Liquid culture production of microsclerotia of Colletotrichum truncatum for use as bioherbicidal propagules

Abstract

Colletotrichum truncatum NRRL 18434 is a host-specific pathogen of hemp sesbania (Sesbania exaltata), a weed in cotton, soybeans and rice. The bioherbicidal potential of C. truncatum microsclerotia (MS) was assessed based on the feasibility of producing and storing microsclerotia and the efficacy of microsclerotia in controlling hemp sesbania. Microsclerotia were produced in highly aerated liquid cultures grown at 28 °C in a basal salts medium with 80 g glucose and 13·2 g Casamino acids l−1. Hyphal aggregation to form microsclerotial initials began on day 4 with maximum concentrations (6 × 106 MS l−1) being attained by day 6. Melanization started on day 6 and was completed by day 10. After air-drying at 22°, microsclerotia: diatomaceous earth preparations were stored at 4°. After storage for 7 months 90% of the dried C. truncatum MS germinated. Efficacy studies showed that the incorporation of either fresh or 1-month-old microsclerotia into potting soil (156 microsclerotia cm−3 soil) killed more than 95% of hemp sesbania seedlings after emergence. Microsclerotia-infested potting mix incited disease of a similar magnitude in a second crop of hemp sesbania seedlings planted 28 d after the first. These studies demonstrate that C. truncatum microsclerotia may have potential for use as an alternative to conidia for biocontrol since they can be produced in submerged culture, remain viable as a dry formulation and effectively control hemp sesbania seedlings when incorporated into potting medium prior to seeding.