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(Fungi). 



Late Blight or Downy Mildew (Phytophthora infestans) : This fun- 

 gus disease is often very destructive in late summer to the potato crop. 

 The margins of the leaves are usually the first portions to become diseased, 

 probably because in periods of excessive moisture the edges are kept 

 longer moist than the inner parts. There is a sharp line of demarcation 

 between the affected and the unaffected areas ; and in moist, warm 



Early Blight. 



Late Blight. 



weather, when the affected areas are enlarging rapidly, there is between 

 the two areas a narrow whitish ring of mildew, which is producing 

 spores in large numbers. The diseased leaves first turn brown, then 

 darker, and finally black. Complete decay of the leaves soon occurs, 

 accompanied by an offensive, yet characteristic, odor. 



It is usually supposed that the fungus attacks the tubers as well as 

 the stalks and leaves, producing a brown rot, but the mode of infection 



