Influence of glyphosate and fungicide coapplications on weed control, spray penetration, soybean response, and yield in glyphosate-resistant soybean

Field experiments were conducted to evaluate the effect of various glyphosate plus fungicide coapplications on weed control, spray penetration, phytotoxicity, and yield in glyphosate-resistant soybean [Glycine max (L.) Merr.] during 2004 and 2005. Glyphosate was applied alone or in combination with the fungicides chlorothalonil [tetrachloroisophthalonitrile], metconazole [5-[(4-chlorophenyl)methyl]-2,2-dimethyl-1-(1H-1,24-triazol-1-yiniethyl) cyclopentanol], myclobutanil [2-(4-chlorophenyl)-2(1H-1,2,4-triazol-1-yl methyl) hexanenitrile], propiconazole (1-[[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-yllmethyl]-1H-1,2,4-triazole), pyraclostrobin (carbamic acid, [2,[[[1-(4-chlorophenyl)-1H-pyrazol-3-yl]oxy]methyl]phenyl]methoxy-, methyl ester), tebuconazole [1-(4-chlorophenyl)-4,4-dimethyl-3-(1,2,4,j-triazol-1-ylmethyl)pentan-3-ol]-tetrafluoroetho, tetraconazole [(1-[2-(2,4-dichlorophenyl)-3-(1,1,2,2-tetrafluoroethoxy)propylj]-1H-1,2,4-triazole)], azoxystrobin (methyl (E)-212-[6-(2-cyanophenoxy) pyrimidin-4-yloxy]phenyl}-3-,methoxyacrylate) plus propiconazole, propiconazole plus trifloxystrobin [methyl (E) -methoxyimino-{(EE)- alpha- [1-alpha, alpha, alpha-trifluoro-m-tolyl) ethy lideneaminooxy]-o-tolyl}acetate], and pyraclostrobin plus boscalid at two application timings recommended for optimum weed control. Across all locations and years, similar levels of visual weed control and weed density at harvest were recorded with all glyphosate plus fungicide combinations compared with glyphosate alone. Little soybean plant injury or leaf area index reduction was observed with any glyphosate and fungicide coapplication when compared with glyphosate alone. Only coapplications of either glyphosate and pyraclostrobin or azoxystrobin plus propiconazole increased yields when compared with glyphosate alone. Greater spray penetration was achieved with early compared with late applications. No more than 10% spray card coverage was achieved at any canopy height with late applications. Although the optimum timing of fungicide application(s) will largely be dependent on the time of disease infection, these results suggest that the timing of fungicide application for control of Asian soybean rust (ASR) will rarely coincide with the optimum timing of herbicides like glyphosate for weed control in soybean.