Management Of Native And Introduced Microorganisms

There are many cases in which soil organisms have been managed to improve plant growth and serve as biological control agents to suppress plant disease, inhibit weeds, control insects, or detoxify environmental pollutants (Table 17.5). Beneficial microorganisms directly promote plant growth through the creation of symbiotic associations with plant roots, release of phytohormones, induction of systemic resistance, suppression of pathogens, production of antibiotics, and reduction of heavy metal toxicity (Bowden and Rovira, 1999). Many of these organisms are naturally present in soil, although under some circumstances it may be necessary to increase their populations either by modifying the soil environment or through inoculation to enhance their abundance and activity.

The positive role of symbiotic associations such as mycorrhiza in plant production is well known (see Chap. 10), with many cases documenting growth and yield enhancement of infected plants. The plant response is due to an increase in effective root area that improves water and nutrient extraction. Other benefits of the association are protection against pathogens, improved tolerance to pollutants, and greater resistance to water stress, high temperatures, and adverse pH. Management strategies that enhance mycorrhizal populations and activity in agricultural fields include reduced tillage, crop rotation, and lower N and P applications. The other major plant symbionts, rhizobia associated with legumes, provide the N required for plant growth and in return obtain photosynthetic products from the plant for their own growth.

Many bacterial species have been used as plant-growth-promoting organisms, predominately pseudomonads (e.g., Pseudomonas fluorescens, P. putida, P. gladioli),

TABLE 17.5 Examples of Bacteria and Fungi Introduced to Improve Agricultural Productivity, Control Pests, and Degrade Toxic Substances in Soil

Example

Microorganisms

Plant growth

Bradyrhizobia, Mycorrhizae, Azotobacter, PGPRBa

Biological control

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