SUMMARYThe effect of three different concentrations (half, equal and twice of the recommended dose) of three herbicides (trifluralin, dinitramine and fluometuron) usually used to control cotton weeds on the development of Rhizoctonia disease in cotton was studied. Trifluralin and dinitramine at the lower concentrations increased pre-and/or post-emergence damping off. Both herbicides at these concentrations increased the saprophytic activity of the fungus, while they did not affect the susceptibility of the cotton plants. Fluometuron at the highest concentration used was highly phytotoxic and increased the post-emergence damping off. This chemical at the recommended dose caused a drastic reduction in the pre-and post-emergence damping off caused by R. solani. Flnometuron at this concentration decreased both the susceptibility of the host plants and the saprophytic activity of the fungus.

The herbicides EPTC and linuron applied to soil in the field at three different concentrations X2X, 1X, and 2X of recommended dose) decreased post‐emergence, but not pre‐emergence daraping‐off in cotton incited by Rhizoctonia solani. Both herbicides at the two high concentrations significantly reduced wilt incited by Fusarium oxysporum f. sp. vasinfectum. Both herbicides reduced germination of chlamydospores of Fusarium in natural soil, but not in steamed soil. The 2X concentration of EPTC and linuron reduced the saprophytic activity of R. solani in soil. The effects of EPTC and linuron on post‐emergence damping‐off and wilt were attributed Ic their ability to suppress the saprophytic ability of R. solani and chlamydospore germinability ofFusarium in soil, respectively.

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