Curvularia spicifera causing black rot on Ipomoea batatas in China.

Abstract

Ipomoea batatas (sweet potato) is an annual herb originating from South America and the large and small Antilles. It is widely cultivated in tropical and subtropical regions around the world and is widely grown in most parts of China. As an edible plant with rich nutrition, I. batatas has high economic and medicinal value (Suhendy et al. 2023). In September 2023, black rot-like disease signs and symptoms were observed on the roots of I. batatas in a farmland (about 6667 m2) located in Kaifeng city, Henan Province, China. The roots showed irregular brown or dark spots on the surface, extending to the internal center, and brown to black necrosis. Additionally, above-ground parts of infected plants showed symptoms such as yellowing and wilting of leaves, brown spots on stems, and stunted growth. Around 80 % of monitored plant roots (n = 200) exhibited the symptoms. Infected roots were cut into pieces and then placed on potato dextrose agar (PDA) (light/dark, 16 h/8 h; temperature, 18°C). After a period of 3-10 days, single hyphal tips of each fungal colony were placed on PDA and incubated for another 5-10 days (Paul et al. 2021). Colonies of the fungal pathogen on PDA reached 50 mm in diameter within 7 days, dark gray on the inner side, dark brown extending to the edge, irregular round edge, with abundant aerial mycelium, cotton-like, irregularly convex upward, undulating. The conidia were brown, ellipsoid to oval, 8 to 19 × 3 to 6 μm (n = 50). Morphologically similar isolates with characteristics consistent with those of Curvularia spicifera (Cui et al. 2020) were recovered from 87% of symptomatic root tissues (n=100). The internal transcribed spacer (ITS) region and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene from three independent isolates (CSZM202101, CSZM202102, and CSZM202103) of the fungus were amplified and sequenced with primers ITS1/ITS4 (Seliger et al. 1990) and GAPDH1/GAPDH3R (Bradshaw et al. 2022) according to a previously reported method (Zhu et al. 2022). The resulting sequences were deposited in GenBank (Accession No. OR885691, PV056889, PV056568, PQ839726, PV072835 and PV02836). BLASTn analysis showed that the ITS and GAPDH sequences had 100% (ITS, 516/516; GAPDH, 508/508) identity with C. spicifera (OQ845826 and CBS 274.52 JN192387) from maize (Ram et al. 2024) and pearl millet (S. et al. 2024), respectively. The phylogenetic analysis clearly illustrated that these isolates clustered with the sequences of a representative reference strain of C. spicifera(CBS 274.52, GenBank accession number JN192387). Therefore, the morphological, molecular and phylogenetic analysis indicated that the pathogen was C. spicifera. To complete Koch's postulates, pathogenicity experiments were carried out by inoculating spore suspension (106 spores mL-1) into the wounded roots (n=10) of I. batatas plants. Sterile distilled water treated wounded roots (n=10) served as control. One to two days after inoculation, visible mycelia were produced at the inoculation sites of I. batatas. Inoculated roots showed black rot signs 21 days post-inoculation; controls were unaffected. This was confirmed in three repeated pathogenicity tests. To our knowledge, this is the first report of black rot caused by C. spicifera on I. batatas in China. The emergence of the black rot pathogen could harm valuable food crops and reduce agricultural productivity in China. Accurate identification of the black rot fungus is essential for devising effective disease management strategies and supporting future control of C. spicifera in China.