Liquid culturing of microsclerotia of Mycoleptodiscus terrestris, a potential biological control agent for the management of hydrilla

Mycoleptodiscus terrestris has potential as an inundative biological control agent for the management of hydrilla, one of the world's worst aquatic weeds. Essential to producing a marketable bioherbicidal product was the development of liquid culture procedures that would yield propagules that maintained biocontrol efficacy. Since M. terrestris did not produce conidia in liquid culture, various nutritional conditions were evaluated as a means to produce high concentrations of stable fungal propagules such as microsclerotia. Evaluations of propagule formation and biomass yield were carried out in liquid culture media containing a basal salts solution amended with corn steep liquor powder or cottonseed meal combined with 4% or 6% glucose. Hyphal aggregation was observed by day 2, and by day 8 abundant melanized microsclerotia were present in the broth cultures. When applied as a liquid inoculum to hydrilla at rates of 0.1 and 0.2 ml/l, the microsclerotial matrix was capable of significantly reducing hydrilla shoot biomass by as much as 99%. Air-dried microsclerotia were capable of hyphal germination in 24 h and sporogenic germination in 72 h. These capabilities have significance for the use of microsclerotia of M. terrestris as the preferred inoculum for biocontrol purposes. Hyphae germinating from microsclerotia on hydrilla plant surfaces can establish initial infection sites followed several days later by secondary infections resulting from the development and release of spores from the surface of the microsclerotia. The capability of microsclerotia of M. terrestris to remain stable as a dry preparation and to germinate both hyphally and sporogenically upon rehydration enhances the potential of this fungus for use as a nonchemical, biological control agent for hydrilla.