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Climatic Constraints to the Potential of Microsphaeropsis amaranthi as a Bioherbicide for Common Waterhemp

The fungal plant pathogen Microsphaeropsis amaranthi is a candidate bioherbicide for the control of weedy Amaranthus species since it grows and sporulates readily in culture, is a pathogen of a number of important weed species, and is host-restricted to the family Amaranthaceae. This study was designed to determine the optimum and limiting environmental conditions for the efficacy of foliar applications of M. amaranthi for the control of common waterhemp. The greatest disease severity and the greatest plant biomass reductions were found when a prolonged leaf wetness period of ≥ 12 h at 18 to 24°C was provided after application. Disease severity and plant biomass reductions were greater when plants were treated at an earlier stage of development. Disease severity and impact were limited when shorter leaf wetness periods were provided, with negligible disease observed on plants provided with ≤6 h leaf wetness. When common waterhemp seedlings were sprayed in the field weekly through the summer of 2004 with conidial suspensions of M. amaranthi, only 7 of 14 applications resulted in disease and only 3 applications resulted in more than slight disease flecks. Moderate levels of disease were observed when applications had been made on days with moderate temperatures and leaf wetness duration in excess of 12 h following application. We confirm the pathogenicity of M. amaranthi against common waterhemp and demonstrate that the unformulated pathogen can cause disease in the field. Nonetheless, the weather conditions of the Midwestern United States, where common waterhemp control would be desired, are infrequently conducive to infection. The continued candidacy of M. amaranthi as a bioherbicide for the control of common waterhemp in the Midwest-ern United States will require that its efficacy under suboptimal environmental conditions be improved.