Botrytis cinerea Pers.:Fr., is a necrotrophic fungus with a broad host range that causes gray mold on hundreds of plant species (2). Control of gray mold mainly depends on fungicides, including the dicarboxamide iprodione. Thirty-nine diseased blackberry fruit were collected from four orchards in South Carolina and the sensitivity of single-spore isolates to iprodione was examined by Spiral Plater assays (1) on potato dextrose agar (PDA). Briefly, a 5.3 cm long paper strip containing mycelia was placed along the concentration gradient of the PDA and 50% inhibition (EC50 value) was calculated after 2 days of incubation with the Spiral Gradient Endpoint (SGE) software (Spiral Biotech, Norwood, MA). Each isolate was tested in duplicates. Sensitivity ranged from 0.043 to 2.596 μg/ml, with a maximum resistance factor of 60.4. Isolates with EC50 values greater than 2 μg/ml were found in two orchards. Those isolates represented 40 and 7.1% of the total isolates from each orchard. Two isolates with high (EC50 value of 2.596 μg/ml) and low (EC50 value of 0.062 μg/ml) values were chosen to determine the efficacy of iprodione formulated product Rovral 4 Fl (Bayer CropSciences, Research Triangle Park, NC) on detached apple fruit. Fifteen apples were used for each isolate and experiment. Each fruit was wounded on the surface in three locations with a sterile syringe and inoculated with 15 μl of a spore suspension (106 conidia/ml) at the wounded sites. Rovral was applied at the recommended label rate either 24 h before (protective treatment) or 48 h after inoculation (curative treatment). The experiment was conducted three times. Blackberry fruit were not found suitable for this assay because of persistent contamination problems likely from latent infections of a symptomatic fruit. Disease incidence and lesion diameter were recorded 7 days after incubation. Disease incidence following inoculation of the sensitive and resistant isolates on non-fungicide-treated fruit was 100 and 86.7%, respectively. Disease incidence on fungicide-treated apples was 4.4% for the sensitive isolate and 75.6% for the resistant isolate with corresponding mean lesion areas of 0.36 mm and 9.37 mm, respectively. Both isolates were controlled effectively in protective treatments, however, indicating low levels of resistance. To our knowledge, this is the first report of iprodione resistance in B. cinerea from blackberry or any other field-grown crop in South Carolina. This finding adds to a study from 1999 (3) documenting resistance to the dicarboxamide fungicide vinclozolin in B. cinerea collected from ornamentals in South Carolinian greenhouses and suggests that resistance to iprodione needs to be considered in the design of gray mold control strategies in commercial blackberry orchards. No cross resistance between the phenylpyrrole fludioxonil and iprodione was found.
References: (1) H. Forster et al. Phytopathology 94:163, 2004. (2) B. Williamson et al. Mol. Plant Pathol. 8:561. 2007. (3) L. F. Yourman and S. N. Jeffers. Plant Dis. 83:569, 1999.