Synergism of Colletotrichum truncatum with herbicides for control of scentless chamomile (Matricaria perforata)

Abstract

Scentless chamomile is a noxious weed in the Canadian prairies and has natural tolerance to most post-emergent herbicides. A fungal pathogen, Colletotrichum truncatum, was identified as a potential biocontrol agent with moderate efficacy when broadcast at a spore concentration of 7x106 spores ml-1 and a water volume of 200 L ha-1. In this study, potential synergy of herbicides with the fungus was investigated by tank-mix applications in the greenhouse. Most of the herbicides tested at full label rates delayed fungal spore germination temporarily, but this inhibition was not observed after 24 hr incubation. On scentless chamomile, the efficacy of the herbicide influenced the interaction between fungus and herbicide, with less effective products more likely exhibiting synergy although weed control might not be optimum. Clopyralid plus MCPA ester and metribuzin provided more effective scentless chamomile control when applied with the fungus and provided ideal tank mixes at the 8-leaf and 11-leaf stages, respectively. The addition of clodinafop to clopyralid plus MCPA ester plus the fungus did not further improve control of scentless chamomile, but this tank-mix option may be practical for targeting a broad weed spectrum in cereals. Increasing fungal dose generally enhanced biocontrol efficacy, but this effect eased rapidly at higher doses. The addition of synergistic herbicides helped alleviate the high fungal dose required for weed control. There were no major differences in herbicide interaction among six fungal isolates tested, with metribuzin and clopyralid plus MCPA ester consistently exhibiting the highest weed control with all the isolates. Field trials with appropriate formulations are needed to validate the feasibility of this control strategy for scentless chamomile in the prairie climate.