Crop rotation and summer fallow often reduce damages from root rots, because many of the pathogens survive in the soil, on refuse of crops, and on wild and cultivated grasses. They also develop on plant remains in and on the surface of the soil. Some, such as species of Fusarium, attack crops other than cereals. Proper crop sequence is important therefore in fighting root rot.

Root rot is usually more prevalent in soils that have been cropped continuously to a given crop than those in which rotation has been practiced. A rotation that includes immune or highly resistant crops suppresses the pathogens and consequently reduces root rot. Take-all, for example, can be reduced materially if the crop sequence includes such nonsusceptible crops as alfalfa, sweetclover, flax, and corn. In regions where the race of the take-all fungus that attacks oats is not prevalent wheat can follow oats.

Summer fallow is also an effective control measure against take-all and to some extent against other types of root rots, with the exception of browning. In fact, summer fallow tends to increase the severity of this disease in wheat.

ANTIBIOSIS is the association of two kinds of organisms in which one inhibits the growth of the other. Antibiosis is important in the control of root rots. Agricultural soil is teeming with microscopic organisms algae, fungi, bacteria, protozoa, and nematodes. Some have no apparent effect on plant growth. Some are beneficial. Some, especially the root-rotting fungi, are destructive. As organisms grow and multiply rapidly in the soil and on plant refuse, they struggle constantly for survival. If it were not for this constant "biological warfare" the soil-borne organisms that cause root rots would multiply so fast that the arable land would become "sick" and then be unsuitable for grain crops.

Many factors other than the prevalence of micro-organisms in the soil determine how long a soil-borne pathogen can persist in the soil temperature, moisture, acidity, type of soil. Nevertheless, soil-borne pathogens cannot compete in the soil with their enemies, the saprophytic organisms. Indeed, many pathogens that cannot survive in normal soil actually grow well in sterilized soil. Some of them can survive in the soil for a short time. Others may live in the soil for years. Those that persist are more tolerant to antibiotic substances and therefore are more difficult to control by cultural methods.

The biological control of soil-borne disease is promising, but we still do not know the extent to which the method can be applied. Adding large amounts of specific antibiotic organisms or their extracts to the soil will control or greatly reduce some soil-borne diseases. Seedling blight of wheat, barley, and rye caused by Helminthosporium species can be controlled by applying cultures of antibiotic organisms to the soil at the time of planting. That practice may not be feasible on a large scale; cultural practices must then be resorted to. We have some evidence that antibiotic substances may be used as seed disinfectants for controlling soil-borne diseases. The application of manure and plant refuse to the soil and cropping sequences of cultivated crops have a pronounced influence on the number of micro-organisms in the soil. Take-all has been controlled by applying manure to the soil. More studies are needed on control of soil-borne diseases by means of antibiosis, especially in relation to crop sequence.

VARIETAL RESISTANCE is perhaps the most promising key to the control of root rot diseases, even though resistance is only relative. The degree of susceptibility of a given variety is influenced tremendously by ecological conditions and cultural practices. Nevertheless, varieties differ greatly in susceptibility to specific organisms when grown under uniform tests. Most varieties of wheat are susceptible to root rots. Apex, Thatcher, and Marquis are among the more resistant spring wheats to the common root rots. Kota, Kubanka, and McMurachy Selection are moderately resistant to take-all.

All varieties of oats that have Victoria as one parent, such as Vicland, Tama, and Boone, are very susceptible to helminthosporium root rot. Many commercial varieties, such as Clinton, Benton, and Bonda, are highly resistant.

A variety may be resistant to one type of rot but susceptible to another. Kubanka wheat is moderately resistant to take-all, but is susceptible to the common root rots. Victoria oats is fairly resistant to root rot caused by Fusarium culmorum but extremely susceptible to H. victoriae. Peatland barley is moderately resistant to helminthosporium root rot but very susceptible to pythium root rot.

Resistance must be bred into the adapted commercial varieties that do not have it. That was done by scientists in Minnesota, who developed varieties of wheat, barley, and oats that are resistant to helminthosporium root rot.

The disease gardens have been of practical help in eliminating inferior varieties and hybrid lines. The idea could well be expanded: Growing a world collection of wheat, oats, and barley in nurseries in different regions of the United States and maybe in other countries would give us a wonderful opportunity for selecting disease-resistant varieties and desirable parental materials.

IN CONCLUSION: Root rots are among the most destructive diseases of cereals in the United States. They are debilitating, insidious, and usually inconspicuous, but sometimes they become epidemic. Their importance varies greatly with season, locality, and the crop. Host plants are subject to attack from the time the seeds are sown until the crops are mature. The pathogens are of diverse types and attack all underground parts of the plants and induce root decay, seedling blight, and premature death of older plants.

Root-rotting pathogens live in the soil and tend to increase in the soils when susceptible crops are grown year after year. Soil is not just soil; it is actually teeming with microscopic life protozoa, bacteria, and fungi. Those organisms are carrying on a biological war, struggling for the survival of the fittest. They destroy many of the root-rotting organisms and hence help to prevent the development of root rot. Their relative prevalence in the soil can be greatly modified by cropping sequences and cultural methods.

Although root rots cannot be completely prevented where cereals are grown intensely, they can be greatly reduced by good agricultural practices. Sound seed of recommended varieties should be treated with a fungicide to eliminate the pathogens from the seed and to give the young seedlings protection against soil-borne organisms. Also, the seed should be sown only deep enough to provide adequate moisture for germination.,

A good cropping sequence must be followed. The seed beds should be well prepared, and appropriate fertilizer applied if there is a deficiency of essential elements. Varieties differ in susceptibility and only the recommended varieties should be grown. In general, good farming is the most feasible method of controlling root rots of cereals.

J. J. CHRISTENSEN is professor of plant pathology in the University of Minnesota, where he obtained his doctor's degree. He has made extensive studies of cereal diseases and the genetics of plant pathogens in relation to breeding for resistance and has participated in cereal breeding programs. In addition to work at Minnesota, Dr. Christensen has studied genetics of plant pathogens in Europe, has been adviser to SCAP on plant diseases in Japan, and has traveled extensively in South America on a study of cereal diseases.