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Effect of Pathogen Aggressiveness and Vinclozolin on Efficacy of Rhodotorula glutinis PM4 Against Botrytis cinerea on Geranium Leaf Disks and Seedlings

Efficacy of the yeast Rhodotorula glutinis isolate PM4 as a biological control agent against 29 isolates of Botrytis cinerea obtained from greenhouse-grown ornamentals was assessed in vitro on geranium leaf disks. Isolates of B. cinerea varied in aggressiveness in the absence of either biological or chemical controls; diameters of lesions produced on leaf disks ranged from 0.8 to 12.3 mm. Efficacy of R. glutinis PM4 against the different isolates of B. cinerea varied greatly; lesion diameters ranged from 0.2 to 10.3 mm when the yeast was present. The yeast significantly reduced lesion development by 16 B. cinerea isolates in each of two replicate trials and by 9 isolates in one of the trials; however, 3 isolates were not inhibited by the yeast on geranium leaf disks. The yeast significantly reduced lesion development by B. cinerea isolate 01, used as a standard for comparison, in four of six trials. Fourteen of the B. cinerea isolates were inoculated onto geranium seedlings and produced a range of lesion sizes (2.9 to 16.4 mm), similar to that produced on leaf disks. Efficacy of the yeast in combination with a reduced rate (0.1×) of the fungicide vinclozolin (50 μg of active ingredient ml^-1) was evaluated on geranium seedlings against 10 isolates of B. cinerea that were resistant to vinclozolin. Addition of vinclozolin to the yeast significantly reduced lesion diameter by five of the isolates compared with diameters of lesions produced in the presence of the yeast alone. Lesions produced by nine of the resistant isolates were 2.6 mm or smaller in both trials on plants treated with the mixture of yeast and vinclozolin. The effect of vinclozolin concentration (0 to 500 μg a.i. ml^-1) on biocontrol efficacy of R. glutinis PM4 was evaluated using three resistant isolates of B. cinerea and geranium seedlings. Disease control was significantly better at higher concentrations of fungicide for two of the isolates. Linear regression of lesion diameter against vinclozolin concentration showed a significant effect on yeast biocontrol efficacy with B. cinerea isolate FL-2-b (y = 6.20 -- 0.63x; r²= 0.74) and isolate BR-1 (y = 4.10 -- 0.32x; r² =0.28) but there was no significant effect with isolate GG-2-b. Overall, PM4 exhibited biocontrol activity on both geranium leaf disks and seedlings against a number of isolates of B. cinerea that varied in aggressiveness. Variability in biocontrol efficacy against isolates resistant to vinclozolin usually was reduced by the addition of vinclozolin.