ARS | Publication request: Development of pilot-scale fermentation and stabilization processes for the production of microsclerotia of the entomopathogenic fungus Metarhizium brunneun strain F52

Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 21, 2012
Publication Date: August 29, 2012
Citation: Jackson, M.A., Jaronski, S. 2012. Development of pilot-scale fermentation and stabilization processes for the production of microsclerotia of the entomopathogenic fungus Metarhizium brunneun strain F52. Biocontrol Science and Technology. 22(1):915-930.

Interpretive Summary: The fungus Metarhizium anisopliae (Ma) infects and kills many important soil-dwelling insect pests including root weevils, soil grubs, rootworms, wireworms, fruit flies, and root maggots. Spores of Ma are the infectious form of the fungus, but their application into soil has proven to be difficult. We have discovered a very stable form of Ma, a microsclerotium, that can be produced using liquid culture fermentation and stabilized as a dry granular formulation. Microsclerotial granules can be easily applied to the soil; and when rehydrated, they grow and produce infective spores of Ma in the soil environment. Developing a low-cost method of producing and stabilizing microsclerotia granules of Ma is an absolute requirement for their commercial use in insect control. In these studies, we evaluated various pilot-scale fermentation conditions and drying processes to determine the feasibility of mass-producing stable microsclerotia of Ma capable of producing high concentrations of infective spores. Results from these studies showed that high concentrations of microsclerotia could be rapidly produced (4-day fermentation time) in 100-L, pilot-scale fermentors by cultures of Ma using low-cost nutritional feedstocks. The microsclerotial granules produced under these liquid fermentation conditions were very stable using the drying protocols tested with no significant loss in conidia production after 12 months of storage. These studies have demonstrated the potential for the commercial production and formulation of microsclerotial granules of Ma for use by homeowners, farmers, and land managers in controlling soil-dwelling insect pests.

Technical Abstract: Using 100L stirred-tank bioreactors, we evaluated the effect of fermentation parameters and drying protocols on the production and stabilization of microsclerotia (MS) of the entomopathogenic fungus Metarhizium brunneum (formerly M. anisopliae F52). Results showed that stirred-tank bioreactors can be used to mass produce stable MS of Metarhizium and that culturing and drying protocols significantly affected MS yield and stability. Length of fermentation (4-7 days) for Metarhizium cultures had no significant impact on biomass accumulation, MS formation, or the storage stability of the air-dried MS granules. Although cultures of Metarhizium grown on media with a carbon-to-nitrogen (C:N) ratio of 30:1 produced significantly more biomass when compared to cultures grown in media with a C:N ratio of 50:1, MS formation and desiccation tolerance following drying were similar. After storage for 1 year at 4°C, conidia production by air-dried MS granules from 50:1 media was significantly higher compared to MS granules from 30:1 media. The addition of diatomaceous earth (DE) to cultures of Metarhizium prior to drying at rates of 0-60 g L-1 had no significant affect on MS desiccation tolerance but did impact conidia production. Air-dried MS granules without DE produced significantly more conidia g-1 during the first 4 months of storage, but after one year, conidia production was similar regardless of DE content of the MS granule. Microsclerotial granules with higher moisture levels (2.6-5.0% w/w) produced significantly more conidia immediately after drying and MS granules with low moisture (0-2.5% w/w) produced more conidia after 12 months storage.