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First Report of Causing Blue Mold on Stored Apples in Pennsylvania

K. A. Peter, I. Vico, V. Gaskins, W. J. Janisiewicz, R. A. Saftner, W. M. Jurick II
Plant disease 2012 v.96 no.12 pp. 1823
DNA, Penicillium, alkaloids, apples, application rate, conidia, conidiophores, controlled atmosphere storage, cultivars, fruits, fungicide resistance, fungicides, genes, loci, new host records, nucleotide sequences, pathogenicity, polymerase chain reaction, postharvest diseases, resistance mechanisms, sequence analysis, signs and symptoms (plants), spoilage, sporulation, thiabendazole, tubulin, yeasts, Pennsylvania
Blue mold decay occurs during long term storage of apples and is predominantly caused by Penicillium expansum Link. Apples harvested in 2010 were stored in a controlled atmosphere at a commercial Pennsylvania apple packing and storage facility, and were examined for occurrence of decay in May 2011. Several decayed apples from different cultivars, exhibiting blue mold symptoms with a sporulating fungus were collected. One isolate recovered from a decayed ‘Golden Delicious’ apple fruit was identified as P. carneum Frisvad. Genomic DNA was isolated, 800 bp of the 3′ end of the β-tubulin locus was amplified using gene specific primers and sequenced (4). The recovered nucleotide sequence (GenBank Accession No. JX127312) indicated 99% sequence identity with P. carneum strain IBT 3472 (GenBank Accession No. JF302650) (3). The P. carneum colonies strongly sporulated and had a blue green color on potato dextrose agar (PDA), Czapek yeast autolysate agar (CYA), malt extract agar (MEA), and yeast extract sucrose agar (YES) media at 25°C after 7 days. The colonies also had a beige color on plate reverse on CYA and YES media. The species tested positive for the production of alkaloids, as indicated by a violet reaction for the Ehrlich test, and grew on CYA at 30°C and on Czapek with 1,000 ppm propionic acid agar at 25°C; all of which are diagnostic characters of this species (2). The conidiophores were hyaline and tetraverticillate with a finely rough stipe. Conida were produced in long columns, blue green, globose, and averaged 2.9 μm in diameter. To prove pathogenicity, Koch's postulates were conducted using 20 ‘Golden Delicious’ apple fruits. Fruits were washed, surface sterilized with 70% ethanol, and placed onto fruit trays. Using a nail, 3-mm wounds were created and inoculated with 50 μl of a 106/ml conidial suspension or water only as a negative control. The fruit trays were placed into boxes and were stored in the laboratory at 20°C for 7 days. The inoculated fruit developed soft watery lesions, with hard defined edges 37 ± 4 mm in diameter. The sporulating fungus was reisolated from infected tissue of all conidia inoculated apples and confirmed to be P. carneum by polymerase chain reaction (PCR) using the β-tubulin locus as described. Water inoculated control apples were symptomless. Originally grouped with P. roqueforti, P. carneum was reclassified in 1996 as a separate species (1). P. carneum is typically associated with meat products, beverages, and bread spoilage and produces patulin, which is not produced by P. roqueforti (1,2). Our isolate of P. carneum was susceptible to the thiabendazole (TBZ) fungicide at 250 ppm, which is below the recommended labeled application rate of 600 ppm. The susceptibility to TBZ suggests that this P. carneum isolate has been recently introduced because resistance to TBZ has evolved rapidly in P. expansum (4). To the best of our knowledge, P. carneum has not previously been described on apple, and this is the first report of P. carneum causing postharvest decay on apple fruits obtained from storage in Pennsylvania.