Cherry (Cerasus avium (Linn.) Moench) is the third most economically important fruit in Yantai, Shandong Province, China. In August 2012, brown spots or necrosis on cherry seedling leaves, with an incidence of 8.2 to 34.3%, were observed in some fields of cherry seedlings in Yantai. Our survey indicated that the economic losses could reach up to 15.3% if disease conditions, such as a cool rainy summer season, were favorable. Conspicuous watery lesions on the stems turned to brown streaks; the leaves all wilted; and finally the plants collapsed. Diseased stem and leaf samples were surface-disinfected in 1% sodium hypochlorite for 1 min, rinsed three times in sterile water, which was absorbed with filter paper, and then transferred to 10% V8 juice agar medium containing 50 μg/ml ampicillin and 5 μg/ml carbendazim (1). The plates were incubated at 22°C in the dark for 5 days. The colonies consisted of white, loose, fluffy aerial mycelia. Eight isolates were obtained, and all were identified as Phytophthora nicotianae based on morphological characteristics and the sequence of the internal transcribed spacer (ITS) region of rDNA. The sporangia were ovoid/spherical, obturbinate with rounded bases and prominent papillae that were 37.5 to 62.5 × 30 to 50 μm (average 46.4 × 37.8 μm, n = 100) in size, with an average length-to-breadth ratio of 1.2. Chlamydospores were terminal, intercalary, and measured 19 to 42 μm (average 30.4 μm), which is typical of P. nicotianae (2). The genomic DNA of the eight isolates was extracted from mycelia. The ITS region of all eight isolates was amplified using primers ITS1 and ITS4, producing specific products that were directly sequenced. The sequence of a representative isolate P1401 (895 bp) was submitted to GenBank (Accession No. KJ754387). It was 100% similar to P. nicotianae strains NV-20T and TARI 22073 (KC768775 and GU111667). To confirm the pathogenicity, at least 10 cherry leaves and new stems were inoculated with mycelial plugs (5 × 5 mm) from each isolate. Necrosis of leaves and stems was observed 4 and 7 days after inoculation, respectively. No symptoms were observed on the control leaves and stems that were inoculated with blank agar plugs. P. nicotianae was re-isolated from the infected leaves, and the ITS sequence was analyzed to confirm its identity. Phytophthora species, such as P. cambivora, P. megasperma, and P. drechsleri, had been previously isolated from cherry (3), but to the best of our knowledge this is the first report of stem rot and leaf necrosis disease caused by P. nicotianae on cherry. Since the economic loss caused by this disease could reach 15% if an outbreak occurred in a rainy summer, control measures should be implemented.
References: (1) Y. Balci et al. Mycol. Res. 112:906, 2008. (2) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. The American Phytopathological Society, St Paul, MN, 1996. (2) S. M. Mircetich and M. E. Matheron. Phytopathology 66:549, 1976.