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First Report of Alternaria tenuissima Causing Postharvest Decay on Apple Fruit from Cold Storage in the United States

Apples are grown and stored for 9 to 12 months under controlled atmosphere conditions in the United States. During storage, apples are susceptible to various fungal pathogens, including several Alternaria species (2). Alternaria tenuissima (Nees) Wiltshire causes dry core rot (DCR) on apples during storage and has recently occurred in South Africa (1). Losses range widely, but typically occur at 6 to 8% annually due to this disease (2). In February 2013, ‘Nittany’ apples with round, dark-colored, dry, spongy lesions were obtained from wooden bins in a commercial cold storage facility located in Pennsylvania. Symptomatic fruits were transported to the lab, rinsed with sterile water, and the lesions were sprayed with 70% ethanol until runoff and wiped dry. The skin was aseptically removed with a scalpel, and asymptomatic tissue was placed onto potato dextrose agar (PDA) and incubated at 25°C. Two single-spore isolates were propagated on PDA and permanent cultures were maintained as slants and stored at 4°C. The fungus produced a cottony white mycelium that turned olive-green to brown with abundant aerial hyphae and had a dark brown to black reverse on PDA. Isolates were identified as Alternaria based on conidial morphology as the spores were slightly melanized and obclavate to obpyriform catentulate with longitudinal and transverse septa attached in unbranched chains on simple short conidiophores. Conidia ranged from 10 to 70 μm long (mean 27.7 μm) and 5 to 15 μm wide (mean 5.25 μm) (n = 50) with 1 to 6 transverse and 0 to 2 longitudinal septa. Conidial beaks, when present, were short (5 μm or less) and tapered. Mycelial genomic DNA was extracted, and a portion of the histone gene (357 bp) was amplified via gene specific primers (Alt-His3-F/R) using conventional PCR (Jurick II, unpublished). The forward and reverse sequences were assembled into a consensus representing 2× coverage and MegaBLAST analysis showed that both isolates were 100% identical to Alternaria tenuissima isolates including CR27 (GenBank Accession No. AF404622.1) that caused DCR on apple fruit during storage in South Africa. Koch's postulates were conducted using 10 organic ‘Gala’ apple fruit that were surface sterilized with soap and water, sprayed with 70% ethanol, and wiped dry. The fruit were aseptically wounded with a nail to a 3 mm depth, inoculated with 50 μl of a conidial suspension (1 × 10⁴ conidia/ml), and stored at 25°C in 80 count boxes on paper trays for 21 days. Mean lesion diameters on inoculated ‘Gala’ apple fruit were 19.1 mm (±7.4), water only controls (n = 10 fruit) were symptomless, and the experiment was repeated. Symptoms observed on artificially inoculated ‘Gala’ apple fruit were similar to the decay observed on ‘Nittany’ apples from cold storage. Based on our findings, it is possible that A. tenuissima can cause decay that originates from wounded tissue in addition to dry core rot, which has been reported (1). Since A. tenuissima produces potent mycotoxins, even low levels of the pathogen could pose a health problem for contaminated fruit destined for processing and may impact export to other countries. To the best of our knowledge, this is the first report of alternaria rot caused by A. tenuissima on apple fruit from cold storage in the United States.

References: (1) J. C. Combrink et al. Decid. Fruit Grow. 34:88, 1984. (2) M. Serdani et al. Mycol. Res. 106:562, 2002. (3) E. E. Stinson et al. J. Agric. Food Chem. 28:960, 1980.