The Rot That Attacks 2,000 Species

Lester M. Blank.

Cotton root rot, caused by the soil-inhabiting fungus Phymatotrichum omnivorum, flourishes on more than 2,000 species of wild and cultivated plants. Cotton grows in most of the area where the fungus is present, and the disease is usually designated as cotton root rot, although it also attacks the roots of trees, shrubs, fruits, and vegetables. The monocots grasses, corn, sorghum, and such are considered immune to it.

The fungus occurs naturally in the alkaline soils of the southwestern United States and northern Mexico. It has not been found in other parts of the world. The disease is serious in parts of Texas, Oklahoma, New Mexico, Arizona, and northern Mexico. It has been reported in California, Nevada, Utah, Arkansas, and Louisiana. It is particularly damaging in the blackland prairie of central Texas.

It has been alarmingly destructive. In Arizona, the average yearly loss between 1924 and 1929 was 10.3 percent. Losses in Texas were estimated at 130,000 bales in 1918, 314,000 in 1919, 630,000 in 1920, 444,000 in 1928, 300,000 in 1937, and 191,000 in 1939. Those were reductions in yields of 5 to 15 percent.

The losses fluctuate from year to year according to acreage in cotton and the moisture conditions under which the crop is grown. High temperature and high soil moisture favor the development of the disease.

Symptoms of infection are sometimes evident 6 to 8 weeks after planting, but ordinarily are most apparent during the period of squaring and fruiting. At first only an occasional plant z"may display symptoms, but within days or weeks the surrounding plants succumb. By midseason and later, irregular or circular spots or patches of brown, dead plants present a characteristic symptom of the presence of the disease. Such spots shade off abruptly from the dead, browned plants through a narrow zone of wilted plants into normal green plants.

The first above-ground symptom on a plant is a slight yellowing or bronzing of the leaves. There is an increase in leaf temperature, which one can feel. Wilting of the leaves occurs in a day or two and is rapidly followed by drying and browning of the foliage. The browned leaves stay on the plant for some time.

Roots of infected cotton plants show various symptoms, depending on the length of time the root has been subjected to the action of the parasite. Roots of apparently healthy plants that are near plants showing early symptoms sometimes reveal the fungus in the first stages of its action. Whitish or tan threads of the fungus mycelium overrun the roots and break down the outer cells of the root, producing slightly discolored, depressed areas or lesions. Development of the lesions destroys the cortical tissues, and the organism penetrates into the woody central cylinder. The mycelium assumes a tan or buff color. Individual threads join together to form fuzzy strands. In more advanced stages, the cortical tissues completely disintegrate, and the infected part of the central cylinder becomes red or brown, quite different from the white to cream color of the adjacent healthy tissues.

This fungus has several forms, or stages. The vegetative stage consists Of individual threads filaments, or hyphae of the fungus, or the threads may fuse to form mycelial strands. Filaments and strands may push through the soil until they reach another plant whose healthy roots they envelop and penetrate; failing that, they grow until their food supply is exhausted. Young filaments and strands are light-colored. Later they become cream to buff to brown.

A second stage is the spore mat, or fruiting, phase, which arises from the vegetative strands. One sometimes sees spore mats in cotton fields but more often in alfalfa fields. In warm weather when the soil is damp, the irregular- or circular mats appear on the surface of the soil. If conditions are to their liking, they may become a foot or more in diameter. The mats develop rapidly. They appear first as a fluffy white fungal growth on the surface. They become creamy white or tan or buff in a few days. The mature mats are composed almost entirely of minute pores. Attempts to germinate the spores so far have been unsuccessful. The function of the spores is unknown.

A third phase is the sclerotial, or resting, stage. Mature sclerotia are light brown to dark brown and as tiny as mustard seed. In shape they are irregularly round. They are produced singly or in chains or clusters. Sclerotia develop on the mycelium or strands of the fungus, appearing first as slight, spindle-shaped swellings. They attain full size in 4 or 5 days and mature in 10 to 14 days. The fungus produces them apparently in response to an abundant food supply. They have been found in quantities to a depth of 8 feet in the Texas blacklands. The greatest concentration occurs at a depth of 1 to 2 feet. If moisture and temperature are right, the sclerotia germinate readily, and produce the typical Phymatotrichum mycelium and strands. Sclerotia may remain viable in the soil for at least 12 years, and upon germination can reinfect the roots of susceptible plants.

