Bulletin 214, Cotton Diseases in Florida


 BACTERIAL BLIGHT OR ANGULAR LEAF SPOT
 Bacterial blight, caused by Pseudomonas malvacearum EFS.,
is a very common disease of cotton in Florida, and injures cotton
in all stages of its development. It is present in practically every
cotton field in Florida, and its direct and indirect injuries are
sufficiently severe to class it as one of the most serious of Florida
cotton diseases. It is practically impossible to properly evaluate
the losses caused by this disease, since its worst damage results
from its association with other parasites; in fact, much of the
damage attributed to other diseases, particularly the boll rots,
might properly be referred to bacterial blight. The United States
Department of Agriculture, however, estimates the losses due to
this disease for 1927 as 2.6 percent of the crop of the entire cotton
belt, or approximately 500,000 bales. During wet seasons the dis-
ease is much worse than it is during dry seasons. This is ex-
plained by the fact that wind-blown rain is one of the chief factors
in spreading this disease in the field, though insects and other
factors also play some part in its dissemination.
 The most frequent and serious attacks of bacterial blight are
on the leaves and bolls, though the seedlings and the stems and
branches of older plants are also injured. The names "black-arm"
and "angular leaf spot" are often used to designate the disease
on the stems and leaves, respectively. The spots on the leaves
are particularly characteristic, and are the most commonly ob-
served symptoms of bacterial blight. Since the symptoms of the
disease on the seedlings and on the bolls are less distinctive, they
have not been designated with special names. The attacks on the
bolls constitute the most serious phase of the disease, because,
in addition to the direct injuries to the bolls, bacterial blight opens
the way for a number of secondary parasites that are themselves
unable to penetrate the uninjured tissues of cotton bolls. Conse-
quently, it is usually very difficult to distinguish blight lesions on
bolls a few days after infection because secondary parasites enter
them shortly after they are formed. Some of the secondary par-
asites after entering outgrow the blight organism and bring about
an even more rapid and complete rot of the bolls than P. malvac-
earunm.
 The disease is carried over winter on the seed and on diseased
parts of the cotton plant. From these sources of infection the bac-