First Report of Corynespora Leaf Spot on Beach Vitex Caused by Corynespora cassiicola in Korea

Abstract

Beach vitex, Vitex rotundifolia L. fil., is a perennial that grows in temperate and tropical areas of the Pacific. In areas where it has been introduced outside of its native range, beach vitex has proven to be an invasive species. This plant dominates dune ecosystems leading to a reduction in the prevalence of native species (1). In October 2010, previously unknown leaf spots were observed on the beach vitex growing on sand dunes in Incheon City of Korea. The same symptoms were repeated in 2011 and 2012. In September 2012, the same leaf spots were found on the beach vitex in Samcheok and Gyeongju in Korea. The symptoms usually started as small, dark brown to purplish leaf spots with more or less concentric rings, eventually causing leaf blights or yellowing with 50% or more defoliation by the end of September. Representative samples (n = 6) were deposited in the Korea University Herbarium (KUS). Conidiophores of the fungus observed microscopically on the leaf spots were erect, brown to dark brown, single or occasionally in clusters, 80 to 500 × 5 to 9 μm, and mostly arose on the abaxial surface of symptomatic leaves. Conidia were borne singly or in short chains of 2 to 4, ranging from cylindrical to broadest at the base and tapering apically, straight to slightly curved, pale olivaceous brown, 1 to 12 pseudoseptate, 50 to 250 × 8 to 18 μm, each with a conspicuous thickened hilum. On potato dextrose agar (PDA), single-spore cultures of two isolates were identified as Corynespora cassiicola (Berk. & M.A. Curtis) C.T. Wei on the basis of morphological and cultural characteristics (3). Two monoconidial isolates were preserved at the Korean Agricultural Culture Collection (Accession Nos. KACC45712 and KACC46953). Isolate KACC45712 was used for molecular works and pathogenicity test. Genomic DNA was extracted using the DNeasy Plant Mini DNA Extraction Kit (Qiagen Inc., Valencia, CA). The complete internal transcribed spacer (ITS) region of rDNA was amplified with the primers ITS1/ITS4 and sequenced. The resulting sequence of 520 bp was deposited in GenBank (Accession No. KC987359). A BLAST search in GenBank revealed that the sequence showed 100% identity with those of C. cassiicola (e.g., JQ801302). To conduct a pathogenicity test, a conidial suspension (ca. 2 × 104 conidia/ml) was prepared by harvesting conidia from 2-week-old cultures, and the suspension was sprayed onto the leaves of three healthy seedlings. Inoculated plants were kept in humid chambers for 48 h in a glasshouse. After 5 days, typical leaf spot symptoms started to develop on the leaves of all three inoculated plants. C. cassiicola was reisolated from the lesions, confirming Koch's postulates. Control plants treated with sterile water remained symptomless. C. cassiicola is cosmopolitan with a very wide host range (2,4). To our knowledge, C. cassiicola has not been reported on Vitex spp. anywhere in the world. According to field observations in Korea, Corynespora leaf spot was most severe in August and September, especially following a prolonged period of moist weather. C. cassiicola may be a potential biocontrol agent for this highly invasive species.