436 
Journal o f Agricultural Research 
Vol. XXIX, No. 9 
It has been shown by Weimer and 
Harter {19) that acids were formed in 
the solutions on which species of 
Rhizopus grew in sufficient quantities to 
cause maceration of the sweet-potato 
disks. In order to be sure that the 
acidity or alkalinity of the media on 
which A. brassicae had grown was not 
such as to interfere with the enzym 
studies, the hydrogen-ion concentra¬ 
tions of the solutions were determined 
(electrometrically) and are given in 
Table IV. The figures in this table 
show that the cabbage decoction 
underwent slightly greater change in 
hydrogen-ion concentration than did 
the sweet-potato decoction. The 
change seemed to be slightly greater in 
the solutions on which the fungus had 
grown in the light. The data show that 
the hydrogen-ion concentration of none 
of the solutions was such as seriously to 
interfere with the pectinase tests made. 
LIFE HISTORY 
There seems to be little doubt that 
the causal organism (A. brassicae) 
lives over winter in the soil and in dis¬ 
eased parts of the host left on the 
ground. The results of experiments 
have shown that the spores will not 
only live but will germinate and grow 
at 2°. Although no experiments have 
been conducted to determine the mini¬ 
mum temperature which the spores 
will survive, there is every reason to 
believe that they will stand quite low 
temperatures. This is supported by 
the fact that this disease is prevalent 
in sections of the country where very 
low temperatures are experienced each 
winter. There is also the possibility 
that the fungus lives over winter on the 
seed as is shown by the following 
experiment: 
Cauliflower seeds were dipped into a 
suspension of spores from a virulent 
culture of A. brassicae , then dried and 
stored in a drawer of a laboratory desk 
for eight months. Tests showed that 
some of the spores were then still 
capable of germinating. Some of the 
seeds dipped into the spore suspension 
were planted in a pan of sterilized soil 
in the greenhouse. A pan filled with 
some of the same soil was sown with 
seed from the same lot which had 
not been inoculated, as a control. A 
damping-off of the seedlings occurred 
in the pan sown with inoculated seed 
and A . brassicae was recovered from the 
diseased tissue a couple of centimeters 
above the surface of the ground. No 
damping-off occurred in the control 
pan. This experiment indicates that 
the disease may develop in the seed bed 
from the spores on the seed. In addi¬ 
tion to attacking leaves, stems, and 
heads of cauliflower, the fungus also 
causes a spotting of the seed pods. It 
has been isolated from seeds taken from 
beneath the lesions on partly mature 
pods. Spores of the fungus have been 
seen on seeds taken from pods which 
had not yet begun to dehisce. This 
shows that the fungus can grow through 
the pods and infect the seeds. The 
fungus may be carried to the seed beds 
with such seeds and a damping-off of 
the seedlings produced. Spores are 
formed abundantly on the dead seed¬ 
lings and serve as a source of inoculum 
for other plants. A few plants having 
small lesions on the lower leaves have 
been seen at transplanting time, show¬ 
ing that the disease may be carried 
from the seed bed to the field on the 
seedlings. This has not been seen very 
frequently and probably occurs only 
under conditions extremely favorable 
for the development of the fungus. 
Inoculation experiments conducted 
in the greenhouse showed that the 
incubation period in the leaves was 
from 24 to 48 hours. The time neces¬ 
sary for infection to become evident 
varies with the temperature and hu¬ 
midity. Cauliflower plants with heads 
from 5 to 10 cm. in diameter growing 
in the field were inoculated on June 
18, 1921. Infection took place and 
well developed lesions were present 
both on the leaves and curds in six 
days. At this time typical Alternaria 
spores were present in abundance both 
on the leafspots and the brownrot 
lesions. These spores served as a 
source of inoculum so that by the end 
of the season the disease was present on 
many of the neighboring uninoculated 
plants. The spores are easily separated 
from their sporophores, so that ample 
opportunity is thus afforded for the 
disease to become widespread in a com¬ 
paratively short time, provided con¬ 
ditions are favorable for dissemination 
and infection. 
MORPHOLOGY 
The mycelium permeates the tissues 
of the head and leaf in all directions 
from the point of infection passing be¬ 
tween and through the cells. The 
hyphae are hyaline at first, later becom¬ 
ing brownish to olivaceous. In cul¬ 
ture mycelial development is sup¬ 
pressed, but on or in the curd of the 
cauliflower long profusely branched 
threads with numerous septations are 
developed. The hyphae vary in di¬ 
ameter on different substrates, rang¬ 
ing from 1.5 to 7.5 tx. They soon come 
to the surface of the host, finding an 
exit through storhata or wounds. 
