Herbicide Synergy

As stand-alone products, mycoherbicides have achieved limited success in the marketplace. Technologies such as formulation and application methods can improve the performance of biocontrol agents, but more noticeable enhancements in weed control have been observed with the combined application of mycoherbicide agents and synergis-tic herbicides (Peng et al. 2000; Sharon et al. 1992; Wymore et al. 1987). Limited information seems to suggest that synergistic interactions are herbicide and pathogen specific. Use of glyphosate on Canada thistle assisted weed control by A. cirsinoxia only marginally, especially under field conditions (Bailey et al. 2000). Peng et al. (2000) compared two groups of herbicides, bentazon and metribuzin, for interaction with a fungal pathogen on scentless chamomile, a noxious weed in the Canadian prairies. Preliminary results revealed significant fresh weight reduction by up to 150% with applications of herbicides plus the pathogen compared to herbicides alone. Tank-mixing of C. coccodes with thidazuron (N-phenyl-N0-1,2,3-thiadiazol-5-yl-urea) also increased the mortality of velvetleaf when compared to the application of the mycoherbicide alone in field trials (Wymore et al. 1987). The application of a sublethal dose of glyphosate with A. cassiae resulted in an increase in susceptibility of the weed sicklepod (Sharon et al. 1992). As a result, equivalent control was achieved with five times less inoculum of the mycoherbicide agent. The herbicide was believed to interfere with the shikimate acid pathway that is involved in the elicitation of phytoalexins, low molecular weight antimicrobial compounds involved in a plant's defense response. Subsequent interference with the plant's defense mechanism resulted in greater susceptibility of the target weed to the mycoherbicide (Hoagland 1996). This synergistic interaction appears to be an attractive mechanism to enhance the effectivity and feasibility of mycoherbicide agents. According to Hoagland (1996), several benefits may be captured with the application of microbe/herbicide synergy: (a) when defense capabilities of weeds are lowered using herbicides, weeds become more susceptible to pathogen attack, (b) the quantity of mycoherbicide agent or the application rate of herbicides may be reduced, and (c) host range of a given mycoherbicide agent may be expanded with the use of selected chemical synergists.

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