Combinations of Fungicides with Phylloplane Yeasts for Improved Control of Botrytis cinerea on Geranium Seedlings

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Abstract
Control of Botrytis cinerea on geranium seedlings was evaluated in treatments with phylloplane yeasts in combination with 10 fungicides used to manage Botrytis blight of ornamental plants. Rhodotorula glutinis PM4 significantly reduced the development of lesions caused by B. cinerea on geranium cotyledons; however, yeast biocontrol efficacy was highly variable between trials. Treatment with the yeast in combination with azoxystrobin or trifloxystrobin at one tenth the labeled rate (7.5 μg a.i. ml-1) or the full labeled rate (7.5 μg a.i. ml-1) reduced lesion development, compared to treatment with the yeast or the fungicide alone. Vinclozolin at half the labeled rate or the full labeled rate (250 or 500 μg a.i. ml-1), in combination with R. glutinis PM4, significantly reduced the development of lesions caused by an isolate of B. cinerea resistant to vinclozolin. Copper hydroxide and iprodione at one-tenth the labeled rates, with or without yeast, were highly effective in limiting lesion development. Mancozeb did not increase the biocontrol efficacy of the yeast, and thiophanate-methyl negatively affected the yeast efficacy. Improved disease control was observed in treatments with vinclozolin at the labeled rate and R. glutinis PM4 at cell densities of 5 × 105 and 1 × 106 cells ml-1, but not 1 × 105 cells ml-1, on seedlings co-inoculated with B. cinerea in a suspension containing 1 × 105 conidia ml-1. Disease control improved in treatments with combinations of vinclozolin and eight other isolates of R. glutinis, two isolates of R. graminis, and two isolates of R. mucilaginosa. Biocontrol was not observed in treatments with two isolates of R. minuta. The combination of yeast and vinclozolin significantly reduced the germination of conidia of B. cinerea and the growth of R. glutinis PM4 in vitro. All combinations of R. glutinis PM4 with azoxystrobin, trifloxystrobin, or vinclozolin provided highly effective and consistent disease control not observed in treatments with the fungicides alone or the yeast alone.