The fungal plant pathogen Microsphaeropsis amaranthi is virulent against a number of key weeds in the Amaranthaceae, including common waterhemp, and is under investigation as a bioherbicide. Common waterhemp has become a key weed in midwestern crop production systems and is a good target for a bioherbicide that could be integrated into weed management systems. We investigated the direct effects of a range of chemical herbicides and adjuvants upon conidia of M. amaranthi and found that many herbicides and most adjuvants were strongly inhibitory to germination. On the other hand, M. amaranthi was compatible with a selection of postemergence herbicides commonly used in midwestern weed management systems, including carfentrazone, chloransulam, and imazethapyr. Most glyphosate products suppressed or abolished germination of M. amaranthi conidia, but by testing adjuvants commonly used in glyphosate products and technical-grade glyphosate salts, it was revealed that this inhibition was due to formulation additives and not the active ingredient. When glyphosate and conidia of M. amaranthi were sprayed onto common waterhemp seedlings, the herbicide predisposed plants to infection by M. amaranthi. When M. amaranthi was applied 1 to 3 d after glyphosate, the glyphosate rate required to control common waterhemp was reduced by half. Similar results were observed on clones propagated from a common waterhemp plant resistant to glyphosate. When M. amaranthi was applied to seedlings 2 d before glyphosate, the efficacy of the herbicide was reduced. These findings demonstrate that positive interactions between herbicides and M. amaranthi exist but reveal practical difficulties that may limit the integration of the strategy in the field.

Nomenclature: Microsphaeropsis amaranthi (Ell. & Barth.); common waterhemp, Amaranthus rudis Sauer, AMATA.