THE INFLUENCE OF SOLID-STATE FERMENTATION INOCULUM ON BIOPESTICIDES' VIABILITY IN GRANULES
DJ DAIGLE1, WJ CONNICK JR1, AB PEPPERMAN1, ET CHAMPAGNE1, CD BOYETTE2 and JW DORNER3
1Southern Regional Research Center, ARS, USDA, PO Box 19687, New Orleans, LA 70179, USA; 2SWSU, ARS, USDA, Stoneville, MS 38776 USA; 3National Peanut Research Laboratory, ARS, USDA, Dawson, GA 31742, USA
Background and objectives
Multi-pound quantities of granules containing biopesticides (PestaTM) have been prepared by twin-screw extrusion. Conidial inoculum of some fungi suffered large (>90%) viability losses because of the low water content of the dough (wheat flour/kaolin) . Fluid bed drying at 35°C reduced viability further. lnoculum prepared by solid-state fermentation (SSF)  and/or surfactants were used to reduce these losses and make the production of PestaTM more commercially acceptable. The bioherbicide agents Colletotrichum truncatum, a pathogen of hemp sesbania (Sesbania exaltata) and Alternaria infectoria, a pathogen of dodder (Cuscuta gronovii), and an atoxigenic strain of Aspergillus flavus that can reduce aflatoxin in peanuts by biocompetition, were studied.
Materials and methods
Fungal inoculum was grown on sterile rice flour in vented plastic bags. After incubation for 2-4 weeks, the fungal biomass-rice flour mixture, wheat flour, kaolin, and water were blended, and the blend processed by twin-screw extrusion at room temperature. Surfactants were mixed with the wheat flour and kaolin prior to blending with the inoculum. The wet granules (1.6x1.5 mm) were dried at 50°C in a fluid bed dryer to a water activity range of 0.25-0.33.
Results and conclusions
Granules made with C. truncatum shake-flask inoculum retained only 1-2% viability. With SSF inoculum, 10-40% of the theoretical inoculum was retained. Shake-flask mycelial inoculum of A. infectoria encapsulated in PestaTM granules retained 2% viability compared with 28-100% viability when the inoculum was rice flour-conidial biomass. These increases in viability were made despite an increase in the drying temperature from 35 to 50°C. The enhanced fungal stability may be the effect of hardier conidia grown by SSF or the rice flour component itself.
1. Daigle DJ, Connick WJ Jr, Boyette CD et al., 1997. World Journal of Microbiology and Biotechnology, in press.
2. Feng MG, Poprawski TJ, Khachatourians GG, 1994. Biocontrol Science and Technology 4, 3-34.