A factor in plant disease control

The amount and type of organic matter in soils plays a role in the extent to which a crop is affected by certain plant diseases. In some cases, organic matter may increase the incidence and severity of a disease but under proper management beneficial effects can usually be realized. This applies particularly to diseases produced by fungi but also to those caused by bacteria, actinomycetes and possibly viruses and nematodes. 

A brief discussion of research trends in the field of plant disease control will be helpful in the consideration of recent developments in nonchemical methods of disease control. 

Since these demonstrations of the importance of antagonism in plant disease control much work has been done with some success but with more failures (Wood and Tveit, 1955). Perhaps the main fact that has been driven home is that the soil is a very complicated environment where so many biological, physical, and chemical factors are involved that even exact experimentation is difficult. If one attempts to isolate the factor 

The control of bacterial diseases of plants has been less satisfactory than that of fungous diseases. Spray applications are seldom effective since many of these diseases are systemic. The use of disease-free seed, seed stock, or disease-resistant varieties has proven most effective. 

This has been and still is an active field of research. 

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The addition of green manures to soils has been shown to effect control or partial control of a number of plant diseases. These include potato scab, take-all disease of wheat, and root rot of cotton and of snap beans. This method is not always successful and many factors, which are not well understood, affect the results. This subject is discussed in Chapter 19. 

While insect control is more often a limiting factor in tropical bean production, there are several diseases that are serious in certain seasons and locations. Bean rust is one of the most widespread diseases, and it can be controlled effectively with protective fungicides, such as elemental sulfur and certain of the carbamic acid derivatives. The economics of bean production usually preclude any costly applications, however, and the problem has generally been turned over to the plant breeder to solve with resistant varieties. At present, the most practical control of bean anthracnose and the bean blights is through the use of clean seed and resistant varieties. Control with fungicides has always proved difficult and of doubtful value. 

An important factor in the redistribution of fungicides throughout a target crop is their level of vapour phase activity. Compounds that are relatively immobile in the plant may be extensively redistributed through the vapour phase and effect commercially acceptable disease control. A good example is fenpropimorph. 

Diet is a strong factor in the control of atherosclerosis relating to general vascular disease, coronary heart disease, and stroke. The interrelated disorders in atherosclerosis of hyperinsulinemia, hyperlipidemia, and hypertension are strongly subject to dietary influence. The type of dietary protein, animal vs. plant, appears to be as important as the type of lipid, animal vs. plant, in atherosclerosis. Dietary protein type, with its differing amino acid ratios, appears to be a major secretagogue of insulin. 

Fungal diseases of weeds are common, but until recently they have not received much research attention plant pathologists have emphasized research to control diseases of economic crops. Weed diseases have often been noted by weed scientists as confounding factors in weed research plots and by growers faced with severe weed infestations in crops for which chemical herbicides are either unavailable or inadequate to control a particular weed. 

The ability to produce antibiotics is common among the actinomycetes and much of the interest in these filamentous bacteria in recent years may be attributed to this fact. This interest has been manifest chiefly in research connected directly or indirectly with medicine. Soil samples from all over the world have been examined in the search for new species and strains that may produce a new antibiotic suitable for medical use. Hundreds of these antibiotics have been isolated but only a few have been found to be sufficiently non-toxic and specific for use in disease control. Among the best known of these that are in use are streptomycin, terramycin, aureomycin, cycloheximide, chlortetracycline, oxytetracycline, chloramphenicol and neomycin. Some of these have been used to a limited extent in disease control in plants. Their use for this purpose is limited by cost, by difficulty in getting the antibiotic into the plant, and by toxicity when used at concentrations adequate to be effective. The limited time that they are effective is also an important factor (Brian, 1957 Zaumeyer, 1958 Pramer, 1959). The role of antibiotics in soils and their effect on microbial ecology is considered in Chapter 18. 

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