574 
Journal of A gricultural Research voi. xxiv, No. 7 
Table V. Effect of soil temperature and soil moisture on infection of Avena nuda by 
Ustilago avenae 
Average temperature. 
Soil 
moisture. 
Average 
smutted 
plants. 
Soil 
moisture. 
Average 
smutt^ 
plants. 
Soil 
moisture. 
Average 
smutted 
plants. 
C. 
Per cent. 
Per cent. 
Per cent. 
Per cent. 
Per cent. 
Per cent. 
io°-i4° . 
30-35 
96.5 
60 
74.0 
80 
56.0 
I4°-i6°. 
i 7 °- 2 i°. 
30-35 
30-35 
30-35 
96- 5 
98. 0 
100. 0 
60 
58. 5 
80 
I. 0 
i8°-22°.. 
60 
93-0 
(Series 
failed) 
31-32°. 
30-35 
24. 0 
60 
0 
80 1 
IS 
When Table V is compared with Tables III and IV, it will be noted 
that a less pronounced temperature relationship is evidenced, especially 
at the lower moisture contents. At a temperature of 31° to 32° C. there 
is a decided falling off in infec¬ 
tion at all soil moistures. These 
results indicate that one would be 
safe in predicting that high soil 
temperatures tend to limit infec¬ 
tion regardless of soil moistures. 
When we consider that 32° C. 
is the maximum temperature 
for germination and growth of 
the fungus, we would expect to 
find this decrease in infection, 
especially when we consider the 
rapidity of germination and elongation of the young seedlings at this 
temperature. 
It will be noted that a temperature of 18® to 22° C. furnishes the 
optimum for infection in all soil moistures. This coincides directly with 
the optimum for germination and 
growth of both host and fungus. 
The high percentage of irfection 
at 10° to 14° C. might correlate mth 
the longer time the seed remains in 
the ground, especially in the case of 
the lower soil moistures. It may 
Fig. I. —^Influence of soil temperature upon the infec- 
^ tion of A'oena nuda by Ustilago avenae when soil 
^ moisture is disr^[arded. 
these conditions for the seedling to 
appear above ground. In this con¬ 
nection, experiments on the relation 
of soil moisture to the germination 
of the spores show that the spores 
germinate as readily in a soil moisture 
35 cent as in a soil moisture 
of 65 per cent. If this is correlated 
with the temperature data already 
cited, it will be seen that there is no 
reason why the fungus might not become fairly well established in the 
soil while the seed remains dormant, or very slowly germinating. 
Such a curve as is shown in figure i may help to interpret ^e com¬ 
mon observation that the early sowing of oats in the spring tends to 
