Formulation ofColletotrichum truncatumMicrosclerotia for Improved Biocontrol of the Weed Hemp Sesbania (Sesbania exaltata)

Submerged cultures of the biocontrol fungusColletotrichum truncatumproduce high concentrations of microsclerotia (MS) when grown under specific nutritional conditions. Previous studies have shown that MS survived drying, remained viable following long-term storage at 4°C, and killed hemp sesbania seedlings when incorporated into potting soil. In this study, dried preparations ofC. truncatumMS were encapsulated in wetted formulations of pregelatinized corn flour, pregelatinized cornstarch, or a mixture of the two (1:1). Germination rates of microsclerotia immediately after formulation and drying were not significantly different. After storage at 4°C for 18 months, MS germination rates were significantly lower for all formulations compared to nonformulated MS. Dried MS were capable of producing mycelia or spores upon germination when plated on water agar. After 3 days of growth on Noble agar, the MS–flour formulations produced significantly more conidia than those made with starch, starch–flour, or nonformulated MS. By the 10th day of incubation, significantly more conidia were produced in formulations containing starch–flour (≈10-fold) and flour (≈50-fold) than in MS–starch formulations and nonformulated MS. Bioassays showed that all MS preparations incorporated into potting soil incited disease in emerging hemp sesbania seedlings. A significantly higher incidence of disease was seen in seedlings grown in potting soil containing flour-formulated MS. These results suggest that corn flour formulations ofC. truncatummicrosclerotia can be used to increase the effectiveness of this bioherbicide for controlling the weed hemp sesbania and that increased conidial production may play a role in improving biocontrol efficacy.