Branches may be cankered or girdled and turn black. This symptom is known as black arm. It occurs commonly in the more susceptible American-Egyptian varieties. Occasionally the Acala upland varieties also show black arm symptoms. Spread of the organism along the midrib and major veins produces the commonly known vein blight. Younger leaves are more susceptible to vein blighting than are fully mature and older leaves.

Infections originating in the corolla may spread to the young boll and cause extensive shedding. On older bolls the lesions are circular and shiny and later become sunken and browned and blackened. Penetration of the blight organism through the husk wall permits staining of the fiber and invasion of the seed. Infected bolls become deformed and open prematurely.

Most of the infections of seedlings in spring are caused by bacteria carried over the winter on seed. The bacteria may be attached to the seed coat or seed fuzz or inside the seed coat. Seed harvested from heavily infested fields may produce seedlings with 25 percent infection.

Such early and heavy infestations often produce blight epidemics by the end of the growing season. The blight organism may also overwinter on plant refuse left in the field in arid areas. Volunteer seedlings from diseased bolls or old plants which survive the winter may carry the disease over and infect the new crop. Bacteria on infested seed are washed to the germinating embryo and initiate the new infections. The primary lesions produce inoculum, which is splashed to the lower leaves to initiate secondary infections. The disease is carried through several generations on the leaves until the bolls are formed. Greatest damage by the disease is caused by boll infections. High temperatures and high humidity are favorable for maximum infection. The infrequent rains accompanied by strong winds of the High Plains area of the Southwest are most favorable for epidemic spread. Relative scarcity of the disease in California is attributed to the higher elevation and average lower temperatures and scarcity of rainfall.

Blight bacteria gain entrance to the host tissue through stomata and wounds. Entrance to leaf tissue is generally on the lower surface, where the stomatal openings are more abundant. Bacteria are transported by water to the stomatal cavity. The organism characteristically invades the parenchymatous tissue, made up of the thin-walled cells. Less frequently they enter the xylem, or water-conducting vessels, of the leaf petiole, stem, or leaf vein. Once inside the stomata, the bacteria multiply and crush adjacent cells and continue to utilize and multiply on the destroyed cell contents. A bacterial exudate is produced and accumulates on the surface of the wound or drops off to other leaves or to the soil. The exudate dries as a thin, yellowish, translucent film. This material is readily dispersed in rain water and serves as inoculum to further spread the disease. Water congestion of the leaf tissue caused by hard, blowing rains facilitates infection. Wounds made by insects also facilitate infections. Hailstorms which injure all parts of the plant may be followed by blight epiphytotics. The incubation period on bolls and leaves is 8 to 10 days, from time of inoculation to earliest appearance of symptoms.

The world-wide distribution of bacterial blight is attributed to the seed-borne bacteria. Spread in the field is largely attributed to wind-blown rain water and splashing from plant to plant and leaf to leaf by falling water. Bacteria are also carried by flowing of surface water. In irrigated fields the spread of the disease may be traced in the direction of the water flow. Experimentally, dissemination of the disease in fields can be correlated with the direction and velocity of the wind during periods of rainfall. The spread of bacteria by dust storms has been observed in Arizona. The wind-blown rains characteristic of the High Plains of Texas and the irrigated valleys of New Mexico and western Texas are more effective in spreading the disease than the rainfalls of the mid-South and Southeast. The movement of fallen diseased leaves by strong winds also spreads the disease.

CONTROL MEASURES for the bacterial blight disease have been directed toward eliminating the overwintering sources of inoculum on the seed and in the field. When F. M. Rolls found blight bacteria adhering to the seed fuzz and seed coat he demonstrated the possibility of removing them with sulfuric acid in experiments at the South Carolina Agricultural Experiment Station in 1915. With concentrated sulfuric acid all fibers are removed from the seed. Any adhering bacteria on the seed coat are also removed. Washing the acid off of the seed may remove any lightweight seed or floaters and give a high-grade planting seed.

Beginning about 1930, organic mercurial dust disinfectants became available for seed treatment, which also largely controlled seed-borne bacteria. A combination of delinting and dust treatment became widely used in areas where serious bacterial blight losses occurred. As an outgrowth of delinting efforts to control blight, commercial processing plants are now operating across the Cotton Belt and delinted seed of all major varieties is available. Many growers prefer the delinted seed for more precise planting with tractors.

Other practices for blight control are the selection of seed grown in disease-free fields and the destruction in early fall of the previous crop refuse by deep, clean plowing. Any volunteer seedlings are also destroyed in spring before planting.

