PLUM AND POTATO DISEASES 279 



Rust (Gymnosporangium globosum). — Having the same habits and 

 appearance as apple rust. 

 Control. — As for Scab. 

 Plum. Black-knot {Plowrightia morbosa). —A black tumorous swell- 

 ing 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 priini-spinosoB) . — 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 begin- 

 ning 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 ir- 

 regular 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 L 



