First Report of Leaf Spot Caused by a Phoma sp. on Schisandra chinensis in Korea

Abstract

Schisandra chinensis (Turcz.) Baill. is a deciduous woody vine native to northern China and the Russian Far East. Its berries have long been used in traditional Asian medicine. In Korea, S. chinensis is one of 10 major medicinal crops and, as of 2011, the production is 6,892 metric tons from 1,749 ha of cultivation area (1). During summer to autumn of 2011 and 2012, leaf spots were observed on S. chinensis (cv. Cheongsun) with disease incidence of 100% in many locations of Jangsu County, Korea. Early symptoms appeared as small, circular, and pale brown spots. Each spot increased in size, became grayish brown and necrotic, and finally developed concentric rings with a definite margin. Some spots coalesced to cover nearly half of the leaves, often becoming torn and giving a shot hole effect. The infected leaf tissue contained blackish pycnidia from which masses of conidia were released in a humid environment. The pycnidia were brown, globose to pyriform, ostiolate, and 45 to 160 μm in diameter. Conidia were hyaline, smooth, oval to ellipsoidal, aseptate or medianly 1-septate, very occasionally 2-septate, slightly constricted at the septa, 4 to 11 × 2.5 to 5 μm, and contained small oil drops. These morphological characteristics were consistent with the generic concept of Phoma (2). Three monoconidial isolates were successfully cultured by diluting conidia mass in sterile water and streaking conidia suspension on potato dextrose agar (PDA). A representative isolate was deposited in the Korean Agricultural Culture Collection (Accession No. KACC47113) and used for pathogenicity test and molecular analysis. Inoculum for a pathogenicity test was prepared by harvesting conidia from 30-day-old cultures (12-h diurnal cycle, 25°C) and a conidial suspension in water (1.1 × 107 conidia/ml) was sprayed onto leaves of three healthy seedlings (cv. Cheongsun). Three seedlings serving as controls were sprayed until runoff with sterile distilled water. The plants were separately covered with plastic bags for 48 h in a glasshouse. After 10 days, typical leaf spot symptoms developed on the leaves inoculated with the fungus. Phoma sp. was re-isolated from those lesions, confirming Koch's postulates. No symptoms were observed on controls. The pathogenicity test was conducted twice. Fungal DNA was extracted, and the complete internal transcribed spacer (ITS) region of rDNA was amplified with the primers ITS1/ITS4 and sequenced directly. The resulting 520-bp sequence was deposited in GenBank (Accession No. KC928322). The sequence showed over 99% similarity with many Phoma species from various substrates, but no exact matches. Phoma leaf spot of S. chinensis was once recorded in Korea without pathogenicity test and culture deposition (3). Phoma glomerata was recorded as a causal fungus of leaf spot disease on S. chinensis in China (4). The Korean isolates differ from P. glomerata in having larger conidia and are separated from it in ITS sequence data. Therefore, we tentatively place the Korean isolates as unidentified Phoma sp. To our knowledge, this is the first confirmed report of leaf spot disease caused by a Phoma sp. in Korea. References: (1) Anonymous. Statistics of Cultivation and Production of Industrial Crops in 2011. Korean Ministry for Food, Agriculture, Forestry and Fisheries. 2012. (2) M. M. Aveskamp et al. Mycologia 101:363, 2009. (3) E. J. Lee et al. Compendium of Medicinal Plant Diseases with Color Plates. Nat. Inst. Agric. Sci., Suwon, Korea. 1991. (4) X. Wang et al. Plant Dis. 96:289, 2012.