First report of choanephora flower blight caused by Choanephora cucurbitarum on Althaea officinalis in Korea

Marsh mallow (Althaea officinalis L., Malvaceae) is used both as a medicinal and an ornamental plant (Shah et al. 2011). In August 2014, Choanephora blight was observed on flowers of A. officinalis in an experimental plot of Jeollabuk-do Agricultural Research and Extension Services (35°56′38.44″ N, 126°59′37.14″ E) in Iksan, Korea. Initially water-soaked lesions appeared on the petals and monosporous sporangiola developed along the edge of the petal, causing discoloration of the flowers. Eventually the flowers were rapidly withered and rotted, leading to flower blight. However, no symptoms were found on the stems and leaves. Approximately 20% of the flowers on plants were affected in the beds. A representative specimen was deposited in the Korea University Herbarium (KUS-F29113). A fungal isolate was established by placing monosporous sporangiola, obtained from infected tissue, on potato dextrose agars (PDA). Fungal colonies covering the plate (diam. 90 mm) in 48 h were white at first with abundant aerial mycelia, and subsequently turned pale yellow with abundant sporangiola. Sporangiola were indehiscent, ellipsoid to broadly fusiform, brown to dark brown, longitudinally striate, and 8 to 12 μm wide and 12 to 25 μm high. Sporangia with a few or many sporangiospores were subglobose to globose, pale brown to brown, and 55 to 145 μm in diameter. Sporangiospores from sporangia were broadly ellipsoid to ovoid, brown to pale brown, striate, 8 to 12 μm wide and 16 to 22 μm high, with hyaline appendages at both ends. Zygospores were not observed. Based on these characteristics, the 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 and sequencing the internal transcribed spacer (ITS) region and the D1/D2 region of the large subunit (LSU) of nuclear ribosomal DNA, respectively (Walther et al. 2013). GenBank BLAST analysis of the D1/D2 sequence showed that the sequence (KU316935) belonged to the genus Choanephora, whereas the ITS sequence (KU316934) identified the species as C. infundibulifera f. cucurbitarum (syn. C. cucurbitarum) based on 100% identity to accessions KM200034 (ex Hosta plantaginea) and KP406599 (ex Phlox paniculata) and >99% similarity to accession JN206234. 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 2 to 3 days postinoculation, water-soaked lesions appeared on the petals of inoculated flowers. No symptoms were observed on controls. C. cucurbitarum was reisolated from inoculated flowers, fulfilling Koch’s postulates. C. infundibulifera as well as C. cucurbitarum have been reported on many malvaceous plants, but not on Althaea species (Farr and Rossman 2015). This is the first report of C. cucurbitarum on A. officinalis globally as well as in Korea.