Cotton

Fusarium and Nematodes on Cotton

Albert L. Smith.

Wilt of cotton resembles the wilt disease of tomato, cowpeas, watermelons, cabbage, and several other crops. It is primarily a disease of the water-conducting vessels of the woody or stem part of the plant and is caused by a fungus that inhabits the soil.

The disease is complicated by nematodes, the eellike microscopic worms that also inhabit the soil and provide the openings through which the wilt fungus enters cotton roots. Nematodes reduce the root growth and increase the susceptibility of cotton plants to fusarium wilt. Thus the disease is considered a wilt-nematode complex.

Some nematodes enter the tip end of young roots. Others enter and feed on the root tissue some distance back from the tip. The root knot nematode makes galls, or knots, on roots, which later decay and leave the ends of the water-conducting vessels open and exposed to soil-borne organisms. The meadow nematode, feeding some distance from the root tips, may cause a pruning off of the small rootlets.

THE WOUNDS made by those and other species of nematode provide numerous openings for the wilt fungus, Fusarium oxysporum f. vasinfectum. Once the wilt pathogen gets into the vascular system, which conducts water to all parts of the plant, it can grow and spread throughout the woody portion. In the vascular ducts it may be found in pure culture, and appears especially adapted to grow in this tissue while most other organisms from the soil are excluded.

The wilt disease of cotton is distributed throughout the world wherever American cottons are grown in acid alluvial sandy soils. Pathologists believe the disease originated in Mexico or Central America in the same locality as upland cotton. The wilt fungus is carried inside cotton seeds and has been transported by the seed to new cotton-growing areas. In the United States the disease occurs in all States from Virginia to eastern Oklahoma and Texas. It is limited in those States by low rainfall and alkaline soils. Greatest losses occur in Coastal Plain soils of the Carolinas, Georgia, and Alabama. Wilt is also an important disease in Arkansas, Louisiana, Mississippi, and southeastern Texas. Although the disease is more severe in the sandy soils, it occurs in the lighter soils scattered throughout some of the heavier soils series of the Piedmont and Mississippi Delta regions.

Losses to the cotton crop from the wilt-nematode disease complex have been greater than those caused by any other disease except possibly Texas root rot. Losses in yields result from reduced stands, stunted plants, small bolls, and poor-quality lint. Before the development, distribution, and general use of wilt-resistant varieties, losses to individual growers often amounted to 75 to90 percent of the crop. The growing of sea-island cotton had ceased on many acres before 1902, when Rivers, a resistant variety, became available. The first wilt-resistant upland variety, Dillon, was released in 1905. Estimates of crop losses beginning in 1920 indicate wilt losses from 1 to 5 percent for the different States. Additional losses from the root knot nematode ranged from a trace to 3 percent. Between 1940 and 1950 the release of improved wilt-resistant varieties brought further reductions in Wilt losses. Now the losses from nematodes and wilt together probably do not exceed 3 or 4 percent in any State.

SYMPTOMS OF WILT may appear on cotton plants at any stage of development. The earliest symptoms to be seen on seedlings and small plants are the yellowing and browning of cotyledons and leaves. The affected parts ultimately die and fall off. The bare stem soon blackens and dies. The first symptom in older plants may be stunting, followed by yellowing, wilting, and dropping of most of the leaves. Leaf discoloration first appears near the margin of the blade near a vein. The affected areas enlarge, and an abscission layer may form at the base of the petiole, causing the leaf to drop.

An outstanding symptom is the browning and blackening of the woody tissue. When a stem or branch is cut crosswise, the discoloration is usually found in a ring just beneath the bark. Sometimes the discoloration is dispersed through the woody cylinder. In advanced cases, discoloration may extend throughout the plant from the roots through the stem, branches, leaf petioles, and peduncles and into the bolls.

Wilting mostly occurs gradually, but after a rain, following a dry period, plants may wilt suddenly and in large numbers. Wilted plants may produce some bolls, which usually are smaller and open prematurely. Plants may die one at a time until the stand is reduced or largely eliminated, depending on the susceptibility of the variety and the amount of infestation.

The wilt fungus survives in the soil on organic matter. It grows as a threadlike mycelium and produces two kinds of spores. One type, known as conidia, is relatively short-lived. The second type, chlamydospores, is a resting stage, which may live longer. Both types give rise to a mycelium that infects the roots of the host plants.

Dissemination by spores probably accounts for the rapid spread of the disease to all parts of a field once the disease is introduced by cotton seed. Spores may be washed about in the field or blown about by the wind or transported by many other means. The wilt fungus lives in the soil indefinitely once it is introduced even though cotton and other susceptible plants are not grown. Fields not planted to cotton for as long as 25 years have shown severe wilting the first year after cotton planting was resumed. No method of eradicating the organism economically from fields is known.

Many laboratory experiments indicate that with large amounts of inoculum the fungus enters healthy cotton roots in the absence of openings made by nematodes. Field experiments show that the openings caused by nematodes largely account for the infections occurring naturally. In experiments at the Alabama Agricultural Experiment Station, I found that wilt is readily controlled with soil fumigants, which reduce or eliminate nematodes before planting.

After entering the small roots, the fungus inhabits the water-conducting vessels and spreads by growth of the mycelium and by movement of spores upward in the water stream. The vascular tubes become browned and later blackened by the formation of gumlike substances and by growth of tyloses. Plugging or partial plugging of the vessels lowers the flow of water and uptake of salts from the soil and stunts the plant or causes wilting. Toxic products, which injure the host cells, are also produced by the fungus. Browning, drying, and killing follow; the ultimate falling of most leaves is a symptom largely produced by toxic materials. When bolls are present, the mycelium may grow through the peduncle into the seed. After the plant dies, the organism invades all its tissues; if enough moisture is present, spores are produced, which may be spread to all parts of fields.

THE ROOT KNOT NEMATODE is the most common of the nematodes that affect cotton roots. It occurs on cotton roots in all lighter soils of the Cotton Belt. The immature larvae infest cotton plants by invasion through the soft root tips. After entering the tip, the larvae push their way between the cells. Then they become stationary and feed by puncturing all the cell walls within reach with a spearlike stylet and sucking out the juices from inside the cell. The affected cells grow much larger and proliferate to form knots, or galls. With susceptible varieties and the feeding of large numbers of nematodes, the knotlike enlargements may become a half-inch in diameter. Tissues in the galls are quite soft and are likely to decay and leave the ends of vascular bundles exposed to the wilt fungus. The enlarged worms are filled with eggs, which hatch and release numerous young nematodes. They, in turn, feed on any new cotton rootlets in the vicinity. Some cotton plants become infected by wilt when quite young; perhaps the mycelium enters the root tissue with the young larvae as well as later when the galls decompose.

If nematodes have made many points of entry for the fungus, multiple infections gradually envelop the root system, even in fusarium-resistant plants of upland cotton.

The meadow or root rot nematode (Pratylenchus pratensis) multiplies rapidly on corn, crabgrass, and other fibrous rooted crops, following which it might become the predominant species on cotton. The meadow nematode is also found abundantly in some soils too heavy in texture to support the root knot nematode. It may enter the soft cortical root tissue at any point near the growing zone. In feeding, the female moves about, destroying cells and depositing eggs throughout a short segment of the root. The young larvae intensify the destruction of the cortical tissues so that usually the rootlet is cut off. Many openings are thus left for the wilt organism to enter directly into the vascular tubes.