192 The Potato 



their death is noticed as a blighting of the leaf. After 

 four or more days, the mycelium has produced fruiting 

 bodies bearing more spores. These are carried by air 

 currents or by other means to healthy leaves, which in 

 turn become infected if the weather is wet, or they fall 

 to the ground, where during rains they are washed into 

 the soil. Thus under favorable weather conditions the 

 fungus may spread from an infected vine to neighboring 

 ones, causing them to blight, and thence to all the vines 

 in the field until all are blighted. The spores, washed 

 into the soil, come into contact with the new tubers and 

 infect them in much the same manner as the leaves were 

 infected, so that they rot in the manner described earlier. 

 If the soil remains wet and soggy, as heavy soils often do 

 during rainy periods of the fall, the fungus grows luxu- 

 riantly in the tuber and produces fruiting bodies on its 

 surface. Spores produced there infect other tubers in 

 the hill and possibly spread to other hills. It has been 

 shown that the fungus may fruit on the surface of the 

 tuber in storage and infect mature tubers there, though 

 this does not often happen. All that is necessary for 

 infection of the tubers is the presence of spores on their 

 surface under moist and fairly mild or cool weather. 

 Infection readily occurs when the blighting vines come 

 into contact with freshly dug tubers. These points are 

 of importance, as it teaches us to leave blighting vines 

 until they are dead and dry before digging unless we can 

 dispose of the crop at once or unless there is danger of 

 the tubers rotting badly in wet soil. Tubers infected 

 shortly before digging time may show no evidencf of it 

 then, but the dry rot will appear later in storage. 



It has also been shown that throwing the soil over the 

 tubers with a shovel plow to the depth of four or five 



