Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 29, 2012
Publication Date: December 1, 2012
Citation: Peter, K.A., Vico, I., Gaskins, V.L., Janisiewicz, W.J., Saftner, R.A., Jurick II, W.M. 2012. First report of Penicillium carneum causing blue mold on stored apples in the United States. Plant Disease. 96:1823. Interpretive Summary: Apples are kept in cold storage from 9 months and up to 1 year to ensure that fruit are available year round for consumption. While in storage, apples are highly susceptible to blue mold decay which is caused by multiple Penicillium species. In order to understand which species cause blue mold, apples displaying symptoms consistent with blue mold were collected from a packinghouse in Pennsylvania. During this process, an isolate of Penicillium carneum was found, which is a previously undescribed pathogen of apple fruit. The isolate was identified using nucleic acid technology coupled with growth on diagnostic media, and examining microscopic features of the fungus. This research provides new knowledge for plant diagnosticians, extension agents, and plant pathologists to design detection and diagnosis methods for this emerging Penicillium species to ensure timely implementation of disease control methods.
Technical Abstract: 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 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, the ß-tubulin locus was amplified using gene specific primers and sequenced. The recovered nucleotide sequence indicated 99% similarity to P. carneum and was deposited into Genbank. P. carneum colonies strongly sporulate and have a blue green color on 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 display 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, both of which are diagnostic characters of this species. The conidiophores were hyaline and tetraverticillate with a finely rough stipe. Conida were produced in long columns, blue green, globose, and measured on average 2.9 µm in diameter. To prove pathogenicity, Koch’s postulates were conducted using 20 ‘Golden Delicious’ apple fruits. Fruits were washed and surface sterilized with 70% ethanol. 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. Apples were stored at 20°C for seven 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 by PCR to be P. carneum. Water inoculated control apples were symptomless. Originally grouped as a variety of P. roqueforti, P. carneum was reclassified in 1996 as a separate species. P. carneum is typically associated with meat products, beverages, and bread spoilage and produces patulin, which is not produced by P. roqueforti. Our isolate of P. carneum grew on PDA amended with the fungicide thiabenzamidazole (TBZ) below 250 ppm, which is well below the labeled application rate. The susceptibility to TBZ suggests that this P. carneum isolate has been recently introduced because resistance to TBZ has evolved rapidly in P. expansum. 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.