158 Report of the Pjotanist of the 



appears during damp, muggy weather in August and September. It 

 first appears on the leaves (usually the lower ones) in the form of 

 small brown spots which rapidly enlarge. In moist weather the mar- 

 gins of the diseased spots are covered, on the under surface, with a 

 fine, frost-like mildew (Plate XVII). In dry weather, this mildew 

 may be difficult to detect. In the later stages of the disease affected 

 plants frequently have the appearance shown in Plate XVI. Under 

 favorable weather conditions a field of potatoes may be almost com- 

 pletely ruined within a few days after the first appearance of 

 the disease. 



Contrary to popular opinion, this form of blight is not caused by 

 wet weather. The real cause is a parasitic fungus. Without the 

 fungus there could be no blight of this kind, no matter what the 

 weather conditions might be. Blight is most virulent in wet weather 

 because the blight fungus thrive best and spreads most rapidly in 

 wet weather. 



In Plate XVTII the potato blight fungus, Pliyfoplitltom infcstans, 

 is illustrated. Figure i is a cross section of a blighted potato leaf. 

 The branching, tree-like affairs hanging down from the undersurface 

 are the spore-stalks of the fungus. It is these which make up the 

 frosty mildew on the undersurface of affected spots. The tgg shaped 

 bodies at the ends of the branches are the spores. When one of these 

 spores falls u])<)n a healthy potato leaf in a drop of water it germi- 

 nates within a few hours (after the manner shown in Fig. 5) and 

 forces a slender, colorless tube into the tissue of the leaf. Once 

 within the leaf the colorless tube branches and penetrates the leaf in 

 all directions (See Fig. i), absorbing nourishment from the cells of 

 the leaf and later killing them. As the leaf tissues dies the fungus 

 forms spore-stalks bearing new spores and the life cycle is complete. 

 Usually about four or five days elapse between the germination of 

 the spore and the production of a new crop of spores. 



The rot of the tubers which frequently follows an attack of blight 



