First Report of Chestnut Ink Disease by Phytophthora katsurae on Chestnut in Korea.

Abstract

Since the late 1970s, chestnut (Castanea spp.) has been one of the most commonly cultivated nuts for human consumption in Korea. In 1998, as much as 100,000 t of chestnut seeds were produced from 79,000 ha of plantations for export (2). Most cultivated chestnuts are hybrids of Castanea crenata and C. mollissima that have resulted in 27 cultivars commonly grown in Korea. In 2004, dead trees of two cultivars (Tsukuba and Ginyose) from different locations were found with black ooze emanating from reddish, sunken tissues on the trunks. When the bark was peeled off, a distinct necrotic region was observed on the basal trunk with the discoloration extending up and girdling the trunk. In November 2006, a Phytophthora species was isolated from the necrotic areas with a CARP selective medium (4). Eleven isolates were obtained from three locations: Hadong, Hapcheon, and Youngqwang. The isolates produced numerous homothallic oogonia (34.0 to 46.2 × 21.9 to 26.7 μm) with warty protuberances on the surface. Antheridia were amphigynous with long, funnel-shaped stalks at the base of the oogonia. The formation of papillate, ovoid to obpyriform sporangia (17.0 to 38.9 × 14.6 to 29.2 μm) was induced by cold treatment in filtered (through 25-μm particle retention) creek water or deionized water. Chlamydospores were not observed. Comparison of the rDNA ITS sequence using ITS1/4 (3) showed 99.6% similarity to P. katsurae (GenBank Accession No. AF266771) with three base pair differences. All eleven isolates showed identical ITS sequences. Pathogenicity studies were conducted on excised three 15-cm-long × 5-cm-diameter logs from each of three chestnut cultivars (Moriwase, Parkmi 2, and Ibuki). Bark (5 mm in diameter) was taken with a cork borer on each log followed by inoculation with a 5-mm agar disk of two isolates, PH.K01 and PH.K07. The logs were placed in a moist chamber for 7 days. Noninoculated controls consisted of log sections from each cultivar treated with agar disks. The pathogencity test was repeated three times. The entire log of cv. Parkmi 2 became necrotic, whereas the other two cultivars developed only small lesions (P < 0.0001). The noninoculated control logs of each cultivar did not develop lesions and no pathogen was reisolated from them. This suggests that cultivars differ in susceptibility. P. katsurae (synonym = P. castaneae) was first described from Japan in 1931. P. cambivora, P. cinnamomi, and P. katsurae are commonly responsible for ink disease on chestnut species in Europe, the United States, and Asia (1). The impact of P. katsurae on chestnut production in Korea is currently unknown.