188 
F. T. Brooks. 
tubers or decaying completely in the soil. Under field conditions 
however, none of the blighted tubers planted experimentally by him 
were seen to throw up shoots invaded by mycelium of Phytophthora 
infestans derived from the seed sets. In the light of these invest¬ 
igations, the blight problem has also been discussed by Horne. 1 
Melhus 2 in America obtained a considerable amount of evidence 
in favour of the view that primary infection arises from mycelium 
which hibernates in the seed tubers. Thus a number of partly 
diseased tubers planted experimentally in the open threw up one 
or more shoots which were infected by Phytophthora and served as 
starting points for an epidemic. 
In connection with the perennation of potato blight by means 
of mycelium in diseased tubers, mention may be made of the 
possibility of shoots becoming infected while the tubers are stored 
in clamps or after being discarded from these and left lying near 
them. Such shoots may give rise to spores in the late spring or 
early summer under favourable weather conditions, and these may 
serve to infect growing crops close at hand, although there is no 
published evidence of the occurrence of an outbreak arising in this way. 
Portions of blighted tubers left in the soil from the previous 
crops or introduced with the manure ( e.g ., from pigs) may also, per¬ 
haps give rise to conidia of Phytophthora when lying near the surface 
of the soil, but such diseased tissues usually become invaded by second¬ 
ary organisms which cause complete disintegration. 
There are of course other ways in which the blight fungus 
might hibernate. Its mycelium may perhaps live saprophytically 
in the soil but this is unlikely for Phytophthora infestans is by no 
means an easy fungus to grow artificially and in competition with 
other organisms would probably soon collapse. The intervention 
of an alternate host has sometimes been invoked but there is no 
evidence that such exists, and other plants e.g., tomatoes, which 
occasionally become infected by Phytophthora infestans in this 
country, are only attacked after an epidemic has begun in potatoes. 
Massee 3 considered that the mycelium hibernating in the tubers 
passed thence into the growing shoots, remaining dormant in the 
stems and leaves until weather conditions suitable for the spolia¬ 
tion of the fungus intervened, but no evidence in support of the 
dormancy of the mycelium in the aerial parts of the plants was 
given. Recently, Eriksson has advanced his mycoplasm theory 
’ Horne, A. S. Potato diseases. Ann. App. Biol., 1914. 
5 Melhus, I. E. Hibernation of Phytophthora infestans in the Irish potato. 
Journ. Agric. Res., 1915, V, p. 71. 
* Massee, G. Diseases of Cultivated Plants and Trees, 1910, p. 123. 
