94 
Journal of Agricultural Research 
Vol. V, Nc. 2 
this low temperature explains clearly how infected tubers are able to 
survive the winter season and are in a condition to make rapid 
growth when placed in the soil. The statements that tubers infected 
with P. infestans very largely rotted in the ground and that a large 
majority grew and produced normal plants are both very prevalent in the 
literature, and the author reports similar experiences in his own experi¬ 
ments. These discrepancies, however, may well have been due to the 
conditions under which the tubers were stored and their state of germina¬ 
tion at planting time. Of course, as will be shown later, the influence of 
moisture and temperature after planting plays an important r 61 e. 
From infected seed tubers growing rapidly the greatest number of 
infected sprouts and shoots were obtained in a saturated atmosphere at 
a relatively high temperature (23 0 to 27 0 C.). A temperature of 27 0 
seemed even more favorable than 23 0 C. This is of interest in view of 
Vochting’s (35) results to the effect that the optimum for the growth of 
the potato tuber is about 27 0 and is not out of harmony with the optimum 
fixed by Jensen (14) for the growth of the mycelium in the potato tuber. 
How the fungus spreads in the stem and sprout tissues at temperatures 
between 23 0 and 27 0 C. has been described in an earlier part of this 
paper. The fungus not only traveled up the stem rapidly but also 
sporulated profusely at such temperatures. In a paper not yet published 
it is also shown that the growth of liberated zoospores is more rapid at 
23 0 to 24 0 C. than at lower temperatures. This is true also where the 
vines have been inoculated with conidia and zoospores. Although no 
experiments have been made to establish the optimum for the growth of 
the mycelium in the diseased tuber, the data cited above show that the 
mycelium is very active at 23 0 to 27 0 C. Whatever may be the optimum 
for the mycelium in the tuber, this point is clear: That temperatures 
between 23 0 and 27 0 C. are more conducive to the growth of the mycelium 
than lower temperatures, other conditions being favorable. 
Although the state of germination of the tuber and the temperature 
are important, they do not take precedence over moisture. It need 
hardly be mentioned that P. infestans , by virtue of its phylogeny, is a 
moisture-loving fungus. To the practical grower it is well known also 
that an epidemic of late-blight need not be feared in a dry season, while 
in our northern potato sections a wet season is a sure sign of such an- 
epidemic. The mycelium grows very slowly and absolutely refuses to 
fruit in a dry atmosphere. It has been shown that the spread of the 
mycelium is materially retarded when tubers infected with P. infestans 
are buried in dry soil. Again, the necessity of moisture is well illus¬ 
trated in the case of the isolated plantlet referred to. The fungus 
made little progress in the stem even after reaching the surface of the 
soil, and it was only by restoring a moist atmosphere that the fungus 
fruited. It has also been shown that a greater number of the infected 
