314 
Journal of Agricultural Research 
Vol. XXX, No. 4 
rather than from direct entry. When 
peas are sterilized superficially and 
grown under sterile conditions in agar 
upon which the fungus is also growing, 
penetration of roots can then be found 
readily in thin razor sections of the 
root surface. The mycelium passing 
along the root surface turns inward at 
the junction of two epidermal cells and 
passes readily into one of these cells, 
whence it advances into an adjoining 
cell or into underlying tissue. (Pl. 3, 
B.) The point of entry and passage 
from one cell to another is not marked 
by any conspicuous constriction of the 
fungus strand. Invaded tissue softens 
immediately even in the absence of 
bacteria, indicating perhaps the pro¬ 
duction of an enzyme which softens cell 
walls and aids in fungal penetration. 
RELATION OF SOIL TEMPERATURE 
AND MOISTURE TO THE DEVELOP¬ 
MENT OF THE DISEASE 
EXPERIMENTAL STUDIES 
Among the many diseases of roots 
of plants few show in the field a greater 
degree of apparently erratic irregularity 
than the one under consideration. 
Even in fields which are very severely 
infested the disease is usually much 
worse in spots, and at its first appear¬ 
ance it occurs in irregular areas which 
are frequently coincident with the 
more moist soil, but at other times are 
not clearly related to any obvious soil 
differences. It appears that the occur¬ 
rence of this disease is greatly influ¬ 
enced by environmental conditions. 
In its seasonal development tempera¬ 
ture is undoubtedly a limiting factor, 
and in its local occurrence the field 
evidence suggests soil moisture as the 
more important limiting factor. In 
order to gain exact experimental evi¬ 
dence of the relations of these factors 
to the development of the disease, 
several series of plantings were made 
in the Wisconsin soil temperature tanks 
where both temperature and moisture 
were controlled within narrow limits. 
Two of these series will be reported in 
detail. 
The soil chosen was a sandy loam 
from a field in which peas had never 
been grown, and as a further precaution 
against the presence of root-destroying 
fungi it was treated with formaldehyde 
a month before use and subsequently 
thoroughly dried until no trace of this 
sterilizing agent was detected. The 
soil was inoculated one week before 
planting by mixing thoroughly with 
small fragments of roll cultures of an 
isolation of this fungus which had 
shown very active parasitism in pre¬ 
vious tests. The viability of the 
oospores in these cultures had also 
been previously demonstrated. The 
inoculated soil was packed in 6-inch 
cans, near the bottom of which an 
irrigation apparatus was arranged to 
permit the addition of water without 
flooding the surface of the soil. On No¬ 
vember 18, 15 Alaska peas were planted 
at the depth of about inches in 
each can. Six cans were placed in 
each of a series of tanks, which were 
maintained at 12°, 18°, 24°, and 30° C., 
respectively. The moisture present in 
the soil at planting was 15 per cent of 
its dry weight, or about 35 per cent of 
the moisture-holding capacity of the 
soil, previously determined to be 42 
per cent of its dry weight. Two of the 
cans in each tank were maintained at 
the original soil moisture, while in two 
others the water was increased to 60 
per cent of the moisture-holding ca¬ 
pacity of the soil, and in the remaining 
two cans it was raised to 80 per cent of 
the moisture-holding capacity. 
Inoculation in this instance was so 
effective that soon after they had 
emerged from the ground a few of the 
plants decayed and wilted down, 
whereupon they were removed. At 
the conclusion of the series, on Decem¬ 
ber 18, the remaining plants were re¬ 
moved from the soil, and record taken 
of those showing decay of the base of 
the stem and of the roots. (See Table 
II.) 
From these data it appears that the 
fungus was restrained little if any in its 
parasitic activity toward peas at 30° C., 
which is within 3° of the upper limit of 
soil temperature at which peas can be 
induced to make an approximately nor¬ 
mal growth. In soil with the highest 
moisture content the progress of the 
disease does not appear to be favored 
as much as might be expected. Since 
some degree of infection took place even 
at the lowest soil temperature main¬ 
tained here, and the activity of the 
parasite at the lower range of tempera¬ 
ture is of greater interest, a new series 
was started December 19, with the pur¬ 
pose of exploring more thoroughly the 
range of temperature which occurs in 
the field at the time when the disease 
normally appears. 
In the second series the soil used be¬ 
fore was mixed, dried somewhat, and 
placed in cans. The moisture in this 
soil was found to be 36 per cent of its 
moisture-holding capacity, and adjust¬ 
ments were made to 60 and 80 per cent 
to correspond with the earlier series. 
(See Table III.) 
