Florida Agricultural Experiment Station


teria attack the cotyledons of the seedlings and later spread to
the new growth as it develops.
 SYMPTOMS
 The disease appears on the cotyledons and leaves as dull
green, water-soaked areas. The spots on the leaves are bounded
by the veins of the leaf and are definitely angular in shape.
The spots later dry out, becoming brown in color, surrounded
by a reddish border. The individual spots rarely become a
quarter of an inch in diameter and they are usually con-
siderably smaller. The lesions may be scattered over the entire
surface of the leaf or may be concentrated in the neighborhood
of the main veins. A number of the spots may coalesce (run to-
gether), causing the rapid death and shedding of the leaves. Un-
der especially favorable conditions the disease may move down
the pedicels of the leaves and attack the young, tender shoots,
causing their death. The leaves and stems killed by this viru-
lent attack become black and shriveled in appearance and remain
on the plant. The name "black-arm" is applied to this type of in-
jury. These lesions on the leaves furnish excellent openings for
the entry of fungi that are unable to attack the uninjured leaf,
and in Florida the greatest defoliation of cotton occurs where a
fungus, Alternaria sp., accompanies angular leaf spot. Com-
plete defoliation has been observed in wet years under such cir-
cumstances, but defoliation from angular leaf-spot alone is rarely
so severe.
 The most serious form of the disease, however, occurs on the
young bolls, where small spots similar to those on the leaves may
occur. These spots enlarge, unlike those on the leaves which are
bounded by the small veins, and a few of them may cover the
entire boll. Such bolls become dry and do not open, or if they do
the lint is usually discolored and rotten. Ordinarily, however,
other fungi get into the lesions and outgrow the angular leaf spot
organism, causing a more rapid rot than would otherwise occur.
This is the indirect injury done by the disease. In fact, the dam-
age from all boll rots would be greatly diminished if angular leaf
spot were controlled, since most of the boll rot fungi are unable
to attack a healthy boll, and even in the case of those that can
penetrate the boll themselves infection is much more abundant
and severe if angular leaf spot lesions are present. In Fig. 5 are
shown two types of injury to cotton leaves caused by the bacter-
ial blight organism.