VEGETABLE AND FIELD CROPS 227 



angular in outline, owing to their limitation by the veins. 

 They seldom exceed 3-6 mm. in diameter unless by the 

 coalescence of several spots. . The spots soon become brown, 

 and the central part turns ashen-white and is seen by the 



Fig. 107. — Cerco.sporose of celery. Original. 



aid of a hand lens to bear numerous hyphae. The leaves 



may bear so many spots as to appear as though scorched, 



and often death results to the entire plant. 



The disease spreads most rapidly in damp warm weather. 

 Marked difference in varietal resistance was reported by 



Tracy ^ as early as 1885, the Boston Market and Golden 



iSee Lamson-Scribner, F., U.S. Dept. Agr. Rpt. for 1886, p. 117. 



