First report of choanephora flower blight on dahlia pinnata caused by Choanephora cucurbitarum in Korea

Dahlia pinnata Cav., commonly named garden dahlia, is a perennial herbaceous plant in the family Asteraceae. This plant is most popularly planted for ornamental purposes and developed with many cultivars. In August 2014, Choanephora blight was observed on the flowers of D. pinnata in a botanical garden (37°25′08.9″ N; 126°49′35.5″ E) in Siheung City, Korea. Initially water-soaked lesions appeared on the petals and monosporous sporangiola developed along the edge of the petal. It was causing discoloration of the flowers. Eventually, white mycelia covered the whole flowers and the flowers rapidly withered and rotted, leading to flower blight. However, stems and leaves were not affected by this fungus. A representative specimen was deposited in the Korea University Herbarium (KUS-F28029). A fungal isolate was established by placing monosporous sporangiola, obtained from infected tissue, on potato dextrose agars (PDA). Fungal colonies growing rapidly on PDA were white at the early stage and subsequently turned yellow with abundant sporangiola. Sporangiola were indehiscent, ellipsoid to broadly fusiform, brown to dark brown, longitudinally striate, and 7 to 13 μm wide and 12 to 22 μm high. Sporangia with a few or many sporangiospores were subglobose to globose, pale brown to brown, and 60 to 125 μm in diameter. Sporangiospores from sporangia were broadly ellipsoid to ovoid, brown to pale brown, striate, 8 to 11 μm wide and 14 to 22 μm high, with hyaline appendages at both ends. Based on the morphological and cultural characteristics, this fungus was identified as Choanephora cucurbitarum (Berk. & Ravenel) Thaxt. (Kirk 1984). To confirm the identification, genomic DNA was extracted with the DNeasy Plant Mini Kit (Qiagen Inc., Valencia, CA). The primers ITS1/ITS4 and NL1/LR3 were used for amplifying the internal transcribed spacer (ITS) and the D1/D2 region of the large subunit (LSU) of nuclear ribosomal DNA (rDNA), respectively. The PCR amplicons were purified and sequenced directly. The resulting 594-bp ITS and 680-bp D1/D2 sequences were submitted to GenBank (Accession Nos. KT581012 and KT581013). As the result of the BLAST search, the sequences of ITS and D1/D2 regions showed 100% identity with C. cucurbitarum (JN943006 and JN939195) (Walther et al. 2013). A pathogenicity test was performed by spraying three cut-flowers in a vase of water with a sporangiola suspension (2 × 10⁴ cells/ml). Another three cut-flowers in the same conditions were treated with sterilized water as controls. The flowers were covered with plastic bags for 24 h and then transferred to a greenhouse (28°C and 60 to 80% RH). On three days postinoculation, water-soaked lesions reappeared on the petals of inoculated flowers. No symptoms were observed on controls. C. cucurbitarum was reisolated from inoculated flowers, fulfilling Koch’s postulates. Two species of Choanephora have been reported on the genus Dahlia. C. cucurbitarum was reported on D. pinnata and D. variabilis in Florida and C. infundibulifera was recorded on D. variabilis in Nepal (Farr and Rossman 2015). This is the first report of C. cucurbitarum on D. pinnata in Korea. Though there are no practical strategies for disease control, low density planting would be helpful for reducing the disease.