First report of leaf spot caused by Corynespora cassiicola on Paulownia coreana in Korea

Paulownia coreana Uyeki (syn. Paulownia tomentosa var. coreana), belonging to the family Scrophulariaceae, is a deciduous tree indigenous to Korea (commonly called Korean paulownia) that has been extensively used in the wood industry and folk medicine (Kim et al. 2015). In September 2016, leaf spots were observed on dozens of P. coreana trees growing in a park in Kimcheon (36°07′05″N; 128°08′05″E), Korea. Symptoms on leaves developed as circular to irregular grayish brown necrotic spots varying from specks to mature lesions (5 to 10 mm in diameter), while the stems were asymptomatic. A representative isolate obtained from a leaf lesion was deposited in the Korean Agricultural Culture Collection (KACC48258). The sample used for obtaining the isolate was deposited in the Korea University Herbarium (KUS-F29543). Conidiophores of the fungus observed microscopically on the leaf spots were erect, brown to dark brown, single or in clusters, on both sides of the leaves but mostly on the abaxial surface, and measured 100 to 320 × 5 to 10 μm. Conidia were borne singly or in short chains, obclavate to cylindrical with a conspicuous thickened hilum, straight to slightly curved, pale olivaceous brown, 5 to 12 pseudoseptate, and measured 60 to 200 × 10 to 18 μm. The fungus was identified as Corynespora cassiicola (Berk. & M.A. Curtis) C.T. Wei on the basis of morphological and cultural characteristics (Dixon et al. 2009; Ellis and Holliday 1971). Genomic DNA was extracted using a DNeasy Plant Mini Kit (Qiagen Inc., Valencia, CA). The internal transcribed spacer (ITS) region was amplified with primers ITS1/ITS4, and sequenced. The obtained sequence of 576 bp was deposited in GenBank (KY764323). A BLAST search in GenBank revealed that the sequence showed >99% identity with those of C. cassiicola isolated from many hosts, e.g., Cucumus sativus (JQ595296). To confirm pathogenicity, a conidial suspension (ca. 2 × 104 conidia/ml) was prepared by harvesting conidia from 2-week-old cultures of KACC48258, and the suspension was sprayed onto the leaves of three healthy 2-year-old plants. Three plants treated with sterilized water served as controls. Plants were kept in humid chambers for the first 48 h and thereafter placed in the glasshouse at 24 to 30°C. After 7 days, typical leaf spot symptoms developed on the leaves of all inoculated plants while the controls remained symptomless. The fungus reisolated only from lesions was morphologically identical to C. cassiicola, thus fulfilling Koch’s postulates. Though C. cassiicola is cosmopolitan with a very wide host range (Dixon et al. 2009; Ellis and Holliday 1971), it has been reported only once as a leaf spot pathogen on P. tomentosa, and this occurred in China (Farr and Rossman 2017). According to our field observations in Korea, the disease was commonly found in young trees in the fall, especially following a prolonged period of moist weather. Since the symptoms of Corynespora leaf spot might be confused with those caused by other leaf spot pathogens, e.g., Cercospora spp., Colletotrichum spp., or Sphaceloma spp. on Paulownia leaves (Farr and Rossman 2017), this report would be helpful for a better diagnosis of leaf spot diseases on this host.