Control. — In nurseries spray with bordeaux mixture, 4-4-50. In the orchard spray as for pear scab, with perhaps one additional application. Leaf-spot (Mycosphaerella sentina).— Small lecticular spots with white centers on leaves. Spots become so numerous as to cause defoliation. The fungus is known only on leaves, and it winters on them.
Control. — Burn fallen leaves. Spray as for Scab. Scab ( Venturia pyrina). — Greenish brown or black spots on leaves and fruit, arresting growth and often causing fruit to crack. Severe on Flemish Beauty. Often attacks pedicles of fruits and causes them to drop, and may even cause defoliation. Is different from apple scab, but behaves much like it. Differs especially in the fact that the fungus winters on the twigs as well as on fallen leaves.
Control. — Owing to the nearness of the fungus (on the twigs) and the slowness with which the pear-leaf unfolds, two applications of spray before the blossoms open are sometimes necessary, and one immediately after they fall. Use lime-sulfur, 1-50, or bordeaux, 3-3-50.
Remarks in regard to apple scab (on page 264) are equally important here.
Rust (Gymnosporangium globosum). - Having the same habits and appearance as apple rust. Control — As for Scab. Plum. Black-knot (Plowrightia morbosa). -A black tumerous swelling from one to several inches in length, appearing on the limbs and twigs of American plums and sour cherries. Point of attack is usually under a bud or in crotches. Confined to America. A very serious disease. In some regions it has destroyed the plum industry. It was once supposed to be caused by an insect.
Control. - Burn all affected parts in the fall. Cut several inches below the swelling. A badly infected tree should be cut down at once, as there is no hope of saving it. Many states have a law requiring the destruction of affected trees.
Brown-rot. - See under Peach (p. 275).
Shot-hole fungus. - See Leaf-spot of Cherry (p. 268).
Leaf-rust (Puccinia pruni-spinosce). - Small circular powdery spots of yellowish brown on the under surface of the leaves, and reddish spots on the upper surface directly above them.
Control. - Early spraying with bordeaux, 3-3-50, or self-boiled lime-sulfur, 8-8-50.
Powdery mildew. - See under Peach (p. 276). Potato. Early blight (Alternaria solani). — A blight of foliage beginning as an even circular spot and coming early in the season, usually in July. Progresses slowly. This disease does not attack the tubers.
Control. - Bordeaux mixture at intervals of ten days, beginning when plants are 6-8 in. high.
Late blight and Potato-rot (Phytophthorainfestans). - The fungus winters in the tuber, which shows a faint pinkish tinge and a dry rot. Diseased tubers are planted, the fungus fruits on the cut surface and its swarm spores pass through the soil-water to the leaves which touch or are buried in the soil. An extensive irregular blighted area covers the leaf, the under surface of which may have a mildewy appearance. The disease spreads very rapidly. Later spores are washed down to the tubers and infect them. Appears late in the season, usually not much before August 1.
Control. - Can be controlled successfully by the use of bordeaux mixture, 5-5-50. It is always profitable to spray at least three times, and in a wet season six or more applications should be made. As the vines increase in size, greater quantities of spray and more nozzles must be used. Use from 40 to 100 gallons of spray mixture per acre.
Dry-Rot and Wilt (Fusarium oxysporum). - A dry rot of the tuber in storage and wilt of plants in the field. Can be detected in the seed tuber before there is any external appearance by examining a section near the stem end. A black ring or chain of dots near the surface is indicative of the rot. Infection frequently takes place through wounds.