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 f umigation to reduce sclerotial viability of Sclerotium rolfsii and the dise ase it causes on snap bean was investigated in the greenhouse. The fertiliz ers ammonium sulphate [(NH4)(2)SO4], ammonium nitrate (NH4NO3), diammonium phosphate [(NH4)(2)HPO4], 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 vir ens (G1 -3) biomass at a rate of 7.5 kg/ha, 15 cm deep (0.05 mg/g) did not reduce t he viability of sclerotia of S. rolfsii (Sr-1) when each was applied alone. However, treatment with fertilizer together with the low rate of G1-3 biom ass significantly reduced the sclerotial viability. The treatments that wer e effective in reducing the viability by more than 75% were the application of(NH4)(2)SO4 or (NH4)(2)PO4 and the low rate of G1-3 biomass. Application of the high rate (0.25 mg/g) of G1-3 biomass alone only reduced the sclero tial 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 comp onent. The disease severity (DSI) on beans was appreciable (> 3.0) in patho gen-infested soil treated with or without the fertilizer (NH4)(2)SO4 and in pathogen-infested soil without fertilizer but with a low rate of G1-3. How ever, in pathogen-infested soil treated with the fertilizer and the low rat e of G1-3 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) indic ated that fumigant rates of less than 23.3 mu g/g of soil were sublethal. I t was also shown that 5.4 mu g/g of metham sodium was inhibitory to G1-3 bi omass but not to conidia. Consequently, the conidia of isolates G1-3, Thm-4 of Trichoderma hamatum, and Tv-1 of Trichoderma viride were used together with metham sodium at 17.1 mu 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 feas ibility of effective disease reduction with an approach utilizing biologica l control in combination with fertilization or fumigation.