Plant Microbes: Beneficial and Detrimental

William E. Fry, professor of plant pathology, Department of Plant Pathology, Cornell University, Ithaca, NY.

Plant microbes affect agriculture both detrimentally and beneficially. Economic losses and human suffering result from plant diseases caused by microbes, so much ecological research deals with plant pathogens (disease-producing agents). Unfortunately, there are so many important crops, diseases, and agroecosystems that only a small proportion can be investigated intensively.

Recently, however, scientists have begun to focus on microbial influences which maintain plant health. (Beneficial microbes also are discussed in the chapter dealing with biotechnology and soil microbes.)

Motivations for Investigating the Ecology of Microbes in Agroecosystems

Much research is aimed at cutting economic losses and human suffering. Recent ecological research has supported the development of disease predictions (specific forecasts and general models to estimate disease loss probabilities) and is providing the knowledge for predicting the impacts of altered agricultural practices on microbes. Biotechnology and genetically engineered microbes are likely to contribute significantly to biological control, but research to assess the ecological impact of engineered organisms is needed.

Some Current Emphases.

Disease Predictions. Accurate forecasting techniques will result in better disease control, reduced use of pesticide with equivalent control, and avoidance of disease. Forecasts have been developed for more than two dozen major diseases, and are in development for another two dozen. Most forecasts depend on accurate knowledge of environmental (especially moisture and temperature) impacts on the pathogen. A few forecasts include impacts of host plants, chemicals, or other biotic components of the ecosystem. General loss estimates are based on knowledge of: a) the most important environmental effects on pathogen and disease development; b) pathogen population size; c) probabilities of future weather; and d) their important interactions. Although application of computer simulation models is still in its infancy, these models have an increasingly important role in loss predictions. Our goal is to create estimates with sufficient lead time so that growers can adjust cropping plans.

Impacts of Altered Agricultural Practices. Changes in agricultural practice can lead to disastrous losses, such as the devastation caused by a leaf blighting fungus on rubber trees when rubber was shifted from forest to plantation agriculture. This shift to monoculture caused a minor problem to become a major one. Knowledge of the ecology of the microbe causing leaf blight of rubber would have enabled scientists to predict the problems associated with the move to monoculture.

In the United States, scientists knew enough about the ecology of major corn pathogens to predict that adoption of conservation tillage would not create an overall serious increase in disease. That prediction appears accurate.

Properly managed conservation tillage cropping systems provide many benefits and do not create an overall serious increase in disease.

Impact of Biotechnology

Benefits. The development of new diagnostic techniques via biotechnology creates new possibilities for research, identification, and the application of disease forecasts. A major limitation to ecological research in agroecosystems is that microbes are difficult to recover and identify. Now genetic engineering is making it easier to discover and monitor microbial Populations through the use of tools with such sophisticated names as: cDNA probes, monoclonal antibodies, and restriction fragment length polymorphisms. Consequently, the influence of various factors on such populations will be much more readily identified.

The new diagnostic technology should enable more reliable application of disease forecasts, and integrated pest management systems. Consequently, pesticides (fungicides, insecticides and nematicides) are more likely to be used only when needed, rather than in unvarying schedules regardless of need. Disease diagnoses will also become more rapid and more accurate.

Biotechnology will enhance our ability to develop biological controls of diseases and of weeds. In general, biocontrol of plant diseases has been difficult.

One of the few successful applications involves cross protection, in which the infection of plant tissues by one virus suppresses the disease caused by another closely related strain of the virus. The protecting strain must have negligible impact on the host. Such strains have been found naturally, but also can be created in the laboratory via biotechnology. Cross protection has been used successfully in protecting citrus trees from severe strains of Citrus tristeza virus.

Another successful example involves the bacterium which causes crown gall of stone fruits and other plants: a nonpathogenic (or "friendly") strain produces an antibiotic which inhibits the pathogenic strain. Because the two strains are closely related, the nonpathogen survives in the same niches as the pathogen, and responds similarly to environmental fluctuations. Consequently, upon deliberate release of the biocontrol agent, close association of the two bacteria is assured.

For crown gall, the biocontrol agent was naturally occurring, but with biotechnology it will be possible to engineer normal resident nonpathogenic microbes into biocontrol agents.

Some microbes can be used as biocontrol agents for weeds. For example, a fungus is used to control Northern jointvetch in rice and soybean fields. Knowledge of the ecology and epidemiology of this fungus contributed to the development of a rational, effective biological control approach. Even though the genetic and biochemical bases of pathogenicity are unknown for this pathogen, it is so specific in its actions and is relatively unable to be dispersed widely that it makes a desirable biocontrol agent. With biotechnology, other pathogens of weeds can be altered for use in biocontrol. The ecology of specific candidates will have to be well described to assure selection of those with the greatest potential for safe, effective use.

Risks. Concern is widespread about the deliberate release of genetically altered organisms into the environment. This concern apparently comes from opinions that biotechnology speeds the natural rate of genetic change, and that genetically engineered microbes may be designed specifically to have modified ecological roles.

The first of the proposed releases of an engineered microbe, a genetically altered leaf-inhabiting bacterium, has generated significant publicity. Concern about the potential hazard to the environment delayed the release of that bacterium for some years. The wild form causes ice nucleation and hence frost damage on plants, while the altered form is non-ice nucleating and suppresses frost injury. Non-ice nucleating forms also occur naturally.

Unfortunately, inaccurate analogies have been drawn between genetical13 engineered microbes proposed for release and exotic microbes known to be dangerous. Certainly, some exotic microbes pose serious threats. Some of these have been accidentally introduced into the United States in spite of quarantine efforts (i.e. the microbes which cause Dutch Elm disease and Chestnut blight). The microbes proposed for release, however, are not of this known dangerous type Instead, they are more like nitrogen-fixing bacteria introduced routinely to legume crops.

Assessment of Biotechnology Impact

Clearly the environmental impact of genetically-engineered organisms must be assessed before their release. Suggestions for attendant regulations include: an environmental impact assessment, subsequent monitoring of organism and system effects, identification of containment procedures, and contingency plans for mitigation. The competitive survival abilities, and relative fitnesses of natural and engineered forms and their effects on ecosystem processes need to be assessed.

Such assessments are not easy because relationships among organisms are altered in complex ways by different environments. Experiments must be conducted in contained facilities under controlled environmental conditions. The development of methods for such assessments must accompany biotechnological advances if we are to reap the full benefits of our technology. At present, it appears unlikely that specific regulations can apply to all releases. Instead, a case-by-case assessment of releases is likely to occur.