170 



FUNGOUS DISEASES OF PLANTS 



be brought in contact with tubers. They penetrate the tubers as 

 readily as the leaves, the dry rot being produced in consequence. 

 An affected tuber which does not show the disease in severe 

 form may be used as seed, and thus the disease may be propa- 

 gated from year to year through the seed tubers. It is now 

 quite certain that the perennial appearance of the disease is due 

 to this use of diseased tubers. It is not always possible, how- 

 ever, to determine if the mycelium is present in the tubers, since, 

 even though they may have been stored for many months, the 



Fig. 63. Control of Late Blight of Potatoes by Bordeaux Mixture 

 (Photograph by F. C. Stewart) 



fungus will not develop further if the conditions are unfavorable. 

 Thus if stored at a low temperature and in a dry atmosphere the 

 rot fungus may not become evident. No oosporic stage of the 

 Phytophthora has been found in the host, but recently Clinton 

 (Conn. Agl. Exp. Sta. Report, 1909-19 10) has obtained mature 

 oospores in cultures of the fungus on oat-juice agar. 



Control. Studies looking toward the prevention of the potato 

 blight were begun during the middle of the past century. At 

 first the greatest success was accomplished only in securing 



