234 DISEASES OF ECONOMIC PLANTS 



To control this disease all infected plants should be 

 destroyed by fire before they have had opportunity to 

 form sclerotia, and the place from which they are taken 

 sprayed with a heavy application of some fungicide. 



Damping off. See p. 60. 



Powdery mildew {Erysiphe polygoni DC). — This mildew 

 has the general characteristics of the powdery mildews, a 

 whitish flourlike coating upon the leaves and stems; at 

 first in circular spots, but later diffuse. Its presence in 

 America was first announced by Humphrey.^ It has 

 since been frequently reported in the field and occa- 

 sionally under glass. The fungus appears chiefly upon 

 the upper surfaces and is most developed upon the older 

 leaves. It renders the fruits bitter and distorts them. 



The disease is not usually serious. If it is so under 

 glass, the treatment suggested under cantaloupe will be 

 found effective. 



Leaf blight, fruit spot (Cladosporium cucwnerinuin Ell. 

 & Arth.). — This spotting of the fruit was first noted at 

 Geneva, N.Y., in 1887,^ where it was so abundant as to 

 ruin the crop for pickling. In 1905-1906 in Wisconsin it 

 caused abandonment of several pickle factories. 



The spots begin when the fruit is only 2-5 cm. long, 

 as gray, slightly sunken places with minutely velvety sur- 

 faces. They are about 3 mm. in diameter, but coalesce 

 into irregular patches, particularly toward the flower end. 

 As the spots age they darken to greenish black, and a 

 gummy exudate often appears. 



Upon leaves where injury is more rare than upon the 



1 Humphrey, J. E., Mass. Agr. Exp. Sta. Rpt. 9, p. 222, 1892. 



2 Arthur, J. C, N.Y. (Geneva) Agr. Exp. Sta. Rpt. 316, 1887. 



