318 
Journal of Agricultural Research 
Vol. XXX, No. 4 
development of the disease which ap¬ 
peared with such remarkable sudden¬ 
ness. In fact, the period of time 
elapsing between the rain and such 
extensive invasion of the plant was so 
brief that it seems unlikely that the 
rain started the fungus to active vege¬ 
tative growth from oospores, but 
rather that it had achieved some 
development during the period of dry 
weather. A heavy rain on June 6 
with continued high soil temperature 
maintained favorable conditions for 
the development of the disease for a 
brief time until it was checked by 
returning drought, which lasted until 
the end of the growing period of the 
crop. It may be added that the shower 
of June 2 was local, not extending to 
the entire pea-growing region of the 
State, and that the general rain of 
June 6 was in many places the only 
precipitation during the entire season 
which wet the soil for even a brief 
period after temperature permitted 
infection of plants. Thus the fact 
that conditions suitable for infection 
did not occur until comparatively late 
in the season and were maintained for 
so brief a period reduced the amount 
of infection and injury from this 
disease to a very small amount that 
year. 
In 1922 the onset of the disease was 
not as sudden or as severe. Some 
infected plants were found first on 
May 29, and thorough decay of entire 
root systems had occurred on June 2. 
Unfortunately no soil-temperature 
records were kept at Madison during 
the month of May of this year which 
can be compared with the records of 
1923 previously cited. However, the 
Madison Monthly Meteorological Sum¬ 
mary shows that the warmest weather 
of the month occurred May 9 to 12 
during a dry period. Precipitation of 
over 2 inches occurred May 23 to 26, 
followed by six days with an average 
mean daily air temperature of 17.7° 
C., an average which is over 3° higher 
than of the six days after the rain of 
May 19, 1923, when no infection 
occurred. If this increase in air 
temperature was accompanied by a 
corresponding increase in soil tempera¬ 
ture, a safe assumption since the 
weather record showed that they were 
days of almost uninterrupted sunshine, 
it would be just sufficient to provide a 
favorable temperature for infection, 
as determined in controlled experi¬ 
ments. It will, however, be necessary 
to accumulate many exact observa¬ 
tions of the date at which disease 
appears and records of soil tempera¬ 
ture and rainfall preceding the appear¬ 
ance of disease before it will be possible 
to predict accurately from weather 
records alone when the disease may be 
expected to appear; but it seems not 
unlikely that the time will come when 
such prediction can be made with 
precision. 
RELATION OF THE PARASITE TO 
THE HOST 
METHODS AND RESULTS OF EXPERIMEN¬ 
TAL INOCULATION IN THE FIELD 
It will not be a matter of surprise 
that a fungus with a mycelium which 
is converted into perishable zoospores 
very readily and which forms oospores 
apparently under such restricted en¬ 
vironmental conditions is not only 
limited in its ability to persist in soils, 
but, when present, is not always in 
active condition ready to invade plants. 
To secure such early and complete ruin 
of plants from inoculations with pure 
cultures, as often occurs in the fields, 
is not always an easy feat. The first 
difficulty which was encountered, 
though it appears to have been quite 
exceptional, deserves passing note. 
The first culture of this species, which 
was isolated, proved in the preliminary 
inoculation experiments to have so slight 
a degree of pathogenicity that the senior 
writer was misled and was delayed in 
recognizing the true cause of this disease. 
There is no apparent reason for the 
lack of pathogenicity of this isolation, 
and none of the seven later isolations 
which have been used in inoculation 
experiments have been thus deficient, 
though there appears to be a slight 
difference in virulence between some 
of them. 
For greenhouse experiments inocu¬ 
lation of the soil by mixture of frag¬ 
ments of culture on corn-meal agar 
which have many mature oospores is 
the most satisfactory. Although such 
soil does not often give more than 50 
per cent severe infection when first 
used, on a second planting will usually 
give better results, especially if care 
is taken to prevent drying out at any 
time. The use of zoospores in soil, 
either at the time of planting or poured 
on the soil when the plants have 
emerged, is uncertain. When plants 
are grown in sand, however, zoospores 
poured around young plants have 
given very thorough infection, pro¬ 
vided great care is taken to keep the 
sand saturated for a time after inocu¬ 
lation is made. 
Field inoculations have been made 
only at Madison, Wis., in 1923, a year 
so dry that it was everywhere un¬ 
favorable for the development of the 
disease. Inoculum was prepared in 
