Nov. 1, 1924 
425 
Alternaria Leafspot of Cauliflower 
tissue. Lesions of the third type 
resembled those of the first,- except 
that they appeared on the larger veins, 
petioles, and stalks, and were linear in 
shape, having the longer diameter 
parallel to the veins, petioles, or stems. 
Under conditions of high humidity 
these lesions sometimes developed 
rapidly, so that the parts attacked were 
soon decayed through. Badly affected 
leaves finally turned yellow and 
dropped off. The area of leaf tissue 
which might have functioned in photo¬ 
synthesis was greatly reduced and the 
plants failed to develop properly. 
Stems of seedlings were frequently 
weakened by the cankers produced by 
this disease, so that they broke over 
and died. Under conditions of high 
humidity the dead areas of the leaves 
were covered with hyphae and spores 
which gave them an olivaceous color. 
In cauliflower leaves inoculated in the 
field (PI. 2, C, D), large areas of leaf 
tissues were killed. This dead tissue 
was papery in texture and bore black 
spore masses. The high humidity and 
temperature at the time of inoculation 
and for several days thereafter no doubt 
account for the severity of the infection. 
The symptoms of the disease on cab¬ 
bage leaves are for the most part the 
same as those on cauliflower. Plate 1, 
B and D, shows the different types of 
lesions on young cabbage leaves inocu¬ 
lated in the greenhouse. 
This disease causes a browning of the 
individual buds or groups of buds of the 
curd. Plate 3, B, shows a very early 
stage of the disease on the curds, result¬ 
ing from 5-day-old artificial inoculation. 
The color of the affected tissues varies 
with age as well as with the conditions 
of temperature and humidity. The 
infected blossoms are at first light 
brown, darkening with age to nearly 
chestnut brown, which is usually fol¬ 
lowed by an olivaceous color, due to 
the development of dark-colored aerial 
hyphae and spores. Plate 3, C, shows 
the same head illustrated in Plate 3, B, 
photographed 13 days after inoculation. 
The diseased spots had enlarged con¬ 
siderably and were of the typical 
olivaceous color. A similar condition 
is shown in Plate 3, D, which is a pho¬ 
tograph of a naturally infected cauli¬ 
flower head obtained from the market. 
Plate 4 shows a head which was inoc¬ 
ulated in the field. Its flower stalks 
are elongated preparatory to the for¬ 
mation of seed. The blackened tissue 
was covered with spores and hyphae. 
Lesions caused by this disease may 
be easily confused, especially in their 
early stages, with bruises and with soft- 
rot produced by bacteria. Micro¬ 
scopical examination is often necessary 
to determine the true nature of some of 
the small lesions. As the decay in¬ 
duced by bacteria develops, the tissue 
becomes soft, wet, and usually slimy. 
The tissue rotted by bacteria is light 
brown and has a water-soaked appear¬ 
ance. 
ETIOLOGY OF THE DISEASE 
In 1918, when this work was begun, 
the etiology of the disease on the curd 
of cauliflower was unknown, at least as 
far as could be ascertained from the 
literature. Isolations were made from 
a large number of affected curds of 
cauliflower from as many sources as 
possible and a pure culture of a species 
of Alternaria was almost always ob¬ 
tained. Careful study of this fungus 
has shown it to be the same as the one 
that causes the leafspot of cabbage 
and related plants, namely, Alternaria 
brassicae (Berk.) Sacc. This fungus 
was first described by Berkeley as 
Macrosporium brassicae ( 1 , p. 339), but 
was later placed in the genus Alternaria 
by Saccardo (14, P • 546). 
PATHOGENICITY 
Although from brownrot lesions 
a single species of Alternaria was 
almost invariably obtained, sometimes 
a species of Fusarium, bacteria, or 
other species of Alternaria appeared in 
the plates. Inoculation experiments 
showed, however, that the species of 
Alternaria so commonly present in the 
lesions was the causal organism. 
The method used in all of the experi¬ 
ments conducted to prove the patho¬ 
genicity of the strain of Alternaria 
isolated from the typical brownrot 
lesions was as follows: The preliminary 
tests were made in the greenhouse. 
Plants of different ages were sprayed 
EXPLANATORY LEGEND FOR PLATE 2 
A, B, and E.—Cauliflower leaves and petioles from greenhouse-grown plants, inoculated with Alternaria 
brassicae showing the circular, light-colored lesions which often coalesce, killing large areas of the leaf and 
also the small, dark spots (leaf B), which may spread but little. These leaves were sprayed with the same 
spore suspension at the same time and were held under identical conditions after being inoculated. 
Photographed 3 days after inoculation. Alternaria brassicae was recovered from both types of lesions. 
About three-fourths natural size 
D.—Leaves of a mature field-grown cauliflower plant sprayed with a suspension of spores of A. brassicae 
during a hot, rainy period. The large, light-colored areas of the leaves were killed by the fungus. About 
one-half natural size. Photographed 6 days after inoculation 
