First Report of Corynespora Leaf Spot on Sweet Potato Caused by Corynespora cassiicola in China

Sweet potato [Ipomoea batatas (L.) Lam.] is an important food and cash crop, and therefore is widely planted in China. In August 2015, a leaf spot disease on sweet potato (cv. Xinnong IV) was observed in Meishan (30°06′57″ N, 104°12′35″ E, elevation 392 m) of Sichuan Province. The disease occurred from mid-August to early September and caused severe yield loss. Leaf symptoms appeared as small gray spots with brown margins at earlier stages, then expanded into suborbicular brown necrotic lesions surrounded by a yellow halo. Subsequently, lesions developed concentric rings with gray and brown bands, and leaves with multiple lesions senesced prematurely. Abundant conidiophores and conidia arose from lesions. Conidiophores were unbranched, brown, single or clustered, 52 to 417 × 4 to 11 µm. Conidia were light brown, obclavate to cylindrical, formed singly or in chains, straight to curved, 28 to 125 × 4 to 18 µm, with 3 to 13 pseudoseptate. Isolates were obtained by single spore isolation. Colonies were gray with thick, white aerial hypha on potato dextrose agar (PDA). To further identify the fungus, the internal transcribed spacers (ITS), β-tubulin, and translation elongation factor (EF-1α) genes were amplified using three corresponding primer pairs: ITS1/ITS4 (5′-TCCGTAGGTGAACCTGCGG and 5′-TCCTCCGCTTATTGATATGC), Bt2a/Bt2b (5′-GGTAACCAAATCGGTGCTGCTTTC and 5′-ACCCTCAGTGTAGTGACCCTTGGC), and EF728F/EF986R (5′-CATCGAGAAGTTCGAGAAGG and 5′-TACTTGAAGGAACCCTTACC), respectively (Cui et al. 2015). Cas2 gene encoding a toxin protein cassiicolin precursor of Corynespora cassiicola was also amplified with specific primer pairs, CasF17/CasR24 (5′-GGATTTGCCTGAGATCCTA and 5′-CAAACAATGCTAACCAAACAAAC) (Déon et al. 2014). ITS, β-tubulin, and EF sequences (GenBank accession nos. KU167046, KU167044, and KU167045, respectively) showed 99 to 100% similarity with C. cassiicola (KT002182, AB539201, and AB539235, respectively), and the Cas2 gene sequence (KU324459) had 88% identity with sequence EF667973 of C. cassiicola. Furthermore, a pathogenicity test was performed on detached leaves and intact plants by a spraying spore suspension (1 × 105 conidia/ml). Controls were sprayed with sterile water. All treatments were incubated at 27°C with alternating light and darkness and high humidity. Typical C. cassiicola-incited symptoms were observed on all inoculated leaves after 7 days, whereas no symptoms developed on control leaves. Three independent experiments were performed with similar results. C. cassiicola was isolated from the inoculated, symptomatic leaves. On the basis of morphological characteristics and molecular identification, the pathogen was identified as C. cassiicola (Berk. & M.A. Curtis) C.T. Wei (Cui et al. 2015; Ellis 1957), which is consistent with previous report in Sri Lanka and Saipan (Dixon et al. 2009; Silva et al. 2000). However, this is the first report of C. cassiicola on sweet potato in China. Considering the serious damage in sweet potato-planting areas of Sichuan as well as the wide host range of C. cassiicola, it is worthwhile to monitor and prevent its spread in other cultivating regions. © 2016, American Phytopathological Society. All rights reserved.