46 U. S. DEPARTMENT OF AGRICULTURE. 



The affected areas often have a metallic or purplish tinge 

 which is especially marked at the margin where the dis- 

 eased tissue joins the healthy. At times, a pinkish color is 

 apparent when the outer skin of affected areas is first re- 

 moved. The diseased tissues underlying the sunken areas 

 are browned, and unless secondary rots have set in, are quite 

 dry and firm. The tissue is merely killed, not disintegrated. 

 In cross section it is seen that the affected tissue tends to be 

 limited to the outer parts of the tuber, and that the advanc- 

 ing edge is irregular and feathery. 



Under storage conditions, especially if the air is very 

 moist, secondary rots due to bacteria or fungi set in. At 

 times, in advanced cases, it is difficult to ascertain whether 

 the primary cause of rotting was freezing injury, with sub- 

 sequent bacterial and fungous infection, or late blight. In 

 such cases the deciding factors are the presence in the ship- 

 ment of clear-cut cases of late blight or of freezing injury. 



Potatoes affected with late blight do not show the hollow 

 brittle regions found in potatoes affected with Fusarium 

 tuber rot, nor the shriveled and wrinkled surfaces caused by 

 Fusarium or freezing injury. The feathery edge of the 

 tissue killed by the late blight fungus also serves to differ- 

 entiate it from tissues killed by Fusarium or freezing. 



Late blight tuber rot occurs most commonly in potatoes 

 grown in the Eastern and North Central States, and in the 

 coastal portions of the Pacific Northwest States. Under cer- 

 tain weather conditions, it also occurs in southern potatoes. 

 During the rainy season of 1918, late blight was prevalent 

 in Florida potatoes. It is not a hot weather disease but is 

 favored by cool, wet weather. 



Late blight is one of the most important field diseases of 

 the potato, causing a severe blight of the vines in addition 

 to its attack on the tubers. 



The fungus overwinters in diseased tubers, and is intro- 

 duced into the field with diseased seed stock. From dis- 

 eased seedlings produced by such seed, the fungus spreads 

 rapidly throughout the field when weather conditions are 

 favorable. Tubers are infected in the field by spores washed 

 down or spattered from the diseased vines. The disease 

 also develops in transit and storage. 



Late blight can be controlled in the field by spraying with 

 Bordeaux mixture. Its development in transit and storage 

 is checked by a dry atmosphere and a low temperature (34 

 to 36 F.; 45 F. is too warm). It is highly desirable that 

 diseased tubers be sorted out as soon as they can be de- 

 tected. In case of a late blight epidemic, digging should be 

 postponed until the tops have dried up. 



Affected tubers are not marketable for table use. They 

 are also a menace to sound stock in transit and storage since 

 secondary rots, such as Fusarium or slimy soft rot, very fre- 

 quently follow late blight. 



Ref. (9); (18); (34); (45). 



