FUNGI AND PLANT DISEASE 



421 



chemicals. Avoidance of heavy seeding, high soil and atmospheric 

 moisture content, and care in shading all help to reduce losses 

 from damping off. Deep plowing, where field crops are to be 

 grown in a soil that is known to contain the pathogens, is bene- 

 ficial since the organisms live in the topmost soil layers. 



The LATE BLIGHT OF POTATOES (fig. 257), referred to in the 

 opening pages of this chapter, is another disease caused by a 

 Phycomycete. Tomatoes are also attacked severely by this disease 

 in the presence of favorable conditions, but ordinarily escape 



A B 



Fig. 257. — Late blight of potatoes: A shows fruiting structures extruded 

 from stomate of leaf, and B shows hyphae of fungus in cells of the potato 

 tuber. 



serious damage. The severity of its attack on the potato crop 

 depends upon seasonal weather conditions; a cool, rainy season 

 providing optimum conditions for infection of both tops and 

 tubers. Initial infection with late blight comes from diseased seed 

 potatoes. When these are planted, the pathogen grows upward 

 through the young sprouts, ultimately producing wind-borne 

 spores which may infect other shoots, or be carried downward 

 in the soil by rain to infect young developing tubers. Of course, 

 the best way to control late blight is to use disease-free seed, thus 

 keeping the parasite from gaining a foothold in the field. Since 

 it is often impossible to be sure of such a freedom from attack, 

 sprays of Bordeaux mixture — a fungicide consisting of lime. 



