| Summary: | <p>Scentless chamomile is a noxious weed in the Canadian prairies and has natural
tolerance to most post-emergent herbicides. A fungal pathogen, <i>Colletotrichum
truncatum</i>, was identified as a potential biocontrol agent with moderate efficacy when
broadcast at a spore concentration of 7x10<sup>6</sup> spores ml<sup>-1</sup> and a water volume of
200 L ha<sup>-1</sup>. 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.</p>
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