AGRICULTURAL EXPERIMENT STATION. 177 



in the field when the potatoes are gathered. The mycelium may 

 live over winter in potato tops and in the few tubers left in the 

 ground. 



The zoospores if they ever occur, may not ordinarily be produced 

 and may not be always essential to the continuance of the disease. 

 The known occurrence of oospores in related species, would in 

 our estimation be stronger evidence of their existence in the Potato 

 Rot, than the fact that they have not been seen by botanists, evi- 

 dence that they do not ever occur. 



Prof. Humphrey (Mass. Ex. 8ta. bull. No. 6, 1889, p. 19), 

 says : "The fungus doubtless survives the winter by the hyber- 

 nation of the threads in potato tubers, but this method alone seems 

 hardly certain enough to constitute the sole reliance of the plant." 



He also says "that the existence of winter spores has never 

 been satisfactorily proved." 



Prof. G. W. Smith of England, claims to have found these 

 spores in the leaves, stems and tubers, and figures them. The 

 relatives of this fungus, the Downy Mildew of the Grape and 

 others are known to produce winter spores, and by analogy, we 

 would expect them to occur in the Potato Rot. 



Prof. Scribner says : "The mycelium is perennial in the tubers 

 and if these, containing this mycelial growth are used for seed, 

 they are almost certain to carry infection to the new crop." 



He does not apparently recognize any other means of survival. 



We are inclined to believe that there is some means of continu- 

 ing the disease in the soil after the crop is harvested. Whether 

 this is by resting spores in the tops, leaves, rotten and small 

 potatoes left on the ground, or whether the mycelium lives over 

 winter in the tops and ungathered potatoes, is an open question. 

 If the mycelium will live in the harvested potatoes we see no rea- 

 son why it may not survive in the small potatoes and tops left on 

 the ground. If potatoes left in the soil grow the next spring cer- 

 tainly the hardier fungus would probably survive. This matter is 

 of fundamental importance. A definite knowledge of the winter 

 conditions of this fungus is essential as a basis for intelligent 

 application of preventive measures. A careful study of the pos- 

 sible winter conditions is of the greatest importance. It will 

 probably be found finally that it has all the resources claimed. 



Life History. 

 Whatever way the plant is infested, the mycelium or threads 

 grow with it, enter the new shoots and finally penetrate the leaves. 



