Submitted to: Biological Control
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/30/2006
Publication Date: 9/1/2006
Citation: Shearer, J.F., Jackson, M.A. 2006. Liquid culture production of microsclerotia of Mycoleptodiscus terrestris: A potential biological control agent for the management of hydrilla. Biological Control. 38:298-306. Interpretive Summary: Hydrilla (Hydrilla verticillata) is considered one of the three most important aquatic weeds in the world. Introduced into Florida from Asia in the 1950's, the invasive, aquatic weed is now found in lakes, ponds, reservoirs, rivers, and canals across the Southern United States. Hydrilla impedes navigation, clogs drainage or irrigation canals, affects water intake systems, interferes with recreational activities, and disrupts wildlife habitats. Hydrilla has been primarily managed by chemical herbicide applications and mechanical removal. The high cost of these control measures, combined with environmental concerns and the recent appearance of herbicide resistance in some populations of hydrilla, has heightened interest in the use of natural enemies of hydrilla for control. During the late 1980's, a survey of hydrilla populations in the Southern United States resulted in the isolation of a mold, Mycoleptodiscus terrestris, that was capable of infecting and killing hydrilla. This mold has demonstrated excellent potential as a biological control agent that can be applied alone or in combination with chemical herbicides for management of hydrilla. Essential to producing a marketable bioherbicidal product was the development of cost-effective methods for mass producing the mold M. terrestris. In this study, deep-tank fermentation processes were developed that produced stable, infective propagules of M. terrestris called microsclerotia. When applied to hydrilla, these microsclerotia significantly reduced hydrilla biomass by as much as 99%. These studies have demonstrated that stable, effective microsclerotia of the mold M. terrestris can be economically mass produced using deep-tank fermentation and have excellent potential for use as a non-chemical control for hydrilla.
Technical Abstract: Mycoleptodiscus terrestris (Gerd.) Ostazeski has shown great potential as an inundative biological control agent for 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 production of conidial M. terrestris propagules in liquid culture was highly unlikely, various nutritional conditions were evaluated for their potential to support the production of 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 salt 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. Applied as a liquid inoculum to hydrilla at rates of 0.1 ml L-1 and 0.2 ml L-1, the microsclerotial matrix was capable of significantly reducing hydrilla shoot biomass as high as 99%. Air-dried microsclerotia were capable of hyphal germination in 24 hours and sporogenic germination in 72 hours. These capabilities have significance for the use of microsclerotia of M. terrestris as a biocontrol pathogen. Hyphal germination of the microsclerotia that impinge on hydrilla plant surfaces can provide initial infection sites followed several days later by secondary infection resulting from the development and release of spores from the surface of the microsclerotia.