Some farmers believe that growing an annual crop for a number of years on the same land hurts the soil. That is not always true, because certain annual crops, such as onions or tobacco, produce as well or better in continuous culture in some localities. Nor is it invariably true that crop rotation will control all soilborne diseases.

Pathogenic organisms that can grow in the soil even in the absence of susceptible plants are little affected by crop rotation. The fungi that cause damping-off, vascular wilts, and potato scab exemplify this group. Other diseases not amenable to control by crop rotation are caused by parasitic fungi, which cannot grow in the soil in the absence of their host roots but can persist almost indefinitely in the soil because of their ability to form dormant or resting bodies like spores or sclerotic. Examples are the Sclerotinia that cause rots of vegetables and the clubroot parasite of crucifers.

Crop rotations are especially useful in controlling the fungi that cannot long survive in the soil in the absence of their host plants. Such organisms as those causing bacterial bean blight, anthracnose of beans, and blackleg of cabbage are controlled by 3- or 4-year rotations with other crops that these organisms do not attack.

Rotations often have been widely used to combat root diseases of tobacco rootknot, or nematode root rot, black shank, Granville wilt, and Fusarium wilt the nematode root diseases of many plants, and a miscellany of other soilborne diseases on vegetables, including black rot of carrot, Phoma rot of celery, black leg of crucifers, Fusarium wilt of watermelons, Fusarium basal rot of onions, Fusarium root rot of peas and beans, and Verticillium wilt of strawberries.

ANTIBIOSIS the inhibition or destruction of one micro-organism by another is important in the control of several root diseases. The soil teems with microscopic organisms, some of which are beneficial to plants because they make nutrients more readily available or favorably affect soil tilth. Others have no such direct effects on plants.

As these nonparasitic organisms multiply in the soil, they exert a natural biological control on many of the parasitic organisms responsible for soil-borne diseases of plants. Most root parasites survive longer in soil that has few nonparasitic micro-organisms than they do in soil containing an abundant microflora. When a parasite and its host plant are introduced experimentally into a sterilized soil, the severity of disease is much greater than when ordinary members of the soil micro-flora are added with the parasite.

Recognition of the competitive and associative effects that micro-organisms exert on one another has led to the discovery and widespread use of antibiotics and to the development of management practices based on the intensification of microbiological interactions in the soil.

This approach is also one of parasite destruction by natural forces, the distinction being that instead of physical factors (such as heat, light, flooding, or drying), less tangible biological factors are involved.

Soil organisms check the activity of plant parasites in several ways. Sometimes they attack and consume the plant parasites. Sometimes they produce destructive toxic materials or antibiotics. In still other instances they compete more successfully for oxygen and nutrients and thus cause suffocation or starvation of the parasite. When fresh organic material, such as green manure, is added to the soil, the nonparasitic micro-organisms multiply rapidly, and whatever ill effects they exert on parasites are intensified.

The significance of antibiosis in soil came to be recognized generally only a generation ago. L. H. Pammel, of the Texas Agricultural Experiment Station, who investigated Texas root rot of cotton, discovered the value of stable manure or other organic materials in controlling the disease.

Biological control has become the method of choice in dealing with the cotton root rot. Practical control of the disease is achieved in irrigated sections of the Southwest by incorporating large amounts of organic matter into the soil. The land is irrigated well before planting time. Control is achieved in nonirrigated areas by growing (and turning under) legume crops in rotation with cotton. The legumes may be grown as winter green manure crops and followed immediately by cotton, or they may be grown to maturity and the residues turned under in summer. The use of early maturing varieties of cotton and early fall plowing are recommended. The latter destroys the remaining live cotton root and so deprives the parasite of its favorite environment.

The take-all disease of wheat is susceptible to biological control if the antibiosis is accomplished during the time the soil is free of wheat. Because it is not feasible to grow green manures in many wheat areas, the application of biological control has to be limited. The take-all disease, however, can be checked in most soils by providing a balanced fertility.

The potato scab organism in heavily infested soil has been reduced by plowing under soybeans, but turning under rye, clover, peas, or barley has not been effective.

Strawberry root rot has been eliminated by incorporating several crops of soybeans, but red clover used in a similar manner has had little effect on the severity of the disease.

In the Columbia River Basin of eastern Washington, the Fusarium root rot disease of beans often is serious in virgin soils where beans are one of the first crops planted and are grown in consecutive years. Other fields in the same area that have been under cultivation for a number of years and on which hay or other crops have been grown for several years before beans are planted produce bean crops practically free of root rot. Such soils presumably develop microbiological properties unfavorable to the organism that causes root rot.