6^ Bulletin 140. 



downy or mouldy growth upon the under surface of the leaves. This 

 is white in color and may be of ^considerable density. The upper 

 surface of the foliage does not show it, but whenever this frost-like 

 growth appears on the under side, it is almost certain that the potato 

 rot fungus is present, especially if the other conditions mentioned 

 above are also present. This external growth consists of spores and 

 of the parts bearing them. The spores, or conidia mature very 

 quickly, and have the power of immediately propagating the fungus. 

 They are small and hght, and may be carried long distances by 

 winds. It is largely owing to these bodies that the progress of this 

 potato disease is so rapid. They are produced in countless numbers 

 and are very energetic in attacking healthy tissue. It appears to be 

 very probable, also, that these conidia, or summer spores, are the 

 cause of the rotting of the tubers. After maturing upon the leaf, 

 some fall to the ground, and by means of water and other mechanical 

 agents they are brought in contact with the tubers growing underneath 

 the surface of the soil. Here they germinate and effect an entrance , 

 in the same manner as occurs above ground. The color of the 

 affected tubers also changes, a brown, dry rot taking the place of the 

 normal white color. The more slowly the tubers decay, the less is the 

 amount of moisture present ; the contrary is also true. The decay 

 does not take place in a uniform manner, but its progress varies in 

 different tubers. In some it is mostly the parts near the surface that 

 are affected, while in others the disease may advance rapidly towards 

 the center of the tuber, causing the exterior to show much smaller 

 amount of disease than is actually present. The discoloration, how- 

 ever, generally presents a uniform appearance. Athough it is by no 

 means impossible for the mycelium to reach the tubers from the leaves 

 by means of the stems, still it is the generally accepted opinion that 

 infection does not take place in this manner. This belief was held 

 many years ago, for in some of the earlier writings recommendations 

 may be found in which very high hilling is advocated so that the 

 spores may be washed past the tubers and away from them, and not 

 through the soil directly to them. 



" There is still another feature of the late blight which it is well to 

 bear in mind. The disease generally appears during August and 

 September, although earlier and later attacks are not very rare. Com- 

 ing so late in the season, all the earlier varieties are comparatively free 



