77 



spread to the healthy tubers. Too frequently, however, 

 it is found on opening a clamp that the whole store is a 

 putrifying mass, simply owing to neglect of the essential 

 facts of ventilation and dryness. 



We have seen that the Phyto-pkthora spreads through- 

 out the summer months with amazing rapidity by air- 

 borne conidia. If, however, these are to produce infection 

 they must germinate immediately, for being only provided 

 with a thin wall they cannot resist drying up or frost. 

 They, therefore, never serve for carrying over the fungus 

 from one season to the next. 



The question then arises as to whether this fungus, like 

 Pythium, produces resting spores that are able to survive 

 the winter in the soil. The most diligent search of many 

 investigators has failed to show that such resistant resting 

 spores are ever produced in Nature. On the other hand, 

 Dr. Pethybridge, in Dublin recently,, has found it possible 

 to obtain resting spores of this fungus by growing it 

 artificially on a medium consisting of cooked Quaker 

 Oats stiffened with a gelatinous substance called agar. 

 The fungus forms a dense, white growth on the surface of 

 this, and after some months produces the resting spores 

 within the medium. These are quite characteristic resting 

 spores and possess a thick resistant wall. Interesting as 

 these artificially-produced resting spores are from the 

 scientific point of view, the fact that their presence has 

 never been demonstrated in Nature, renders it necessary 

 to consider other means by which the fungus may survive 

 the winter. It has been repeatedly shown that diseased 

 tubers kept over winter in the open may give rise to a 

 growth of Phytophthora bearing spores in the warm, moist 

 days of early summer. During the colder months fila- 

 ments of the fungus in diseased tubers grow very slowly 

 indeed, especially if the tubers are kept dry. If, how- 

 ever, such tubers are kept warm and moist the fungus 

 rapidly extends through the whole tuber and even pro- 

 duces a network of threads bearing powdery conidia on 

 the outside. Such conidia, carried in the air, infect leaves 

 -of potato 1 plants in the vicinity, and in this way start an 

 epidemic. It has frequently been noticed that portions 

 of a potato field, near to old potato pits or refuse heaps, 

 have been the starting points for the disease. This mode 

 -of initial infection is, however, scarcely enough to account 

 for the very wide-spread frequency of the disease. 



