72 
Journal of Agricultural Research 
Vol. V, No. a 
marized their work, offered the alternate-host theory as a final resting 
place for this unsolved problem. 
The fact that none of these investigators was able to confirm De Bary 
(i) and the announcement of W. G. Smith in 1875 (30) that he had found 
the oospores of P. infestans doubtless influenced the Royal Agricultural 
Society to ask De Bary to again take up a study of how this fungus 
perpetuates itself. In a report to the Society in 1876 De Bary (4) makes 
the following general statement (p. 265), based on his observations and 
experiments, which shows plainly his thorough understanding of the 
habits of P . infestans. 
I was, perhaps, the first to call attention distinctly to the fact that the mycelium of 
Phytophthora , like that of parasites living in many other perennial plants, can be 
perennial in the surviving parts of the hosts, i. e. in the case of the potato, in its tubers. 
It has already been pointed out that Berkeley (5) first suggested that 
the mycelium of P. infestans is perennial in the potato tuber. Many 
attempts have since been made by Jensen (14), Boehm (6), Smorawski 
(32), Hecke (12), and others to duplicate De Bary's (4) experiments both 
in the laboratory and in the field, but no one except Jensen has obtained 
confirmatory evidence, and his evidence has failed to strengthen the 
perennial-mycelium theory. 
Naturally the accumulated negative evidence has led many to doubt 
the perennial capacity of the mycelium and to substitute widely different 
hypotheses. At least six theories as to the yearly advent of this disease 
have been advanced at various times: (1) That the mycelium lives over 
winter in the soil; (2) that mycelium is perennial in the diseased tuber; 
(3) that resting spores are produced which function in renewing infec¬ 
tion; (4) that the mycelium is latent in the potato plant; (5) that the 
fungus fruits on the parent tuber in the soil and the spores reach the sur¬ 
face and cause infection of the foliage; and (6) that sclerotia-like bodies 
or a mucoplasm gives rise to infection. The second of these is the only 
one supported by any amount of experimental data, the other five being 
based chiefly on negative evidence, of which there is considerable. 
In this paper are recorded data obtained in the laboratory and field 
supporting the perennial-mycelium theory. 
EXPERIMENTAL STUDIES 
The present study has to do largely with the function of the mycelium 
of P . infestans in infected tubers and its relation to the progeny of the 
host plant. The spread of the mycelium in tubers and sprouts was con¬ 
sidered first and followed by further experiments to determine the rela¬ 
tion of the mytelium to the shoots and young plants. Later, infected 
tubers were planted in the field and the progeny watched for any 
evidence of the disease. 
