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First Report of Choanephora Rot Caused by Choanephora cucurbitarum on Hosta plantaginea in Korea

Hosta plantaginea (Lam.) Asch. is an herbaceous perennial plant with ornamental value. In August 2013, water-soaked spots and wet rot were found on flowers of H. plantaginea in a garden bedded out for landscaping in Hongcheon County, Korea. Symptoms initially appeared as water-soaked spots at the tips of flowers. Dark gray spots on flower petals often coalesced and led to rotting of flowers, with abundant sporulation. However, no symptoms were found on the leaves. Approximately 30% of the flowers were affected in the landscape bed. A fungal isolate was obtained by plating surface-disinfested diseased flower tissue on potato dextrose agar (PDA). Fungal colonies covering the plate (diam. 90 mm) in 48 h were white at first, with abundant aerial mycelia, but later turned pale yellow with abundant sporangiola. Sporangiophores bearing sporangiola were aseptate, hyaline, and usually arose from infected tissue. Sporangiola were ellipsoid to ovoid, indehiscent, brown to dark brown, pediculate, 7 to 12 μm wide and 9 to 20 μm high, and showed longitudinal striations at high magnification. Sporangia were few-spored to multispored, pale brown to brown, and 50 to 150 μm. Sporangiospores from sporangia were broadly ellipsoid, brown to pale brown, with hyaline polar appendages, 8 to 10 μm wide and 15 to 22 μm high. Zygospores were not observed. The morphological and cultural characteristics, especially based on shape and striation of sporangiola, were identical with those of Choanephora cucurbitarum (Berk. & Ravenel) Thaxt. (2,3). A representative specimen was deposited in the Korea University Herbarium (KUS-F27540). Genomic DNA was extracted using a DNeasy Plant Mini Kit (Qiagen Inc., Valencia, CA). The primers ITS1/ITS4 and NL1/LR3 were used to amplify the internal transcribed spacer (ITS) region of rDNA and the D1/D2 region of the large subunit (LSU), respectively (4). The PCR products were purified and directly sequenced. The resulting 594-bp ITS and 680-bp D1/D2 sequences were submitted to GenBank (Accession Nos. KM200034 and KM200035). A GenBank BLAST search of the fungal database showed that the sequences of ITS and D1/D2 regions matched those of C. cucurbitrarum (JN943006 and JN939195) with 100% similarity. A pathogenicity test was conducted by spraying three healthy potted plants (2 months old) with a sporangiola suspension (2 × 10⁴ conidia/ml). Another three potted plants of the same age were treated with sterile water and served as controls. The plants were kept in humid chambers for 2 days and placed in a greenhouse (28°C and 60 to 80% RH). After 4 to 5 days, water-soaked spots were evident on the flowers of inoculated plants. No symptoms were observed on control plants. A pathogenicity test was conducted twice with the same results, fulfilling Koch's postulates. C. cucurbitarum has a wide host range but has not been previously reported to cause disease on H. plantaginea (1). To our knowledge, this is the first report of C. cucurbitarum on H. plantaginea globally as well as in Korea. Choanephora rot of flowers is an issue under high-moisture conditions, so allowing for adequate airflow and a dry plant canopy should aid in disease suppression.

References: (1) D. F. Farr and A. Y. Rossman. Fungal Databases. Syst. Mycol. Microbiol. Lab. Online publication, ARS, USDA, retrieved July 11, 2014. (2) P. M. Kirk. Mycol. Pap. 152:1, 1984. (3) A. Saroj et al. Plant Dis. 96:293, 2012. (4) G. Walther et al. Persoonia 30:11, 2013.