FEATURE ARTICLE

Biological Control Of Plant Pathogens

A Note From the Project Director: In the May 1995 issue of MBCN we acknowledged that while weed and plant disease biological control are very important, our grant specifies a newsletter devoted to insect biological control. The feature article of that issue was an introduction to the biological control of weeds. In this issue, we are pleased to have an article by Dr. Jennifer Parke of the University of Wisconsin, Department of Plant Pathology, introducing the subject of biological control of plant diseases.

Plant diseases are caused mainly by fungi, bacteria, viruses and nematodes. Biocontrol of plant disease involves the use of an organism or organisms to reduce disease. In this article, I will emphasize biocontrol of diseases caused by fungi and bacteria. Biocontrol includes management of resident populations of organisms (the 'black box' approach) and introductions of specific organisms (the 'silver bullet' approach) to reduce disease.

The 'Black Box' Approach. The phenomenon of disease suppressive soils has fascinated plant pathologists for decades. Observed in many locations around the world, suppressive soils are those in which a specific pathogen does not persist despite favorable environmental conditions, the pathogen establishes but doesn't cause disease, or disease occurs but diminishes with continuous monoculture of the same crop species. The phenomenon is believed to be biological in nature because fumigation or heat-sterilization of the soil eliminates the suppressive effect, and disease is severe if the pathogen is re-introduced.

A classic example is soil suppressive to the take-all disease of wheat. At first, take-all increases in severity with each successive wheat crop, but with continued monoculture, disease stabilizes at a low level. The suppressive effect is lost with crop rotation. Disease suppression has been attributed to an increase in nonpathogenic microorganisms which are well-adapted to growth on wheat roots. These bacteria utilize root exudates, depriving the take-all fungus of a potential food source. Many of these root-colonizing bacteria may also produce antibiotics which further inhibit growth of the pathogen. In fact, plants may select for communities of microorganisms that protect them from pathogens; this may explain why disease suppression increases with continued monoculture. Monoculture of wheat year after year is not a recommended agronomic practice for several reasons, nor would it be economically practical for a grower to endure several years of severe disease losses while waiting for suppression to develop. But suppressive soils are living laboratories where the complex interactions among microorganisms that result in disease suppression might someday be unraveled. Characterization of biological communities in soil has proved to be a formidable challenge, and the nature of disease-suppressive soils remains largely an enigma. Suppressive soils have nevertheless proved to be sources of some important antagonists and they continue to provide clues useful in developing biocontrol strategies.

There are many other examples of biological control involving complex microbial communities where the mechanism of biological control is not understood. This includes the use of green manures to control soilborne pathogens. For example, incorporation of green manures from sudan grass or corn seems to reduce the severity of potato early dying caused by Verticillium. In a few cases, green manures are thought to have a direct, harmful effect on soilborne pathogens. Sulfur-containing compounds released during the breakdown of crucifer tissues may act as soil fumigants, resulting in less disease. Intercropping two or more plant species together can also result in disease suppression. Oats intercropped with peas seems to reduce the severity of Aphanomyces root rot of peas. The suppressive effect may result from oat root exudates which cause the motile spores of this fungus to stop swimming, preventing them from reaching and infecting pea roots. Alternatively, oat root exudates may influence some other component of the soil microflora which in turn affects the fungus. There are also several successful examples of biocontrol by application of composts and compost extracts to suppress a wide range of soilborne and foliar diseases. The assemblages of microorganisms present in composts are complex and dynamic and can be extremely effective in reducing disease, but they constitute a tremendous challenge for the biologist interested in understanding how this occurs.

The 'Silver Bullet' Approach. The difficulties in understanding the complex interactions of the 'black box' approach to biological control have led some researchers to instead introduce individual strains of microorganisms as biocontrol agents. This "silver bullet" approach, while simplistic, has yielded some practical solutions to plant disease problems and resulted in the development of several commercially available biopesticide products. Although the mode of action is understood for relatively few biocontrol systems, research with specific antagonists has led to important discoveries about biocontrol mechanisms.

There are three main mechanisms by which one microorganism may limit the growth of another microorganism: antibiosis, mycoparasitism, and competition for resources. Antibiosis is defined as inhibition of the growth of one microorganism by another as a result of diffusion of an antibiotic. Antibiotic production is very common among soil-dwelling bacteria and fungi, and in fact many of our most widely used medical antibiotics (e.g., streptomycin) are made by soil microorganisms. Antibiotic production appears to be important to the survival of microorganisms through elimination of microbial competition for food sources, which are usually very limited in soil.

