Cardamine violifolia, also called Cardamine hupingshanensis, is an economically important medicinal plant renowned for accumulating selenium (Guo et al., 2022). Selenium is an essential trace element with anti-oxidant, anti-inflammatory, anti-cancer, and immune regulatory functions. In July 2023, an outbreak of powdery mildew was detected, infecting the leaves of numerous C. violifolia plants in Enshi (30°11'5.27''N; 109°48'48.45''E), Hubei Province, China. This disease caused severe damage to plant leaves and stems, starting as individual spots and merging into a large mold that covers the entire leaf. It affected nearly 25% all C. violifolia plants, resulting in significant yield loss, disruption of normal metabolism, and premature aging. The lower leaf blades and underside of the leaves were particularly vulnerable. The affected leaves were collected and subjected to morphological diagnostic analysis (Mori et al., 2000) (Fig. S1). The powdery mildew species aggressively spread throughout the leaves, pedicels, and pods, persisting until present and often covering the entire surface. The conidiophores were upright, cylindrical, composed of 3 to 4 cells, and measured 92.3 ± 12.9 × 9.2 ± 0.6 μm (n = 30). Conidial pedicels had 21.6 ± 3.4 μm (n = 50) long cylindrical podocytes. The monoconidia were columnar or barrel-columnar, 30.60-55.59 × 9.11-20.00 μm in size. Conidia lacked an obvious cellulose body. The bud tubes formed from the end of conidia, and papillary appressoria developed on the epiphytic mycelia. ITS region sequences were amplified using the specific powdery mildew universal primers ITS1 (5'-TCCGTAGGTGAACCTGCGG-3'), PM6 (5'-GYCRCYCTGTCGCGAG-3') for partial sequences of 18S and 28S ribosomal DNA genes (Takamatsu et al., 2001). The sequence was deposited in the GenBank under the accession number OR506156 and aligned with available sequences on NCBI, which were 99.2%(528/532) identical to the E. cruciferarum (MT309701, MF192845, and KY660929) sequences (Fig. S2). The ITS sequence from GenBank was used to conduct maximum likelihood phylogenetic analysis using MEGA 11.0. The analysis results showed both the strain and E. cruciferarum clustered on the same branch. To confirm Koch's postulates, pathogenicity testing was carried out using an illuminating incubator. Infected leaves were attached to healthy leaves of C. violifolia seedlings (n=8). All the plants were incubated under 25℃ and >80% relative humidity. After one month, all inoculated plants presented the same symptoms as those initially observed in the field. Morphological and molecular analysis confirmed the isolated fungi's identity as the same pathogen. Therefore, C. violifolia is a suitable host for E. cruciferarum in China. The growers must be informed of these findings to prevent serious economic losses caused by this pathogenic white powder and to prepare for proper management practices. To our knowledge, this is the first report of E. cruciferarum infecting C. violifolia in China.
Keywords: Cardamine violifolia; DNA sequences identification; Erysiphe cruciferarum Morphological character.