316 
Joumal of Agricultural Research 
Vol. XXX, No. 4 
presented in detail. The paucity of 
infection in this experiment is not easily 
explained. 
The data presented from the two 
previous series indicate clearly that 
though occasional infection may take 
place through practically the entire 
range of temperature at which peas 
will grow, infection is not abundant nor 
does invasion proceed rapidly at a tem¬ 
perature below 15° C. The optimum 
temperature for infection is between 
15° and 30°. Within this range in¬ 
fection appears to be approximately 
uniformly abundant, but at the lower 
temperatures there is a retardation in 
rate of progress of the fungus through 
the plant that is even greater than the 
retarding effect of temperature upon 
root growth as it occurs under the con¬ 
ditions of experiment. It is probable, 
however, that in the field in the spring 
with longer days root growth is more 
rapid at lower temperatures than under 
the conditions of experiment, and if that 
is the case the apparent retarding effect 
of low temperature upon the progress 
of the disease in the field will be greater 
than is shown here. 
The effect of soil moisture upon in¬ 
fection in these experiments was not 
as great as anticipated. In fact, they 
do not give any decisive indication that 
soil moisture within the limits used in 
these experiments is a factor at all in 
determining the amount of infection. 
Although infected plants in the more 
moist soil uniformly begin to die a 
little earlier than those in drier soil, the 
number is in most cases approximately 
equal. 
Further consideration of these ex¬ 
perimental data may serve to bring out 
more clearly their limitations when an 
attempt is made to interpret field ex¬ 
perience in their light. In these ex¬ 
periments it may be assumed that an 
approximately equal amount of viable 
fungus mycelium was brought in con¬ 
tact with the root systems of plants. 
The number of infections obtained is 
an indication of the ability of the fun¬ 
gus to infect under these conditions. 
It is possible that in the field soil mois¬ 
ture exerts an important effect, not only 
upon infection, but upon the survival 
of the fungus from year to year. The 
favorable effect of wet soil upon the 
disease may be due in large part, if not 
wholly, to the favorable environment 
which moisture provides for the fungus 
in its resting condition as oospores, or 
in a possible saprophytic life in the soil. 
Such an effect of soil moisture increas¬ 
ing the active vegetative growth of the 
fungus in the field may account for the 
association of disease with wet soil, 
rather than any effect of moisture upon 
the penetration of the plant by the 
parasite. 
These experimental data should be 
compared carefully with observations 
of the development of the disease in 
the field on infested land to determine 
whether the inferences which may be 
drawn from the experiments are sup¬ 
ported by practical experience. Un¬ 
fortunately, there is but a limited 
amount of exact field observation 
which will serve for comparison. The 
development of the disease in experi¬ 
mental plats at Madison, Wis., on 
ground thoroughly infested with the 
fungus has been followed more closely 
than elsewhere. Planting in this plat 
was made in 1922 and 1923 on the same 
date, April 28. In 1923, the roots from 
these plants remained free from any 
trace of disease until May 30, when a 
very few rootlets showing typical decay 
due to Aphanomyces were found. On 
June 5 the entire root systems of these 
plants had begun to decay from a great 
number of infections distributed from 
the surface of the ground to the deepest 
roots. In this brief space of time 
abundant oogonia and a few mature 
oospores were formed. 
This sudden appearance of disease 
followed promptly after the first favor¬ 
able period of temperature and mois¬ 
ture that had occurred since the crop 
was planted. (Fig. 1.) The month 
of May of that year was exceedingly 
dry. The only rains sufficient to wet 
the soil for a brief period had fallen on 
May 15, 19, and June 2. The first 
rain occurred when the plants were 
emerging from the ground, and was 
followed by cold weather, the soil tem¬ 
perature 6 at a depth of 2 inches falling 
at night as low as 1° to 7° C. The 
second rain on the 19th was followed 
by weather hardly warmer, the mean 
soil temperature for the next five days 
ranging from 9° to 13.3°, a temperature 
at which oospores would germinate 
very slowly 
The temperature rose very slowly 
after this date until June 2, when a 
heavy thunder storm gave a precipi¬ 
tation of 0.92 inch, wetting the soil 
in the plats to a depth of about 5 
inches. After this day the minimum 
soil temperature did not fall below 15° 
C., and the mean daily soil tempera¬ 
ture rose in the following five days 
from 17.5° to 20.5°. This coincidence 
of moisture with high soil temperature 
furnished optimum conditions for the 
6 The writer is indebted to J. G. Dickson for the use of soil temperature records quoted here. 
