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 tumerous 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 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 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 1. 



