This study was conducted to determine the effects of three commonly applied soybean (Glycine max) herbicides (glyphosate, imazethapyr, and pendimethalin) on the mycelial growth, sclerotial production, and viability of Rhizoctonia solani isolates [anastomosis groups AG-1, AG-2-2, and AG-4] under controlled conditions. Pendimethalin significantly reduced mycelial growth of all three R. solani isolates investigated, whereas effects of the herbicides imazethapyr and glyphosate were not significant. Sclerotial production was affected differently by the three herbicides. Isolates AG-1 and AG-2-2 produced sclerotia both in vitro and in vivo, whereas isolate AG-4 did not produce sclerotia in vitro. In vitro AG-1 isolate showed a decrease, and AG-2-2 isolate showed an increase in sclerotial production in the presence of herbicide. In contrast, both AG-1 and AG-2-2 isolates showed reduction in sclerotial production in vivo compared to AG-4 isolate, which showed an increase in sclerotial production in the presence of herbicides. Sclerotial production was generally higher in vivo than in vitro. The number of sclerotia produced per unit sclerotial weight was often higher in the presence of herbicides. Viability of sclerotia produced in the presence of herbicides was not significantly different from the no-herbicide control.

Nomenclature: Glyphosate; imazethapyr; pendimethalin; Rhizoctonia solani Kuhn [teleomorph Thanatephorus cucumeris (A.B. Frank) Donk] AG-1, AG-2-2, and AG-4; soybean, Glycine max (L.) Merr.