Mar. 19, 1917 
Dissemination of Angular Leafspot of Cotton 
461 
Definite experimental data are wanting on the agencies by which Citrus canker is 
spread * * *. It is evident that rain and dew are important factors in carrying 
the disease to unaffected leaves, twigs, and fruits of trees in which the disease is already 
present. 
Even though Stevens (1914) thought the disease to be due to a fungus, 
he writes (p. 41): 
The disease seems to develop and spread rapidly during rainy weather, but it is 
more or less retarded during periods of drought or in a dry season. 
Brown and Jamieson (1913) present their work on a disease of nas¬ 
turtium and sugar-beet leaves caused by the same organism, which they 
named “Bacterium aptatum” While the authors state they had no 
opportunity to study this disease under field conditions, their experiments 
would suggest that infection takes place only in bruised or wounded 
tissue caused by insects or mechanical injury. 
A number of preliminary reports of work on bacterial diseases have 
been made in recent years; and, since in most of these the subject of dis¬ 
semination is not mentioned, the present author takes it that this phase 
will be discussed later and, therefore, he will not review this literature. 
The facts in the dissemination of certain types of fungus diseases have 
a close bearing on the subject; yet this literature is so voluminous as to 
be impossible of review here. However, because of its relation to one of 
the most serious cotton diseases, anthracnose, Whetzel’s (1906) paper on 
bean diseases is mentioned. Of anthracnose, the author writes (p. 205): 
The spores may be scattered by the cultivator, the pickers, by animals, or by the 
wind in damp or rainy weather. 
EXPERIMENTAL INVESTIGATIONS 
Infection of the leaves of cotton by Bacterium malvacearum can easily 
be brought about by superficial inoculation in the presence of sufficient 
moisture. This has been done by applying small amounts of agar-slant 
cultures of the organism with glass rods and spreading with rubber- 
gloved fingers when the dew was on the leaves, by applying water sus¬ 
pensions of the bacteria with cotton swabs at all times of the day or 
evening, spraying such suspensions with an atomizer, or by painting them 
upon the leaves with camel’s-hair brushes. The first signs of infection 
are minute, dark-green, angular (triangular or quadrilateral) spots on the 
underside, whether the inoculation was made upon the upper or lower 
surface of the leaf, usually in 7 to 10 days, though often not earlier than 
15 days. In a day or two following the first appearance on the underside 
of the leaf the same dark-green water-soaked spot will appear on the 
upper side, though less conspicuously angular. When held up to the light, 
such spots show a translucency as contrasted with the light, impervious 
normal leaf and the irregular transparency of some insect injuries. The 
spot usually increases in size simultaneously on both sides of the leaf, but 
never crosses the veins. Single infections seldom increase to a size larger 
than 3 to 4 mm. in the longest dimension. The larger spots, so con- 
