Twin-screw extrusion of ‘Pesta’-encapsulated biocontrol agents

Abstract

'Pesta' granules in which fungal propagules are encapsulated in a wheat gluten matrix were prepared in multi-pound quantities by twin-screw extrusion and fluid bed drying. Dough formulations for extrusion contained wheat flour and kaolin, or wheat flour, kaolin and rice flour, plus water and fungal inoculum. Conidial inoculum of Colletotrichum truncatum, a pathogen of the weed hemp sesbania (Sesbania exaltata), survived laboratory scale dough preparation [100% retention of colony-forming units (c.f.u.)] better than dough preparation for twin-screw extrusion (8-10% c.f.u. retention). The loss in viability was linked to the lower water content of dough used in the twin-screw extruder. Fluid bed drying reduced viability further to 1%. Retention of viability after twin-screw extrusion and fluid bed drying at 35-50 °C was 35% with Alternaria conjuncta/infectoria, a pathogen of swamp dodder (Cuscuta gronovii). Retention was 86-100% with atoxigenic strains of Aspergillus flavus and Aspergillus parasiticus used as biocompetitors to reduce aflatoxin levels in peanuts. In the greenhouse, twin-screw-extruded granules containing C. truncatum (at about 5 x 10 4 c.f.u. g -1 ) caused high levels of infection and mortality in hemp sesbania seedlings.