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Efficacy of Seed Treatment Chemicals for Black Root Rot, Caused by Thielaviopsis basicola, on Cotton

The efficacy of triazole and host resistance--inducing seed treatment chemicals was examined for black root rot on cotton caused by Thielaviopsis basicola in both artificially and naturally infested soils with and without nematodes. In naturally infested soil, myclobutanil was effective in reducing root and hypocotyl discoloration over a wide range of soil population densities. Treatments containing high rates (42 g a.i./100 kg seed) of myclobutanil provided greater reductions in disease than low rates (21 g a.i./100 kg seed) in some experiments. Acibenzolar-S-methyl applied to the seed reduced black root rot or colonization by T. basicola on seedlings in artificially infested soils. Rates of acibenzolar-S-methyl did not differ in efficacy. In controlled studies, root colonization by T. basicola was significantly lower when seeds were treated with both myclobutanil and acibenzolar-S-methyl than with either chemical alone. In naturally infested soil under low (24 CFU/g soil) and high (154 CFU/g soil) populations of T. basicola, a combination of myclobutanil and acibenzolar-S-methyl at the high rate resulted in the lowest root discoloration and colonization. The nematicide seed treatment abamectin improved the control of black root rot in the presence of Meloidogyne incognita. The semi-selective medium TB-CEN allowed the importance of T. basicola to be evaluated in the presence of other pathogens that contribute to the seedling disease complex on cotton by quantifying the isolation frequency and percent colonization of T. basicola.