Biological Control Agents in Combination with Fertilization or Fumigation to Reduce Sclerotial Viability of Sclerotium rolfsii and Disease of Snap Beans in the Greenhouse

The use of biological control agents in combination with fertilization or fumigation to reduce sclerotial viability of Sclerotium rolfsii and the disease it causes on snap bean was investigated in the greenhouse. The fertilizers ammonium sulphate (NH₄)₂SO₄], ammonium nitrate (NH₄NO₃), diammonium phosphate (NH₄)₂HPO₄], or urea applied to soil at a field rate of 135 kg/ha, 15 cm deep of nitrogen (N) (0.09 mg of N/g) or Gliocladium virens (Gl-3) biomass at a rate of 7.5 kg/ha, 15 cm deep (0.05 mg/g) did not reduce the viability of sclerotia of S. rolfsii (Sr-1) when each was applied alone. However, treatment with fertilizer together with the low rate of Gl-3 biomass significantly reduced the sclerotial viability. The treatments that were effective in reducing the viability by more than 75% were the application of (NH₄)₂SO₄ or (NH₄)₂PO₄ and the low rate of Gl-3 biomass. Application of the high rate (0.25 mg/g) of Gl-3 biomass alone only reduced the sclerotial viability by 25%. The addition of any of the fertilizers with the low rate of biomass generally resulted in bean seed germination in the pathogen-infested soil that was higher than that achieved with each individual component. The disease severity (DSI) on beans was appreciable (<3.0) in pathogen-infested soil treated with or without the fertilizer (NH₄)₂SO₄ and in pathogen-infested soil without fertilizer but with a low rate of Gl-3. However, in pathogen-infested soil treated with the fertilizer and the low rate of Gl-3 biomass together, the disease was reduced to a DSI value of less than 1.0. In fumigation studies with metham sodium (Vapam), a dose-response study to investigate the viability of sclerotia of S. rolfsii (Sr-3) indicated that fumigant rates of less than 23.3 μ g/g of soil were sublethal. It was also shown that 5.4 μ g/g of metham sodium was inhibitory to Gl-3 biomass but not to conidia. Consequently, the conidia of isolates Gl-3, Thm-4 of Trichoderma hamatum, and Tv-1 of Trichoderma viride were used together with metham sodium at 17.1 μ g/g of soil. Conidia that were applied to the soil 2 days prior to metham sodium reduced the viability of sclerotia more than each individual component. The results of this study suggest the feasibility of effective disease reduction with an approach utilizing biological control in combination with fertilization or fumigation.