and cracks, releasing a gummy substance. Control measures consist in spraying with fungicides in nurseries and orchards and pruning diseased shoots.

Collar rot of apple trees mainly affects the collar of the trees, where the bark acquires a blue-violet coloring and cracks, revealing a tissue that is chocolate brown in color. Control measures consist in gathering and destroying drops, on which the fungus winters. Trees may be treated by cleaning and disinfecting wounds or by cauterizing affected parts.