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First Report of Leaf Blight Caused by Phomopsis ipomoeae-batatas on Sweet Potato in Korea

Ipomoea batatas (L.) Lam., belonging to the Convolvulaceae, is widely cultivated and used as an industrial resource as well as for food and feed worldwide (2). In September 2010, an unknown leaf blight was observed on leaves in Hoengseong County and Jecheon City in Korea. Symptoms were mostly observed in older leaves as cream to tan-brown lesions surrounded by purplish brown-to-dark brown margin. Each lesion was circular to irregular, not exceeding 10 mm, but coalesced to form larger lesions. Necrotic tissue fell out giving rise to shot-holes. A number of black pycnidia were present in the lesions of diseased leaves. The same symptoms were observed at several localities in Korea during 2010 and 2011 seasons. The voucher specimens (n = 5) were preserved in the Korea University Herbarium (KUS). Two isolates were obtained from the two samples (KUS-F25274 and KUS-F25361) and deposited in the Korean Agricultural Culture Collection (Accession Nos. KACC45680 and KACC45702). Pycnidia were amphigenous, but mostly epigenous, scattered, dark brown-to-rusty brown, globose, embedded in host tissue or partly erumpent, 110 to 170 μm in diameter, and with an ostiole of 25 to 40 μm in diameter. Alpha conidia were aseptate, lageniform, biguttulate, hyaline, and 5.5 to 8.0 × 3.5 to 4.5 μm. Beta conidia were absent. Based on the morphological characteristics, the fungus was consistent with Phomopsis ipomoeae-batatas Punith. (1,3). Preliminary identification of the fungal isolate was confirmed by molecular data. Genomic DNA was extracted from the two isolates. The D1/D2 region of 28S rDNA was amplified using the primers LROR and LR7, and sequenced. The resulting sequences of the two isolates were identical to each other, and were deposited in GenBank (Accession Nos. JX157848 and JX157849). A BLAST search showed that there was no matching sequence of P. ipomoeae-batatas. Therefore, these were the first 28S sequences for the species submitted to GenBank. The present sequences showed >98% similarity with 24 entries of Phomopsis spp. and Diaporthe spp. (teleomorph of Phomopsis spp.), indicating their close phylogenetic relationship. Pathogenicity was tested by spraying leaves of three potted plants with a conidial suspension (2 × 10⁶ conidia/ml), which was harvested from a 3-week-old culture on potato dextrose agar. Control leaves were sprayed with sterile water. The plants were placed in a dew chamber at 24°C in darkness and continuous dew for the first 24 h and then moved to a greenhouse bench. After 10 days, leaf blight symptoms that were identical to those observed in the field started to develop on the leaves inoculated with the fungus. No symptoms were observed on control plants. P. ipomoeae-batatas was reisolated from the lesions of inoculated plants, confirming Koch's postulates. Occurrence of leaf blight caused by P. ipomoeae-batatas on sweet potato has been reported in many countries (1,3). To our knowledge, this is the first report of the disease in Korea. The economic losses are of minor importance, because the disease is mostly present toward the end of growing season; however, attention must be paid considering that the pathogen may reduce the quality of vines used as fodder.

References: (1) C. A. Clark and J. W. Moyer. Compendium of Sweet Potato Diseases. The American Phytopathological Society. St. Paul, MN, 1988. (2) I. G. Mok et al. J. Plant Biotechnol. 36:202, 2009. (3) E. Punithalingam. Phomopsis ipomoeae-batatas. IMI Descriptions of Fungi and Bacteria. Sheet 739, 1982.