The loss of organic material from the roots provides the energy for the development of active microbial populations in the rhizosphere around the root. Generally, saproptrophs or biotrophs such as mycorrhizal fungi grow in the rhizosphere in response to this carbon loss, but plant pathogens may also develop and infect a susceptible host, resulting in disease. This review examines the microbial interactions that can take place in the rhizosphere and that are involved in biological disease control. The interactions of bacteria used as biocontrol agents of bacterial and fungal plant pathogens, and fungi used as biocontrol agents of protozoan, bacterial and fungal plant pathogens are considered. Whenever possible, modes of action involved in each type of interaction are assessed with particular emphasis on antibiosis, competition, parasitism, and induced resistance. The significance of plant growth promotion and rhizosphere competence in biocontrol is also considered. Multiple microbial interactions involving bacteria and fungi in the rhizosphere are shown to provide enhanced biocontrol in many cases in comparison with biocontrol agents used singly. The extreme complexity of interactions that can occur in the rhizosphere is highlighted and some potential areas for future research in this area are discussed briefly.

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Microbial interactions and biocontrol in the rhizosphere

▪ Abstract Agricultural soils suppressive to soilborne plant pathogens occur worldwide, and for several of these soils the biological basis of suppressiveness has been described. Two classical types of suppressiveness are known. General suppression owes its activity to the total microbial biomass in soil and is not transferable between soils. Specific suppression owes its activity to the effects of individual or select groups of microorganisms and is transferable. The microbial basis of specific suppression to four diseases, Fusarium wilts, potato scab, apple replant disease, and take-all, is discussed. One of the best-described examples occurs in take-all decline soils. In Washington State, take-all decline results from the buildup of fluorescent Pseudomonas spp. that produce the antifungal metabolite 2,4-diacetylphloroglucinol. Producers of this metabolite may have a broader role in disease-suppressive soils worldwide. By coupling molecular technologies with traditional approaches used in plant patholog...

Microbial populations responsible for specific soil suppressiveness to plant pathogens.

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Myths and Dogmas of Biocontrol Changes in Perceptions Derived from Research on Trichoderma harzinum T-22.

Background
Inoculation of plants to enhance yield of crops and performance of other plants is a century old, proven technology for rhizobia and a newer venue for plant growth-promoting bacteria and other plant symbionts. The two main aspects dominating the success of inoculation are the effectiveness of the bacterial isolate and the proper application technology.

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Advances in plant growth-promoting bacterial inoculant technology: formulations and practical perspectives (1998–2013)

Publisher Summary The chapter presents a discussion on both rhizosphere and rhizoplane. Rhizosphere is defined as the soil adjacent to roots with a different physical, chemical, and biological environment from the bulk soil. The rhizoplane is defined as the actual root surface or soil interface. The chapter describes plant responses, the nature and dynamics of rhizosphere populations and their determinants, the management of the plant, and management of microorganisms. The chapter also explores the impact of soil conditions and agronomic practices on the rhizoshere microflora and suggests areas that require more study in order to achieve effective management. The chapter also illustrates different methods that are used by agronomists, plant pathologists, and microbiologists in order to modify the rhizosphere. In most cases, when microbiologists and plant pathologists needs to change the rhizosphere in order to improve plant growth they often think in terms of introducing microorganisms that possess the desired characteristics, be they antibiotic or siderophore production, or the ability to stimulate plant growth.

The rhizosphere and its management to improve plant growth

Microorganisms play an enormously important role in plant disease control As naturally occurring resident antagonists, they can be managed or exploited to achieve the desired results They are responsible for the “crop rotation effect,” which is possibly still the single most important disease management tool used worldwide Simply not growing the same crop in the same field more than every second or third year allows time for the resident antagonists to lower the inoculum potential of root- and many foliar-infecting pathogens of that crop below some economic threshold before that crop is again grown in that field With the possible exception of hyperparasites of sclerotia

From Discovery to Use: Traversing the World of Commercializing Biocontrol Agents for Plant Disease Control

(...) During bio-priming, bacterial populations increased form 10 to over 10,000-fold, depending on initial inoculum level. Bio-priming provided protection against damping-off as good as or better than seed treatment with metalaxyl when the seeds were planted in cold soil. this process may be of interest and value to growers who wish to plant sweet corn or other temperature-sensitive crops into cold soils where damping-off is a problem and the use of chemical seed treatments is not desired

Bio-priming seed treatment for biological control of Pythium ultimum preemergence damping-off in sh2 sweet corn.

