Anthracnose and Some Blights

Albert L. Smith.

Cotton anthracnose, caused by Glomerella gossypii, is the primary cause of seedling blight, boll rot, and fiber deterioration in the more humid cotton-growing States of the South and Southeast.

The boll rot phase was first discovered in Louisiana in 1890. The seedling blight and other phases of the disease were described in 1892 by George F. Atkinson at the Alabama Agricultural Experiment Station.

From that time until about 1920 the pink boll rot phase of the disease was extremely destructive. Anthracnose boll rot losses diminished with the coming of the boll weevil and the change to earlier, smaller, less vegetative varietal types.

Upper left: Cotton anthracnose, here shown on bolls, reduces yields and discolors lint. Upper right: Symptoms of bacterial blight of cotton on stems, leaves, and bolls. Lower left: Enlarged lesions of the frogeye fungus on soybean. Lower right: Lower and upper surfaces of soybean leaves affected with bacterial blight.

The well-known sore shin of cotton seedlings is caused mostly by anthracnose. The fungus is mainly a secondary invader, entering through wounds, and is adapted to a semisaprophytic existence on injured or dead parts of the cotton plant. It is apparently universally present in all fields throughout the area of its distribution. Anthracnose and ascochyta blight have many features in common although caused by different pathogens.

Anthracnose exists in all humid cotton-growing areas. It has been distributed on seeds to all cotton-producing countries. In the United States it is widespread from Virginia to Texas and Oklahoma. It is delimited by the 40-inch rainfall line. The line extends north and south through eastern Texas and Oklahoma. West of that line, low rainfall and low humidity are unfavorable for the fungus. The disease reaches its maximum intensity along the Atlantic and Gulf coasts and tapers off toward the inland areas in the Southeast, where somewhat lower humidity prevails. The fungus may be recovered in the laboratory from a high percentage of healthy-appearing leaves, stems, and bracts of most plants selected at random in fields from April to October particularly during periods of continued wet weather. Consequently the fungus is apparently present most of the time even on healthy-looking plant parts. No other disease of cotton is more widely prevalent.

Anthracnose seedling blight losses were serious for many years before 1935-1945, when seed-treatment disinfectants came into general use. A 4-year (1938-1941) survey of seedling diseases indicated that anthracnose was the predominant cause of seedling blights. The causal organism was recovered from 81.2 percent of the diseased seedling samples. In many fields the characteristic blight lesions could be seen on the below-ground stems of all seedlings. Stand reductions are brought about by preemergence and postemergence blighting-off.

Skippy stands reduce yields. Where planting-over becomes necessary, there are additional costs for seed, labor, and reduced yields caused by lateness and greater weevil damage. Cotton planted over may yield only 60 to 80 percent of the first planting.

Losses from anthracnose boll rot were serious for many years after discovery of the disease in 1890. Commonly, damage was estimated from 10 to 70 percent of the crop. Losses were much less after coming of the boll weevil and are now estimated at from 0.5 to 3 percent, depending on weather conditions.

The anthracnose fungus is often associated with Alternaria, Fusarium,other fungi, and bacteria in boll rot lesions. A 4-year survey made by Paul R. Miller and Richard Weindling of the Department of Agriculture showed that the anthracnose fungus was present in 67.8 percent of diseased boll samples. They considered it to be the major boll rot disease. Losses are caused by a direct invasion of the unopened bolls with destruction of the seed and lint, by invasion of the partially open bolls causing hard locks, and by weakening and staining of the fiber.

Symptoms on cotyledons are usually diseased areas on margins or small reddish or light-colored spots. Diseased seedlings show reddish-brown lesions below ground. The lesions may be on one side of the stem, or they may surround it and extend down on the root.

Many seedlings are killed before or after they emerge. Others survive with the change to more favorable growing conditions. With the falling of diseased cotyledons, anthracnose symptoms disappear over summer until lesions appear on bolls.

ON THE BOLLS the disease appears as small, round, water-soaked spots,which enlarge and become sunken and brownish in color. A sticky mass of spores comes over the surface of lesions on bolls. The lesions may occur near the tip or at any other point. Often they are associated with wounds made by boll weevils. The bacterial blight pathogen often may be the primary invader, with anthracnose following in the same wound and often associated with other fungi. The lint and seed are rapidly invaded once the disease gets through the husk of the boll. P. B. Marsh and his associates, in studies in 1950 at the Pee Dee Agricultural Experiment Station, described "tight lock," caused by anthracnose and other fungi. Invasion of the lint and seed after cracking of the bur in wet weather produced a hardened, discolored lock. Long periods of rainy weather retard drying and fluffing of the lint and provide favorable conditions for development of fungi on the seed and lint.

Planting seed of cotton are almost always infested with the anthracnose fungus. H. W. Barre in studies at the South Carolina Agricultural Experiment Station found the fungus growing through the seed coat into the embryo. Seed fully invaded failed to germinate. Those that were partly invaded produced diseased seedlings.

C. W. Edgerton, in studies at the Louisiana Agricultural Experiment Station, found by washing the seeds and counting the spores that as many as 8,000 spores were carried by each seed.

Paul R. Miller, in studies at the Clemson Agricultural Experiment Station, made other studies on spore load. Samples of cottonseed collected at random from gins in South Carolina in 1941 showed as many as 80,000 spores to a seed, although the average was much lower. Germination tests from the same lots of seed indicated that most of the seedlings had anthracnose infections even though few surface-borne spores were present. The small amount of boll infections in many fields was not enough to account for the heavy spore loads carried by seed. Mr. Miller found that some of the spores came from the trash brought to the gin with the seed cotton. He also determined that clean cotton lots became contaminated with anthracnose conidia when ginned after infested lots. Thus the gin caused thorough contamination of all seed in an individual lot and served to contaminate several lots of clean seed ginned immediately following the ginning of infested cotton.