To screen for antibiosis, plant pathogenic fungi or bacteria can be grown in a petri dish, and pure cultures of other microorganisms may be placed near them to see if they inhibit the growth of the pathogen. Inhibition in the petri dish may be the result of antibiosis, but it is not easy to show that this antibiosis is actually responsible for disease suppression. First, the antibiotic must be extracted, purified and identified chemically. Then it is necessary to show that the microorganism grows in the microhabitat of the pathogen, and that the antibiotic is produced in the right place, at the right time, and in sufficient amounts to control disease. It is also necessary to demonstrate that the pathogen is sensitive to the antibiotic. Antibiotic production by strains of the bacteria Pseudomonas and Bacillus has been shown to be important to successful biocontrol of several crop diseases. For example, the antibiotic zwittermicin A, produced by the biocontrol agent Bacillus cereus UW85, appears to be important in biocontrol of Phytophthora root rot of alfalfa.

Because there is often little correlation between the ability of a microorganism to inhibit the growth of a pathogen in a petri dish and its effectiveness in disease suppression in the field, many researchers use an alternative approach to find potential biocontrol agents. In this approach, the mechanism of biocontrol is not presumed. Large numbers of test microorganisms are screened in a plant bioassay for their ability to suppress disease, and those which are effective are studied further to figure out the mechanism of biocontrol.

Another mechanism of biocontrol is destructive mycoparasitism. This is parasitism of a pathogenic fungus by another fungus. It involves direct contact between the fungi resulting in death of the plant pathogen, and nutrient absorption by the parasite. The electron microscope has afforded us stunning views of mycoparasites coiled around the hyphal strands of pathogenic fungi. Mycoparasites produce cell wall-degrading enzymes which allow them to bore holes into other fungi and extract nutrients for their own growth. But many so-called mycoparasites also produce antibiotics which may first weaken the fungi they parasitize. The fungus Trichoderma harzianum, available commercially as the seed treatment product Bio-Trek (Wilbur-Ellis, CA) is a mycoparasite of several damping-off pathogens including Pythium, Rhizoctonia, and Fusarium. (See also Fruit Crops News, this issue, for more on T. harzianum.)

Microorganisms compete with each other for carbon, nitrogen, oxygen, iron and other micronutrients. Nutrient competition is likely to be the most common way by which one organism limits the growth of another, but demonstrating that this is actually responsible for biological control is quite challenging. One of the most recent EPA registrations for a microbial pesticide is the product VICTUS (Sylvan Spawn, PA) which contains the bacterium Pseudomonas fluorescens. This organism, when applied to commercially grown mushrooms, helps prevent bacterial blotch caused by a closely related but pathogenic species, Pseudomonas tolaasii. Apparently P. fluorescens is a better competitor for nutrients than P. tolaasii, and when applied to mushroom caps, excludes the pathogen by utilizing all the available food. There is no evidence of antibiosis. The product Kodiak (Gustafson, TX), which contains the bacterium Bacillus subtilis, is believed to control fungal diseases through competition for nutrients, in this case exudates from seeds and roots.

Occasionally, the specific object of competition is known. For example, in most terrestrial habitats, microbial competition for the soluble form of iron, Fe3+, is keen. Some fungi and bacteria produce very large molecules called siderophores which are efficient at chelating Fe3+. Individual strains can have their own particular siderophores and receptors which can bind Fe3+ in such a way that the iron becomes inaccessible to other microorganisms, including pathogens. In some cases, siderophore production and competitive success in acquiring Fe3+ is the mechanism by which biocontrol agents control plant diseases. Siderophores produced by certain strains of Pseudomonas have been implicated in disease suppression of several fungal diseases, but none of these biocontrol organisms have yet been developed commercially.

We are making rapid progress in understanding the mechanisms of biocontrol, and several biocontrol products are now available for practical, widespread use in plant disease control. The regulatory climate for biopesticides has improved, which has streamlined and accelerated the registration process. Of the 40 new pesticides registered by the U.S. Environmental Protection Agency in 1995, fully half are biopesticides, including several microbial biopesticides for plant disease control. In the past few years there has been a proliferation of many small companies interested in bringing new biocontrol products to the marketplace. Many of these companies are working with university researchers and other public sector scientists who have been encouraged to develop their biocontrol research to provide practical alternatives to chemical pesticides. "Traditional" pesticide companies are also taking a strong interest in biocontrol products because the registration costs are so much less than for traditional chemical pesticides. These are exciting times for biocontrol of plant diseases!

- Jennifer L. Parke Department of Plant Pathology, University of Wisconsin-Madison


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