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First Report of Alternaria panax Causing Leaf Spot of Aralia cordata in Japan

Udo, Aralia cordata Thumb, Araliaceae, is a traditional Japanese perennial vegetable and used in Chinese herbal medicines. During the last 10 days of July 2008, before the period of flower, leaf spots were observed on udo growing under pine trees in Nasushiobara, Tochigi, Japan. Leaf spots affected more than 40% of the plants. Early symptoms appeared as small, round or irregular, water-soaked, dark brown lesions on the leaves. These areas expanded to 15 to 30 mm in diameter, were irregular and pale brown in the central area and the margin of the lesions were water soaked and dark brown. Later, some lesions coalesced. In continuously wet or humid conditions, conidiophores with conidia appeared on the surface of leaf spots. Conidiophores were medium brown and simple (approximately 70 to 160 × 6 to 8 μm). Well-developed conidia were long-obclavate, base obtuse, straight, yellowish brown, smooth walled, with six to nine transverse septa and three to five longitudinal or oblique septa, constricted at some main septa, some cells easily swelled, conidium body was 72 to 100 × 19 to 34 μm, and the rostra extension was 40 to 90 × 4 to 5 μm. The pathogen was identified as Alternaria panax on the basis of the morphology and sequence of ITS1-5.8s-ITS2 of rDNA. The sequence (GenBank Accession No. FJ607183) exactly matched the sequences of two A. panax (e.g., GenBank Accession Nos. AY898639 and AY898640) (2--4). The fungus was isolated on V8 agar from a single conidium found on symptomatic leaf tissue. Colonies of A. panax were gray-to-black and did not easily produce conidia on the agar. Koch's postulates were performed with the leaves of three branches on a field plant of Aralia cordata. Leaves were inoculated with a mycelial plug harvested from the periphery of a 7-day-old colony; an equal number of leaves on the same plant inoculated with plugs of V8 medium served as the control. All test leaves were covered with plastic bags for 24 h to maintain high relative humidity and incubated at a natural temperature (approximately 24 to 28°C). After 7 days, all inoculated leaves showed symptoms identical to those observed in natural conditions, whereas the controls remained symptom free. Reisolation of the fungus from lesions on inoculated leaves confirmed that the causal agent was A. panax. This species has been previously reported on Aralia cordata in Korea (1). To our knowledge, this is the first report of leaf spots caused by A. panax on Aralia cordata in Japan.

References: (1) W. D. Cho and H. D. Shin. List of Plant Diseases in Korea. 4th ed. Korean Society of Plant Pathology, 2004. (2) E. G. Simmons. Mycotaxon 14:17, 1982. (3) E. G. Simmons. Alternaria: An Identification Manual. CBS Fungal Biodiversity Centre, Utrecht, the Netherlands, 2007. (4) T. Y. Zhang et al. Flora Fungorum Sinicorum: Alternaria. Vol. 16. Science Press (in Chinese), Beijing, 2003.