Production and formulation of high quality conidia of Microsphaeropsis amaranthi for the biological control of weedy Amaranthus species

The fungal pathogen, Microsphaeropsis amaranthi, is under consideration as a bioherbicide for the control of weeds in the genus Amaranthus. This organism has been shown to be virulent against a number of important Amaranthus species but has not yet demonstrated sufficient aggressiveness or reliability in the field to be commercially developed. We report a sequence of experiments evaluating a range of solid media for the production of high quality inoculum, methodologies for the enhancement of aggressiveness in culture, and the effects of a variety of spray adjuvants. Additionally, we report preliminary results testing the efficacy of the bioherbicide applied as granules, prior to weed emergence. The fungus grew well and sporulated profusely on a range of different solid substrates. Conidia produced on corn stover were larger than those produced on other substrates, had thicker cell walls, and exhibited particularly high aggressiveness. Repeated inoculation and re-isolation of the fungus from Amaranthus tuberculatus plants increased its aggressiveness. In addition, application in Sunspray oils resulted in improved disease impact under low moisture conditions. Preemergence application of granular formulations, especially where the fungus was grown and applied in ground barley grains, resulted in good control of emerging A. tuberculatus. Further progress has been made in improving the M. amaranthi bioherbicide by refining the conditions for the production and deployment of quality inoculum.