Nov. 1, 1924 
Alternaria Leaf spot of Cauliflower 
433 
At 28.5° lesions were abundant in 24 
hours. These were 1 mm. in diameter 
in 3 days, at which time bacterial rot 
was also present. The curd was nearly 
completely decayed by bacteria in 7 
days. 
At 30.7° lesions were abundant in 
24 hours. These were about 1 mm. 
in diameter but were being obscured 
by bacteria in 2 days. 
At 32° a few brownrot lesions, poorly 
developed, were evident in 3 days. 
Bacterial rot was present. 
At 34.7° no brownrot developed in 3 
days. The curd was decayed by bac¬ 
teria. 
At 37.2° no brownrot developed in 3 
days. The curd was decayed by bac¬ 
teria and Rhizopus. 
Infection took place at all of the 
temperatures tried except 34.7° and 
37.2° C. The disease developed more 
slowly at the lower than at the higher 
temperatures. The optimum tem¬ 
perature for infection lies somewhere 
between 25° and 30°. However, the 
optimum development of the disease 
occurred at about 25° since at the 
higher temperatures the heads were 
soon infected and decayed by bacteria. 
The number of lesions formed at any 
of the temperatures varied little except 
perhaps at 32°, where they were fewer. 
The infected areas in all cases were 
light brown in the early stages, grad¬ 
ually becoming darker, until they were 
olivaceous to nearly chestnut brown 
in color. The time necessary for the 
development of the aerial mycelium 
and spores which gave the olivaceous 
to brown color to the lesions varied 
with the temperature from about two 
days at the optimum to about 30 days 
at 1.8°. At 1.8° infection did not 
become apparent for from 5 to 7 days, 
at which time the lesions appeared as 
very minute lightbrown discolorations 
just visible to the unaided eye. Little 
progress had been made by the disease 
at the end of 19 days, as was indicated 
by a slight darkening in the color of 
the infected tissue. After 35 days the 
lesions had begun to take on the 
characteristic olivaceous color due to 
the presence of dark-colored aerial 
hyphae and spores. Nevertheless, at 
this time the decay had not penetrated 
over 1 to 2 mm. deep. 
Infection was apparent in from 4 to 
5 days at 7° and in 19 days the decay 
had extended into the tissue 1 to 2 
mm. However, in 35 days the lesions 
were about 1 cm. in diameter and 1 to 
1.5 cm. deep. At 9.5° to 15.5°, 18°, 
and 19.5° the maximum number of 
infections became evident in 3, 2, and 
1 days, respectively. Although little 
actual decay was caused by this disease 
in two weeks below 10°, no doubt some 
decrease in the commercial value of the 
infected cauliflowers resulted from the 
browning of the curd. 
The results of the experiments dis¬ 
cussed above show quite definitely to 
what extent temperature may affect 
the development of brownrot on 
cauliflower under conditions of high 
humidity. An experiment was con¬ 
ducted in which the cauliflowers were 
sprayed with a heavy suspension of 
spores in water with an atomizer and 
then placed in wooden crates and held 
in large refrigerator rooms in a cold- 
storage house. The curds were fairly 
well enveloped by leaves. The tem¬ 
peratures of the rooms were 1°, 2.5°, 
2.5°, and 4.5°, with a variation of 
perhaps 1° C. and a relative humidity 
of 81, 74, 91, and 87 per cent, respec¬ 
tively. Infection was visible in 10 
days at all of the temperatures. But 
only a few lesions developed and these 
were still small at the end of a month. 
Under these conditions comparatively 
little damage resulted from this disease. 
This experiment was repeated later, the 
method being varied only in that the 
cauliflowers were placed in moist 
chambers with the expectation that the 
humidity of the air about the heads 
would thereby be raised. In this case 
the number of infections was greatly 
increased and the damage was pro¬ 
portionally greater. However, the fun¬ 
gus had not penetrated the tissues more 
than 0.5 cm. at any of the temperatures, 
there being little apparent difference in 
the stage of decay on the heads held at 
the different temperatures after five 
weeks. 
EFFECT OF THE FUNGUS ON THE HOST 
The blossoms of the cauliflower 
attacked by A. brassicae at first turn 
brown and take on a water-soaked 
appearance. This latter condition is 
especially noticeable in the larger 
branches of the curd. Very soon the 
lesions become olivaceous in color 
because of the presence of dark-colored 
aerial mycelium and spores. The 
mycelium is at first intercellular but 
later penetrates the cell walls and the 
diseased tissue is permeated with 
hyphae. The affected tissue is some¬ 
what softened, although not so much 
so as in the bacterial rot. The decayed 
tissues lose water rather rapidly, so 
that the older lesions often have a dry 
and shriveled appearance. The fungus 
enters the leaves through the stomata 
and kills the tissues. This was demon¬ 
strated by placing drops of a suspen¬ 
sion of spores upon a leaf in a moist 
