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First Report of Penicillium expansum Isolates With Reduced Sensitivity to Fludioxonil From a Commercial Packinghouse in Pennsylvania

V. L. Gaskins, I. Vico, J. Yu, W. M. Jurick III
Plant disease 2015 v.99 no.8 pp. 1182
DNA, Penicillium expansum, active ingredients, agar, apples, conidia, fludioxonil, fungal diseases of plants, fungicide resistance, genetic databases, inoculum, molds (fungi), mycotoxins, new geographic records, packing houses, pesticide application, plant pathogenic fungi, plant protection, polymerase chain reaction, postharvest diseases, tubulin, North Carolina, Pennsylvania
Blue mold is caused by Penicillium expansum and is among the most economically significant disease of stored apples worldwide (1). The fungus gains ingress through cracks, natural openings, and wounds in the fruit and produces mycotoxins that contaminate processed apple products. All commercial apples are susceptible to blue mold leaving producers to rely on cultural practices and fungicides for control. Scholar® (Syngenta Crop Protection, Greensboro, NC), introduced in the United States in 2004 to control Penicillium spp. decay on pome fruits, is a protectant, contact postharvest fungicide (active ingredient, fludioxonil) with little to no systemic activity. In February 2012, water samples were obtained from a presizing flume at a commercial facility in Pennsylvania with five isolates identified as P. expansum according to Pitt (2). Conidial suspensions from each isolate were grown in triplicate on Potato Dextrose Agar amended with a discriminatory dose of 0.5 μg/ml technical grade fludioxonil and repeated according to Li and Xiao (1). Four isolates with reduced sensitivity (R) grew, while one sensitive (S) isolate did not. Species-level identity was conducted for a representative R and S isolate via genomic DNA extraction followed by conventional PCR using β-tubulin specific primers according to Sholberg et al (3). MegaBLAST analysis of the 2X consensus amplicon sequences for the S isolate was 100% identical to P. expansum Genbank accession # FJ012853.1 (E value = 0.0), while the R isolate was 100% identical to P. expansum Genbank accession #JN872743.1 (E value 0.0). To determine if Scholar® applied at the labeled rate could control an R or S P. expansum isolate, ‘Golden Delicious’ apples were dipped in Scholar SC® or sterile water, wounded, and inoculated with a 20µl conidial suspension (1x104 conidia ml-1) of either isolate, and stored at 25ºC for 9 days. Twenty apples were used for each experiment which was repeated. Water only inoculated and uninoculated fungicide treated fruit, served as negative controls, were symptomless. Water-dipped P. expansum inoculated fruit had 100% decay while P. expansum inoculated fruit treated with Scholar SC® were symptomless. Conversely, when fruit were wounded, inoculated with either the R or S isolate, and then dipped in the labeled rate of the Scholar SC fungicide®, 78% decay incidence was observed for both isolates 8 dpi. This is the first report of P. expansum with reduced sensitivity to fludioxonil obtained from a commercial facility in Pennsylvania with a history of Scholar® use. Additionally, results from this study show for the first time that pre-sizing water can harbor viable P. expansum inoculum. Repeated exposure to Scholar® by P. expansum isolates with reduced sensitivity to fludioxonil may facilitate the development of resistant isolates able to cause decay on fruit during storage. Findings from this investigation also demonstrate that the timing of postharvest fungicide application is critical for effective blue mold control regardless of isolate sensitivity, and illuminates a potential pathway for Scholar-resistant P. expansum isolates to mainfest in the packinghouse.