During August to October 2012, several cherry packers in central Washington State reported that a significant volume of sweet cherries (Prunus avium) (cvs. Staccato, Sweetheart, and Lapin) were rotten by an unknown fungal pathogen after packing. Of 14 boxes (9 kg per box) of commercially packed cherries rejected by a retailer, the average incidence of the decay was 68%. Initial symptoms on infected fruit appeared as soft, slippery skin with tan discoloration and later skin cracking, epidermal breakdown, and severe pitting were observed. To isolate the causal agent, decayed fruit were rinsed with water, sprayed with 70% ethanol, and air-dried in a laminar hood. After removing the fruit skin with a sterile scalpel, small fragments of fruit flesh between decayed and healthy tissue were cut and placed on potato dextrose agar (PDA) acidified with 0.1% lactic acid. The plates were incubated at 20°C for 7 days and sub-cultured on PDA to obtain pure cultures. The colonies initially appeared white to cream, yeast-like, and later turned to light yellow to pink or brown with age. Conidia were hyaline, smooth-walled, single-celled, and ellipsoidal with variable shape and size. The fungus was identified as Aureobasidium pullulans (de Bary) G. Arnaud based on its morphology (1). The identity of three representative isolates were further confirmed by analysis of nucleotide sequences of the internal transcribed spacer (ITS) regions amplified using the primers ITS1/ITS4. A BLAST search showed that the sequences had 99% homology (E-value = 0.0) with that of A. pullulans deposited at GenBank (Accession No. JF440584.1). The nucleotide sequence of the isolate, A625, has been assigned GenBank Accession No. KF569512. To test pathogenicity, three single-spore isolates were grown on PDA at 20°C. Cultures grown on 10-day-old PDA were flooded with 20 ml of sterile deionized water, and the resulting conidial suspensions were filtered through two layers of cheesecloth and adjusted to 5 × 105 conidia/ml with a hemacytometer. Organic cherry fruit (cv. Bing for isolate A625 and cv. Sweetheart for isolates A755 and A757) were surface-disinfested in 0.6% sodium hypochlorite solution for 5 min, rinsed twice with deionized water, and air-dried. Ten fruit per replicate, four replications per treatment were inoculated with the conidial suspension using a hand sprayer and placed on sterilized wet paper towel in a plastic container. Control fruit were sprayed with sterile water. All fruit were incubated at 22 ± 1°C for 5 days. The experiments were conducted twice. The same symptoms of skin cracking and epidermal breakdown developed on 73% of the inoculated fruit, while no such symptoms appeared on the control fruit. Koch's postulates were fulfilled by re-isolating the fungus from the symptomatic fruit. A. pullulans, a ubiquitous saprophytic fungus on many fruits, has been reported as a causal agent of melting decay in grapes (2). To the best of our knowledge, this is the first report of postharvest fruit rot in sweet cherries caused by A. pullulans.
References: (1) E. J. Hermanides-Nijhof. Aureobasidium and related genera. Pages 141-181 in: The Black Yeasts and Allied Hyphomycetes. Stud. Mycol. No. 15. Centraalbureau voor Schimmelcultures, Baarn, The Netherlands, 1977. (2) D. P. Morgan and T. J. Michailides. Plant Dis. 88:1047, 2004.
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