In field experiments, bio-priming and coating with Pseudomonas fluorescens AB254 consistently protected sweet corn (Zea mays L.) seeds from preemergence damp- ing-off caused by Pythium ultimum Trow. The bio-priming seed treatment was evalu- ated under various disease pressures and with seeds of three sweet corn genotypes: shrunken-2 supersweet (sh-2), sugary enhancer (se), and sugary (su). While no damping- off occurred in the su sweet corn, bio-priming protected sh-2 and se sweet corn seeds at a level equivalent to that obtained by treatment with the fungicide metalaxyl. Bio- priming increased seedling height of all three sweet corn genotypes at 4 weeks post- planting. Coating of sweet corn seeds with P. fluorescens AB254 provided an equivalent degree of protection from damping-off under all but the most severe conditions.

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Field Performance of Sweet Corn Seed Bio-primed and Coated with Pseudomonas fluorescens AB254

Experiments were conducted in both the glasshouse and in the field to determine if biological and chemical control agents could be used together on sweet corn (Zea mays L.) seed to control Pythium ultimum damping-off or Penicillium oxalicum seedling disease. Pseudomonas aureofaciens AB254 and Pseudomonas sp. AB842 were used for control of P. ultimum and P. oxalicum, respectively. Metalaxyl seed treatment for control of P. ultimum was used at rates from 100 to 0.01% of the recommended rate either alone or in combination with P. aureofaciens AB254. Imazalil seed treatment for control of P. oxalicum was used at rates from 100 to 1% of the recommended rate either alone or in combination with Pseudomonas sp. AB842. In field tests, combining a chemical treatment with the biological agent did not affect the efficacy of disease control. Nor did combining low rates of chemical with the biocontrol agent increase the efficacy or reliability of disease control.

Combined Biological and Chemical Seed Treatments for Control of Two Seedling Diseases of SH2 Sweet Corn

Babadoost, M., Mathre, D. E., Johnston, R. H., and Bonde, M. R. 2004. Survival of teliospores of Tilletia indica in soil. Plant Dis. 88:56-62. This study was conducted to assess survival of Tilletia indica teliospores in a location in the northern United States. Soils differing in texture and other characteristics were collected from four locations, equilibrated to –0.3 MPa, and infested with teliospores of T. indica to give a density of 10 3 teliospores per gram of dry soil. Samples (22 g) of the infested soil were placed in 20-µm mesh polyester bags, which were sealed and placed at 2-, 10-, and 25-cm depths in polyvinyl chloride tubes containing the same field soil as the infested bags. Tubes were buried vertically in the ground at Bozeman, MT, in October 1997. Soil samples were assayed for recovery and germination of T. indica teliospores 1 day and 8, 20, and 32 months after incorporation of teliospores into soil. The rates of teliospores recovered from soil samples were 90.2, 18.7, 16.1, and 13.3% after 1 day and 8, 20, and 32 months after incorporation of teliospores into soil, respectively, and was significantly (P < 0.01) affected by soil source. The percentage of teliospore recovery from soil was the greatest in loam soil and lowest from a silt loam soil. The rate of teliospores recovered from soil was not significantly affected by depth of burial and the soil source–depth interaction during the 32-month period. The percentage of germination of teliospores was significantly (P < 0.01) affected by soil source and depth of burial over the 32month period. The mean percentage of teliospore germination at 1 day, and 8, 20, and 32 months after incorporation into soils was 51.3, 15.1, 16.4, and 16.5%, respectively. In another experiment, samples of silty clay loam soil with 5 × 10 3 teliospores of T. indica per gram of soil were stored at different temperatures in the laboratory. After 37 months of incubation at 22, 4, –5, and –18°C, the rates of teliospore recovered from soil were 1.6, 2.0, 5.7, and 11.3%, respectively. The percentage of spore germination from soil samples was highest at –5°C. Microscopy studies revealed that disintegration of teliospores begin after breakdown of the sheath-covering teliospore. The results of this study showed that teliospores of T. indica can survive in Montana for more than 32 months and remain viable. Additional keywords: Neovossia indica, Triticum aestivum

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D. E. Mathre

Papers

From Discovery to Use: Traversing the World of Commercializing Biocontrol Agents for Plant Disease Control

Bio-priming seed treatment for biological control of Pythium ultimum preemergence damping-off in sh2 sweet corn.

Field Performance of Sweet Corn Seed Bio-primed and Coated with Pseudomonas fluorescens AB254

Combined Biological and Chemical Seed Treatments for Control of Two Seedling Diseases of SH2 Sweet Corn

Survival of Teliospores of Tilletia indica in Soil.