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About 2–5% of rhizobacteria, when present in large number, are able to promote plant growth. Production of plant hormones and improving mineral nutrition are examples of direct promotion, while protecting plants against many diseases caused by bacteria, fungi, viruses, nematodes, or insects is indirect mechanism of growth promotion., 0

About 2–5% of rhizobacteria, when present in large number, are able to promote plant growth. Production of plant hormones and improving mineral nutrition are examples of direct promotion, while protecting plants against many diseases caused by bacteria, fungi, viruses, nematodes, or insects is indirect mechanism of growth promotion., 0

We investigated the in vitro influence of humic substances (HS) extracted from de-inking paper sludge compost on the inhibition of Pythium ultimum by two compost bacteria, Rhizobium radiobacter (Agrobacterium radiobacter) and Pseudomonas aeruginosa. When low concentrations (5 or 50 mgzl−1) of HS were added to the culture medium, fungal inhibition by R. radiobacter significantly increased (P < 0.01) by 2–3%. In contrast, these low levels of HS had no effect on P. ultimum inhibition by P. aeruginosa. The Fe, chelated by HS, was in part responsible for the decrease of P. ultimum inhibition by the bacteria when increasing amounts of HS were added in the culture medium. The addition of 500 mgl−1 of humic acids isolated from de-inking paper sludge compost or from fossil origin completely eliminated the inhibition of P. ultimum by R. radiobacter. This Fe effect also stimulated growth of R. radiobacter and reduced its siderophore production in a minimal medium supplemented with HS as sole source of Fe. The results showed that HS influence microbial antagonism when added to a culture medium. However, this effect varies with different factors such as the type of bacteria, concentration of HS, molecular weight and Fe content.

Phosphorus (P) is vital for plant growth. However, most added soluble P forms insoluble phosphates. Therefore, inorganic P (Pi) accumulates in soils. Soil organic P (Po) is another important reserve (20 to 80% of total soil P). To be available for plant Po must first be mineralized by soil microorganisms. The variability of the results of inoculation trials with P solubilizing microorganisms (PSMs) clearly reflects the complexity of the interactions occurring in the soil-plant-microbes-fauna ecosystem. Important points overlooked in previous studies will be presented and perspectives of the use of PSMs to allow the plant to benefit from soil reserves in Pi and Po will be discussed.

Compost teas are fermented aqueous extracts of composted materials that are used for their ability to control plant pathogens. It had been previously reported that this inhibition by compost teas is at least partially attributed to the presence of live microorganisms. In this study, the inhibitory effects of bacteria from suppressive compost tea were examined against mycelial growth of Alternaria solani and Botrytis cinerea as well as disease development on tomato fruit. Isolation of antifungal extracts and identification of antifungal compounds from the most effective bacterial strains were also performed. Results showed that the bacteria had the ability to greatly inhibit the mycelial growth of B. cinerea and/or A. solani by up to 70%. The two most effective isolates, Brevibacterium linens (IC 10) and Bacillus subtilis, showed that co-application of bacterial antagonists (5 × 105 or 5 × 106 cells) with the pathogens on tomato fruit demonstrated inhibition of the development of B. cinerea lesions by up to 61%. A preventive application of the bacteria (5 × 105 or 5 × 106 cells) was more effective than co-application, allowing a significant reduction in lesions of A. solani and improving efficacy of low bacterial concentrations in reducing B. cinerea lesions. A combined B. linens/B. subtilis treatment was generally more inhibitory than either bacterium alone indicating possible synergistic effects. Antifungal compounds, including surfactins, were found in the bacterial extracts indicating that antibiosis is a main mechanism of action.

Compost teas are fermented aqueous extracts of composted materials that are used for their ability to control plant pathogens. It had been previously reported that this inhibition by compost teas is at least partially attributed to the presence of live microorganisms. In this study, the inhibitory effects of bacteria from suppressive compost tea were examined against mycelial growth of Alternaria solani and Botrytis cinerea as well as disease development on tomato fruit. Isolation of antifungal extracts and identification of antifungal compounds from the most effective bacterial strains were also performed. Results showed that the bacteria had the ability to greatly inhibit the mycelial growth of B. cinerea and/or A. solani by up to 70%. The two most effective isolates, Brevibacterium linens (IC 10) and Bacillus subtilis, showed that co-application of bacterial antagonists (5 105 or 5 106 cells) with the pathogens on tomato fruit demonstrated inhibition of the development of B. cinerea lesions by up to 61%. A preventive application of the bacteria (5 105 or 5 106 cells) was more effective than co-application, allowing a significant reduction in lesions of A. solani and improving efficacy of low bacterial concentrations in reducing B. cinerea lesions. A combined B. linens/B. subtilis treatment was generally more inhibitory than either bacterium alone indicating possible synergistic effects. Antifungal compounds, including surfactins, were found in the bacterial extracts indicating that antibiosis is a main mechanism of action.

About 2–5% of rhizobacteria, when present in large number, are able to promote plant growth. Production of plant hormones and improving mineral nutrition are examples of direct promotion, while protecting plants against many diseases caused by bacteria, fungi, viruses, nematodes, or insects is indirect mechanism of growth promotion.

Phosphorus (P) is vital for plant growth. However, most added soluble P forms insoluble phosphates. Therefore, inorganic P (Pi) accumulates in soils. Soil organic P (Po) is another important reserve (20 to 80% of total soil P). To be available for plant Po must first be mineralized by soil microorganisms. The variability of the results of inoculation trials with P solubilizing microorganisms (PSMs) clearly reflects the complexity of the interactions occurring in the soil-plant-microbes-fauna ecosystem. Important points overlooked in previous studies will be presented and perspectives of the use of PSMs to allow the plant to benefit from soil reserves in Pi and Po will be discussed.

We investigated the in vitro influence of humic substances (HS) extracted from de-inking paper sludge compost on the inhibition of Pythium ultimum by two compost bacteria, Rhizobium radiobacter (Agrobacterium radiobacter) and Pseudomonas aeruginosa. When low concentrations (5 or 50 mgzl−1) of HS were added to the culture medium, fungal inhibition by R. radiobacter significantly increased (P < 0.01) by 2–3%. In contrast, these low levels of HS had no effect on P. ultimum inhibition by P. aeruginosa. The Fe, chelated by HS, was in part responsible for the decrease of P. ultimum inhibition by the bacteria when increasing amounts of HS were added in the culture medium. The addition of 500 mgl−1 of humic acids isolated from de-inking paper sludge compost or from fossil origin completely eliminated the inhibition of P. ultimum by R. radiobacter. This Fe effect also stimulated growth of R. radiobacter and reduced its siderophore production in a minimal medium supplemented with HS as sole source of Fe. The results showed that HS influence microbial antagonism when added to a culture medium. However, this effect varies with different factors such as the type of bacteria, concentration of HS, molecular weight and Fe content.