Although a fair degree of success in controlling blight by sanitation, delinting, and seed treatment is obtained, the results are not always satisfactory. Internally borne infections often survive the treatments and initiate disease centers. Because all growers sometimes do not cooperate in the blight-control program, the spread of infection from an adjoining farm may cause losses. Pathologists believe therefore that the only fully satisfactory possibility for control is the development of blight-resistant varieties.

THE SEVERE LOSSES from bacterial blight in the Gezira of the Sudan caused British workers to make an intensive search for resistance to bacterial blight. The work was done by R. L. Knight and associates in studies conducted at the Agricultural Research Institute, Khartoum, Sudan. An upland-type cotton, Ugandi B31, was found to have a high degree of resistance. It has been used in breeding programs.

A blight-resistant upland, Stoneville 20, was discovered in 1939 by D. M. Simpson and Richard Weindling in breeding plots at the Tennessee Agricultural Experiment Station. Stoneville 20 is not a desirable commercial type, but it has no bad characteristics and is valuable as a source of resistance. In 1953 it was being used as a source of resistance in several breeding programs in the Cotton Belt.

A satisfactory technique for artificial inoculation had to be developed before progress could be made in selecting resistant plants in the breeding program. Mr. Knight at Khartoum learned that he could get uniform infections by soaking 10 pounds of diseased leaves in 40 gallons of water and using the liquid as a spray. The technique was improved by Richard Weindling in research work he did at the South Carolina Agricultural Experiment Station. He found that a single potato-dextrose agar culture grown in a petri plate and diluted with 2 1/2 gallons of water provided a more satisfactory source of inoculum. He also discovered the best time to spray was from mid-morning until noon on sunny days when the stomata were wider open. The lower leaf surface due to the larger number of stomata also gave more infections than the upper surface. Additional improvements in the inoculation technique now indicate that by using an orchard sprayer, with 400 pounds pressure to the square inch, and forcing the bacterial suspension into the stomata very satisfactory infections are obtained on a field scale.

A technique for infecting greenhouse seedlings has also proved useful in the search for blight resistance within established varieties. The seed are first soaked in a bacterial suspension, planted in a greenhouse bench, and examined on emergence for size of bacterial lesions. The lesions are nonexistent, or very small, on resistant plants. Resistance to leaf blight, black arm, and boll blight were all found to be positively correlated, and therefore the leaf symptoms alone could be used as an index to resistance within the individual plant in breeding work.

The inheritance of resistance to blight has been studied by Mr. Knight. In a survey of all cotton species and a large number of species types and varieties, he found five factors for resistance, B₁ to B₅. The factors varied in the amount of resistance they contributed and were generally additive or accumulative in contributing to the resistance of the plant. In segregating populations the resistance factors segregated as dominants or partial dominants.

The resistance in Stoneville 20 is largely determined by a single gene. Resistance is recessive and susceptibility is dominant. In addition to the major gene in Stoneville 20, several minor genes contribute to the resistance with additive effects. The minor genes account for varying amounts of blight tolerance in commercial upland varieties. Deltapine has a high level of minor genes, Stoneville 2B somewhat less, and Acala a very low level. Resistance can be established in American uplands by the accumulation of several minor genes centered about a major gene, as found in Stoneville 20.

The breeding of blight-resistant varieties adapted to all sections of the Cotton Belt is an extensive project and was well under way in 1953. L. S. Bird and L. M. Blank demonstrated the possibilities in breeding for resistance and outlined a backcross method in studies conducted at the Texas Agricultural Experiment Station. Four varieties Stoneville 2B, Deltapine, Empire, and Coker 100 wilt were each crossed with Stoneville 20. Using the backcross method and the inoculation technique developed by Weindling, they were able to obtain satisfactory blight-resistant commercial types within 5 years. A search was started for naturally occurring resistant individual plants within a number of additional varieties. The work is done by growing several thousand plants in a field planting and inoculating them with a power sprayer. Promising plants have been found in several varieties. Pathologists are confident that all commercial varieties will sometime be bred resistant to bacterial blight.

ALBERT L. SMITH is a graduate of Oklahoma Agricultural and Mechanical College and the University of Wisconsin. He joined the Department of Agriculture as a pathologist in 1936 to work on cotton diseases and breeding- for disease resistance. He is currently conducting studies on the use of fungicides for the control of ascochyta blight. Most of his time has been devoted to studies on fusarium wilt and nematode disease of cotton. He is stationed at the Alabama Polytechnic Institute in Auburn.