162 



DISEASES OF ECONOMIC PLANTS 



about 3-8 mm. in diameter, many or few 

 according to the severity of the infection. 

 In or near the centers of the older spots 

 upon the upper surface of the leaves are 

 seen the nearly microscopic pycnidia 

 which are often arranged in concentric 

 circles. 



On the young shoots the spots are some- 

 what more reddish and are often cracked 

 longitudinally, but otherwise they are as 

 upon the leaves. 



This rot is much worse in warm, himiid 

 weather than in dry times, and a few days 

 of muggy weather may so favor the 

 growth of the causal fungus as to develop 

 an epidemic, which may in turn be 

 checked by a dry spell. Often the dis- 

 ease appears in distinct waves during the 

 season, each wave corresponding to a 

 climatic condition favorable to infection 

 and usually lagging behind such favor- 

 able condition a period of 8 to 14 days, 

 the time necessary after infection for the 

 disease to become conspicuous enough to 

 attract the attention of the vineyard- 

 ist. The first infections of each season 

 occur upon the stem- leaf, petioles, and 

 tendrils. 

 While the Scuppernong occasionally shows a few spots 



upon leaf or berry and the blossom is sometimes affected, 



this variety is practically immune. 



^"'^^^ 