The fungus spreads mainly by the growth of the mycelial strands from root to root of the host plants or by free growth through the soil.. There seems to be little spread by ordinary tillage practices or on farm equipment. A danger exists, however, of spread in the movement of nursery plants from an infested area, either as mycelium on the roots or as sclerotia in the ball of soil. Irrigation water or movement of surface water after moderate rains apparently does not spread the fungus, but heavy rains and gullying of the soil could possibly move sclerotia and set up new areas of infections.

EXPERIMENTS with a number of field crops have established that rotations in which cotton is grown in alternate years are of little value in reducing losses from root rot. Rotations in which nonsusceptible crops occupy the land for 2 of 3 years give moderate protection. Marked and consistent control is obtained in rotations with nonsusceptible crops in which cotton is grown i year in 4. Because many of the tap-rooted weeds are susceptible to the fungus, a rigid weed-control program is necessary during the rotation.

Other practices have value, although in themselves they are not considered to be economical or adequate control measures. Clean fallow for 2 years does not give consistently effective control in the cotton crop following the fallow period. After 8 years of clean fallow, the disease appears on the first crop of cotton and becomes increasingly severe on successive crops. Deep tillage has given some evidence of control the next year, but the effect does not consistently persist beyond 1 year. Early-fall plowing of infested fields reduces somewhat the amount of disease in the following crop, but it is not cumulative in effect.

A slightly better control is obtained by the combination of nitrogen fertilizer and early-fall plowing. Inorganic fertilizers alone, particularly those high in nitrogen and phosphorus, increase the yield of cotton on infested soils, but their economic value has not been proved. Many soil disinfectants or fungicides have been tested against root rot, but the cost of materials or application precludes their use under ordinary field conditions.

The search for varieties of cotton resistant to the disease has yielded negative results. Early-maturing varieties of upland cotton, however, will set a partial crop of bolls before the period of greatest mortality, and their use is recommended on infested soils.

Practical control of root rot in the irrigated Southwest has been obtained by the incorporation of large amounts of corral or other organic material into trenches or furrows. The furrows are then covered over, beds are prepared above them, and the land is irrigated well before planting time. Yearly treatment of experimental areas in that way has reduced the disease greatly, although complete eradication is not attained. In nonirrigated places in the central blacklands of Texas, that principle has been applied successfully through the growing and turning under of legume crops in rotation with cotton. Sweetclovers, such as Hubam and sourclover (Melilotus indica) as winter and winter-summer crops increase the nitrogen and organic content of the soils, reduce the amount of root rot, and increase the yield of cotton.

In both irrigated and nonirrigated areas, control through the application of organic materials to the soil appears to be linked with the increased microbiological activity incidental to the decomposition of the organic matter.

Under laboratory conditions, both sclerotia and mycelium of the fungus are largely eliminated in organic-amended soils. In field experiments that used organic amendments (barnyard manure or sorghum fodder) and October plowing, the incidence of root rot was much less on the following crop of cotton, and no sclerotia were found in the upper 12 inches of soil. In adjacent plots that received only normal bedding operations, root rot was severe, and sclerotia occurred abundantly at the 6-inch to 12-inch level.

The importance of microbial action in the elimination of sclerotia and mycelium from organic-amended soils is strongly suggested by those data, but we cannot overlook the importance of the increased nitrogen content and the improved physical condition of the amended soils.

In summary: The greatest promise of practical control of root rot appears to lie in the use of organic manure, particularly in the form of legumes. Legumes may be used as winter green-manure crops and followed immediately by cotton, or they may be grown to maturity and the residues turned under during the summer. The use of organic manures, plus rotations, early-fall plowing, and early-maturing varieties, should provide satisfactory control of root rot.

LESTER M. BLANK is in charge of the cotton disease investigations for the Bureau of Plant Industry, Soils, and Agricultural Engineering at the New Mexico Agricultural Experiment Station, located at State College, N. Mex. He received degrees from Washburn University and the University of Wisconsin. He joined the Department of Agriculture in 1929. His investigations have-been concerned primarily with phymatotrichum root rot, bacterial blight, and verticillium wilt.