Diseases of the Potato 189 



Early blight (Plate X) 



Early blight, caused by the fungus Alternaria solani, 

 appears as angular, dark brown dead spots on the leaves, 

 which may become so numerous as to kill the entire leaf. 

 The spots are marked with concentric rings of elevation, 

 giving the dead area a target-board appearance. The 

 vines usually do not become aflFected until after blossom- 

 ing has occurred, but may then become so severe as to 

 cause the vines to die. The spots often occur around a 

 flea-beetle puncture or other injury, but may also be 

 found on an otherwise uninjured surface. The fungus 

 commonly attacks the older and weaker leaves first, but 

 may spread from them to the younger ones. 



The disease may be confused with tip-burn or with 

 arsenical or other injury to the foliage. The fungus 

 may attack such injured or dead area first and later 

 spread to healthy tissue. The spores are large, thick- 

 walled, many-celled bodies produced on both surfaces of 

 the older dead areas. The fungus is able to live over 

 winter as a mycelium in the dead leaf tissue, and the spores 

 themselves will remain alive for more than a year. They 

 germinate in water, each cell being capable of sending 

 out a germ-tube. These are able to enter the leaf through 

 stomates or penetrate the epidermis directly. The 

 fungus attacks the foliage only, but as a result of the 

 attack the tubers may be small and thus the loss will be 

 considerable. This will depend upon the virulence of 

 the attack and may amount to one-half or more of the 

 crop. 



The disease can be held in check by thorough appli- 

 cations of bordeaux mixture. 



