MORPHOLOGICAL CHARACTERS OF ALTERNARIA MALI 
ROBERTS 1 
By John ’W. Roberts 
Pathologist, Fruit-Disease Investigations , Bureau of Plant Industry, United States 
Department of Agriculture 
Some years ago Alternaria mali was described by the writer (16) 2 and 
shown to be involved in a peculiar spotting of apple leaves. Alternaria 
mali does not originate these spots, but, gaining entrance through areas 
killed by Physalospora cydoniae (,Sphaeropsis malorum), by chemicals 
such as sprays, or by other means, enlarges the dead spots, giving them 
a peculiar and characteristic appearance. A spot thus formed consists 
of the original dead area with semicircular or crescent-shaped enlarge¬ 
ments about it (PI. i, B). One might conceive of such a spot as being 
formed by infections taking place indiscriminately within the tissues of 
the original dead area. The fungus growing at equal pace in all direc¬ 
tions from each of these infection centers would occupy circular areas, 
only part of which would project beyond the original spot. Where 
there is a single infection, in the center of a circular spot, the enlarge¬ 
ment appears as a differently colored zone surrounding the original spot 
and concentric with it. By means of new infections the increase in size 
may be such that one spot may involve almost an entire leaf, appearing 
as a more or less, circular (original) spot surrounded by consecutive 
crescent-shaped enlargements. The striations or zones within the 
spots greatly resemble those produced by species of Alternaria on the 
leaves of other hosts. 
Crabill (7) does not agree that Alternaria mali is the cause of the 
enlargement of spots initiated by Physalospora cydoniae . His inocula¬ 
tion experiments were apparently very few in number and his results 
indecisive. Of the writer’s work Crabill states: “His photographs, 
however, show that the enlargements produced by artificial inoculation 
with A. mali are not at all typical. They are too uniformly spreading 
to resemble the clear-cut crescents of the typical frog-eye spot.” Yet 
these photographs show a remarkable resemblance to figure 4 of his 
bulletin, labeled “Frog-eye leaf spots as they appear in midsummer.” 
As a matter of fact, both Crabill’s photographs and those of the writer 
would have shown the crescents more clearly cut had the leaves been 
allowed to become older and dry before photographing. The writer’s 
experiments involved more than 3,000 spots, and the results in all experi¬ 
ments involving Alternaria mali were quite decisive. The fungus was 
found constantly associated with characteristic spots on apple leaves 
from Virginia, Maryland, Tennessee, Arkansas, and Missouri. 
THE GENUS ALTERNARIA 
The difficulties underlying attempts to identify species of Alternaria 
are appreciated by all those who have ever undertaken the task. Many 
1 Received for publication February 20, 1924. 
* Reference is made by numbei (italic) to ‘ ‘ Literature cited, ” p. 707-708. 
Journal of Agricultural Research, 
Washington, D. C. 
(699) 
Vol. XXVII, No 9 
Mar. 1, 1924 
Key No. G-376 
