432 
Vol XXIX, No. 9 
Journal of Agricultural Research 
THE EFFECT OF TEMPERATURE AND 
HUMIDITY ON INFECTION 
Young detached cauliflower leaves 
were employed to determine the in¬ 
fluence of temperature upon infection. 
Because of continued catabolic ac¬ 
tivities and the retardation of ana¬ 
bolism, the leaves, after being removed 
from the plant, no doubt changed 
physiologically rather rapidly, espe¬ 
cially at the higher temperatures. But, 
uninoculated control leaves at all but 
the highest temperatures remained 
turgid and retained their normal green 
color for several days longer than was 
necessary to obtain the desired results. 
Regardless of the change in the de¬ 
tached leaves, they more nearly ap¬ 
proximated the normal plant than 
would any artificial culture medium 
which might have been used. The 
leaves were placed on moist cotton in 
small moist chambers in incubators at 
temperatures of 7°, 10°, 14°, 17°, 20°, 
22°, 23.5°, 25.6°, 30°, 31°, 32.5°, 
34°, 35.5°, 36.8°, and 38.2° C. The 
experiment was repeated with similar 
results. Abundant infection was evi¬ 
dent after two days at all temperatures 
from 10° to 35.5° inclusive. Distinct 
lesions were present at 7° three days 
later, but no infection took place at 
36.8° or above. The maximum for 
infection was about 36° since infection 
was obtained at 35.5° but none at 
36.8°. The optimum for infection lay 
between 28? and 31°. Although there 
was not m&ch difference in the number 
of lesions, they differed somewhat in 
size, being largest at the optimum and 
gradually becoming smaller above and 
below this temperature. At the opti¬ 
mum temperature aerial mycelium 
grew over the surface and the leaves 
gradually became yellow, while at the 
other temperatures the formation of 
aerial hyphae and the yellowing of the 
leaves was considerably slower. The 
optimum for infection of detached 
leaves under the conditions of these 
experiments was slightly higher than 
that for the growth of the mycelium 
on Irish potato agar and somewhat 
lower than that obtained for the germi¬ 
nation of the spores. 
Several experiments were conducted 
to determine the effect of temperature 
on infection and on the development of 
brownrot on the curd of cauliflower. 
The cauliflowers used were obtained 
from the city market at Washington, 
D. C., or were grown .at the Arling¬ 
ton Experiment Farm at Rosslyn, Va. 
The curds were rinsed in tap water, 
placed in moist chambers on moist filter 
paper, sprayed with a heavy suspension 
of spores of Alternaria brassicae in dis¬ 
tilled water, and then placed in a series of 
incubators whose temperatures ranged 
from 1.8° to 37.2° C. The incubators 
used were the same ones used in the 
spore germination and mycelial growth 
studies. Following are details of the 
effect of temperature on the infection of 
cauliflower curds by Alternaria bras¬ 
sicae and the development of brownrot: 
At 1.8° infection became apparent in 
5 to 7 days. After 14 days the lesions 
were chestnut brown, about 1 mm. in 
diameter and 0.5 mm. deep. At the 
end of 35 days the lesions had changed 
to an olivaceous color, due to the 
formation of spores. They were about 
1 mm. in diameter by 1 to 2 mm. in 
depth. 
At 7° numerous minute light brown- 
lesions were apparent in 4 to 5 days. 
These were about 1 mm. in diameter by 
1 to 2 mm. in depth in 19 days. In 35 
days the lesions were 1 cm. in diameter 
by 1 to 1.5 cm. in depth. 
At 9.5° abundant infection became 
evident in 3 days; these lesions were 
about 0.5 mm. in diameter and slightly 
more developed in 10 days. In 35 days 
the lesions were 2 to 3 cm. in depth 
and about half of the curd was in¬ 
volved. 
At 10.5° infection became evident in 
3 days and typical brownrot lesions 
0.5 to 1.5 cm. in diameter developed 
in 20 days. 
At 11° abundant but very small and 
superficial lesions became evident in 3 
days. Tissues decayed to a depth of 
0.5 to 1 mm. in 10 days. Practically 
the entire curd was involved in 35 days, 
the tissues being affected to a depth of 
2 to 3 cm. 
At 15.5° infection became evident in 
3 days; the lesions were still quite small 
and superficial in 7 days, three-fourths 
of the curd being affected in 14 days. 
At 18° lesions 0.5 mm. in diameter 
were evident in 48 hours, lesions 1 to 
1.5 cm. in diameter were evident in 7 
days, and the curd three-fourths in¬ 
volved in 14 days, the tissues being 
decayed to a depth of 1 cm. at the 
end of this time. 
At 19.5° infection was just apparent 
in 24 hours; the lesions reaching 1 to 3 
mm. in diameter in 3 days. 
At 22.5° lesions were abundant but 
very small in 24 hours, about one-half 
of the curd being involved in 4 days. 
Nearly the entire curd was affected, the 
decay reaching a depth of 5 to 8 mm. 
in 7 days. 
At 26.8° lesions were abundant in 
24 hours and curd two-thirds involved 
in 4 days. Nearly the entire curd was 
affected, decay being 1 cm. deep in 7 
days. 
