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 Phytophthora 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 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. 
