First Report of Black Spot Disease Caused by Alternaria alternata on Cherry Fruits in China.

Cherry (Cerasus pseudocerasus) has become an economically important fruit in China in recent years. In June 2010, fruit spots were discovered on fruit grown in Dalian City, Liaoning Province, China and 30% of local-orchard trees were infected with the disease, reducing yield and fruit quality. Disease incidence increased up to 75% in 2011. At the initial stage of the infection, some small, light brown spots appeared on the fruit that gradually became round or irregular and dark brown, and a black-brown concentric ring formed in the advanced stage of the infection. As disease progressed, the lesions expanded, causing the fruit surface to become pitted, withered, and dead. The pathogen was isolated from infected fruit of four orchards by a tissue isolation method (1) and cultured on potato dextrose agar (PDA) at 25°C in the dark for one week. Colonies on PDA were initially white and became grayish brown over time. Conidiophores were single or fasciculate, straight or knee curved, gray-brown with regular septa, branched or unbranched, and 12.5 to 90.0 × 2.0 to 5.0 μm. Conidia were oval, obclavate, or obpyriform, brown or dark brown, surface smooth or spinulose with short columnar beaks, and 20.0 to 42.0 × 7.5 to 14.5 μm with three to eight transverse septa and zero to three longitudinal or oblique septa. The sporulation pattern appeared in bush branches. According to the morphology, the pathogen was identified as Alternaria alternata (Fr:Fr.) Keissler (2,3). The internal transcribed spacer (ITS) region of rDNA was amplified using the primers ITS1/ITS4 and the ITS sequence was 99% identical to A. alternata (GenBank Accession No. FJ228163). Pathogenicity tests were performed on detached, asymptomatic fruit. Six fruit were inoculated by placing a PDA plug containing mycelia on the upper surface of the fruit. Another six fruit received sterile PDA plugs. Fruit were cultured in petri dishes with a 12-h photoperiod at 25°C and 90% relative humidity. Black spot symptoms were observed on inoculated fruit but not control fruit after 5 days. The pathogen was reisolated from inoculated fruit and confirmed to be A. alternata. The pathogenicity test was repeated once. A. alternata has a broad host range, but to our knowledge, this is the first report of A. alternata infecting cherries in China. References: (1) Z. D. Fang. Research Methods of Plant Disease, 124, 1998. (2) E. G. Simmons. Alternaria themes and variations. Mycotaxon 37: 79, 1990. (3) T. Y. Zhang et al. Fungi Notes-Genera Alternaria in China, 16:32, 2003.