'the potato disease' 701 



In the most frequent and ordinary attacks of the parasite, the 

 stems and leaves of the crop are more or less injured, while the 

 tubers show but insignificant traces of any form of disease. 



Attempts to produce on healthy potatoes dead patches similar 

 to those on tubers dug from the ground, by infection with 

 spores of Phytophthora rarely, if ever, succeed, and we are of 

 the opinion that the view that such well-known dead patches are 

 directly due to this fungus has not been satisfactorily proven. 



There is, however, little doubt that Phytophthora infestans is 

 occasionally to be found in the tubers of diseased crops, and 

 in such cases two views are held in regard lo its mode of 

 access, namely : — 



(i) By growth of the mycelium down through the stem and 

 along the rhizomes which bear the tubers ; or 



(ii) By spores which fall from the diseased leaves, and are 

 carried by rain or other agents down to the tubers which they 

 infect. Probably both opinions are correct in part. 



How the fungus passes the winter and commences its ravages 

 in the following summer is a much-debated question. The 

 conidia and zoospores are short-lived, and the active mycelium 

 appears to be met with almost entirely in the living tissues of 

 the leaves and stems. As soon as the latter are killed the 

 mycelium is much diminished or disappears altogether from 

 those parts ; nevertheless, how far the fungus can carry on 

 existence as a saprophyte is not clear. It is generally assumed 

 that the mycelium hibernates in the dormant tubers during 

 winter, and after planting the tubers the following summer 

 the hyphae of the fungus grow into the new young shoots of 

 potato, and finally make their way -upward into the leaves above 

 ground. From such diseased plants the parasite spreads to 

 the surrounding crop by means of spores in the manner pre- 

 viously described. 



Several workers at the subject nevertheless maintain that 

 from disease-spotted tubers either healthy plants arise or none 



