266 DISEASES OF ECONOMIC PLANTS 



Field trials by William Stuart ' have shown that certain 

 German and English varieties are more highly resistant 

 than the American varieties. , It is a hopeful sign, there- 

 fore, that American potato breeders are now giving atten- 

 tion to disease resistance and that new varieties of better 

 value in this respect may be expected in the future. 



Potatoes free from the disease should be used for seed. 

 Diseased potatoes may be recognized by their reddish 

 brown surface color, which extends toward the center of 

 the potato; also by the slight shrinking of the surface in 

 such parts, or by the black spotting described above. 



The crop should also be sprayed as recommended under 

 the general heading, p. 281. Though the disease may 

 not develop every year in severe epidemic form, spraying 

 is necessary each year as a preventive measure. Since the 

 blight develops so rapidly, it is quite impracticable to wait 

 for signs of the disease before spraying is begun. Moreover, 

 experiments extending over several years have shown that 

 even in years when the blight does not become epidemic, 

 sufficient benefit is derived from spraying to warrant its 

 adoption. 



The Geneva Experiment Station says : '^ " Judging from 

 the experiments thus far made, it appears that spraying 

 for blight is an operation which no potato grower in New 

 York can afford to neglect. Forty-eight farmers' business 

 experiments made during the past four years show an 

 average net profit of $20.51 per acre due to sprajdng. 



» Jones, L. R.,. and Morae, W. J., Vt. Agr. Exp. Sta. Rpt. 16, p. 163, 

 1902-1903. 



» Stewart, F. C, and others, N.Y. (Geneva) Agr. Exp. Sta. Bui. 290, p. 

 240, June, 1907. 



