May 19. 1923 
575 
Relation of Soil Factors to Infection of Oats 
reduce the amount of infection by loose smut. On the other hand, 
fall sowing, while the soil temperature is high, would tend also to cut 
down infection. Such a curve corresponds more nearly to the actual 
conditions obtaining in nature than a curve derived from series wherein 
the various factors are considered separately. 
The graph in Figure 2 again serves to interpret what is found under 
field conditions. Conditions of high moisture combined with low tem¬ 
peratures in the spring undoubtedly would tend materially to reduce 
infection. This conclusion was borne out by some field experiments. 
Plats containing 28,000 heavily inoculated seeds of Avena nuda were 
prepared in April and May of 1919. The spring was very wet and cold. 
The average soil temperature was ii.i° C. with a range from 2° to 
24° C. during the germination period. The soil moisture averaged 
from 45 to 50 per cent of the soil saturation point, and ranged as high 
as 70 per cent at times. The plants matured normally but contained 
less than one-tenth of i per cent of smutted plants. Seed from the 
same lot treated with the same inoculum, but grown under controlled 
conditions favorable to maximum infection, produced 100 per cent of 
smutted plants. 
SUMMARY 
(1) The temperature range for growth of Ustilago avenae when cultured 
on potato-dextrose agar with a +10 reaction is, minimum 8° C.; 
optimum 20^^ C.; maximum 32° C. 
(2) The temperature range for germination of the spores in beef 
broth with a +10 reaction is, minimum 5° C.; optimum 15° to 28° C.; 
maximum 31° to 34° C. 
(3) The temperature range for sporidial production under the same 
conditions as for spore germination is, minimum 5° C.; optimum 15° to 
28° C.; maximum 30° C. 
(4) The temperature range for germination of the seeds of Avena 
nuda is, minimum 3° C. with no measurable subsequent growth; optimum 
18° to 20*^ C.; maximum 33^^ C. plus, not definitely established. 
(5) The temperature at which the seed is germinated and the seed¬ 
lings maintained for two weeks appears to influence the subsequent 
vigor of the plants. The most vigorous plants were secured from seed¬ 
lings which had been kept at 16® to 24° C. 
(6) High soil temperatures were accompanied by a marked reduction 
in the percentage of infection under the conditions of our experiments. 
(7) Tow soil temperatures also were accompanied by a reduction in 
the percentages of infection, but to a much less marked degree than in 
the case of high temperatures. 
(8) High soil moistures, combined with high soil temperatures, result 
in complete elimination of the fungus. 
(9) Low soil moistures, within a certain temperature range, are 
accompanied by relatively high percentages of smut infection. 
