First Report of Leaf Blight on Dumasia villosa Caused by Boeremia exigua in China

Dumasia (Fabaceae, tribe Phaseoleae, subtribe Glycininae), a genus of trifoliate vines, is widely distributed in tropical and subtropical regions of Asia and Africa (Pan & Zhu 2010). In October 2021, lesions were observed on Dumasia villosa leaves on Longwen mountain of Guizhou Normal University, Guiyang City, Guizhou Province, China. The incidence of leaf blight on observed D. villosa leaves was 10%. All necrotic spots were close to insect-feeding sites. Necrotic spots were grey or black, and circular (2 to 10 mm in diameter) or irregular (2 to 20 mm long) in shape. Stems and pods showed no disease symptoms. Pathogen isolation was conducted following surface sterilization with ethanol. After growth for 5 days on potato dextrose agar in a moist climate chamber at 26°C under a 16 h/8 h light/dark cycle, colonies were 5.2 to 5.6 cm in diameter, olivaceous grey in colour, and sparse, white, aerial mycelia with irregular margins were evident. Conidia were hyaline, ellipsoidal to oblong, mostly 1-septate, and occasionally aseptate with dimensions ranging from 3.5 to 7.0 × 1.5 to 3.5 μm (n = 30). To confirm the species of the isolate, ribosomal DNA internal transcribed spacer (ITS) and β-tubulin (tub2) genes were amplified and sequenced using primers ITS1/ITS4 and Btub2Fd/Btub4Rd (White et al. 1990), respectively. The obtained 529 bp ITS sequence (GenBank accession no. OL872186) shares 99.61% identity with the sequences of Boeremia exigua (MF662797 and GU395499). The obtained 299 bp tub2 sequence (OM830712) shares 100% identity with the sequence of B. exigua (KR653201). Based on morphology and DNA sequence analysis, the isolate was identified as B. exigua. To fulfil Koch's postulates, healthy leaves of 8-week-old D. villosa plants (n = 6) were wounded with a sterilized hypodermic needle and inoculated with 2 μL of a conidial suspension (106 conidia/mL). Six plants were inoculated with 2 μL of sterile distilled water as controls. After 3 days in a moist climate chamber at 26°C under a 16 h/8 h light/dark cycle, dark spots were only present on conidia-inoculated leaves, while controls remained healthy. Boeremia exigua was reisolated from disease spots and confirmed using the same morphological and molecular methods described above. To our knowledge, this is the first report of leaf blight on D. villosa caused by B. exigua, although this fungus has been reported to infect leguminous plants, including field pea (Li et al. 2012), white clover (Wang et al. 2020), and soybean (Schaffrath et al. 2020). In China, B. exigua has also been reported to infect other plants, such as walnut (Cai et al. 2021; Wang et al. 2021) and Japanese ginseng (You et al. 2015). This identification suggests that D. villosa could be a potential reservoir for the pathogen affecting other leguminous crops that might be economically important.