190 The Potato 



Late blight (Plate IX) 



Late blight, also known as downy mildew, is caused 

 by the fungus Phytophthora infestans. It occurs in north- 

 eastern United States and adjacent parts of Canada and 

 sporadically farther south and on the Pacific Coast. It 

 has also been known for years in many countries of Europe, 

 and has been reported as occurring in Australia and in 

 the high plateaus of South America where potatoes are 

 grown. It prevails in those regions where the weather is 

 relatively cool and moist, conditions most favorable for 

 the growth of the potato itself, and, even there, is de- 

 structive only during especially favorable seasons. For 

 this reason the disease appears in epidemic form in 

 many sections in occasionaF periods of years, the inter- 

 vening periods being comparatively free from it. 



The disease shows on the leaves as water-soaked or 

 brown areas that may occur on any part of the leaf. In 

 moist weather these spots rapidly enlarge so that the 

 entire leaf may become affected, and over the recently 

 affected area, usually on the lower surface, a fine white 

 mildewy growth may be noticed. The stem may be- 

 come affected in a similar manner. In moist weather 

 the affected areas soften instead of drying and give off a 

 disagreeable odor characteristic of decaying vines. 



The fungus causing the late blight may attack the 

 tubers (Plate IX), the upper surface of those lying nearest 

 the surface of the soil being first infected, producing on 

 them a discolored, water-soaked area which may be small 

 or, if the soil is moist, may extend over much of the tuber. 

 The affected area becomes somewhat sunken at digging 

 time or later in storage and the flesh has a rusty-brown 

 color. This condition is known as dry rot. The flesh 



