Orychophragmus violaceus is an annual or perennial herb in the Brassicaceae family. It is widely planted in China and used as an ornamental and green manure plant (Luo et al. 2022). In September 2022, a survey conducted in a 600 m2 garden in Lanzhou (36°06'N, 103°43'E) found that over 70% of O. violaceus plants were infected with powdery mildew, with 80% of the leaf area on the upper surface of infected leaves was infected. The white colonies on the upper surface of the leaves gradually expanded, thickened, and spread to cover the stem surface. In severe cases, entire foliage withered and the plants died. Fungal structures were taken from the leaves with adhesive tape and placed in sterile water for microscopic observation. 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. Monoconidia were cylindrical or oval in shape, 32.9 ± 6.1 μm long and 15.1 ± 2.1 μm wide (n=80). Conidia lacked an obvious cellulose body. The bud tubes formed from the end of conidia, and papillary appressoria developed on the epiphytic mycelia. Based on these morphological characteristics, the pathogen was initially identified as Erysiphe cruciferarum (Braun et al. 2012). To validate the identity, the internal transcribed spacer (ITS) region of an isolate EYL was amplified by PCR and sequenced using both ITS1/ITS4 and ITS5/PM6 primers (Takamatsu et al. 2001). The resulting sequences were deposited at GenBank (accession nos: OR437967 and OR437969). The ITS sequence of the isolate EYL (OR437967) is 99% (451/453) identical to E. cruciferarum (KP730001) on Brassica napus in China and that of the isolate EYL (OR437969) is 100% (509/509) identical to E. cruciferarum (KM260718) on B. juncea in Vietnam. Pathogenicity experiments were performed on six-week-old plants with an average of 10 ± 0.8 leaves. In the inoculated group, five healthy plants were inoculated by gently pressing the upper surface of diseased leaves against the upper surface of leaves of healthy plants for about 5 to 10 seconds. In the control group, the leaves of five healthy plants were treated with asymptomatic using the same method as described above. The plants were maintained in a greenhouse set at 25℃, 14-h photoperiod, and ≥ 70% humidity. After 13 days, all inoculated plants showed symptoms of powdery mildew, while the plants in the control group had no symptoms. The fungus on the inoculated plant was re-isolated and identified as E. cruciferarum based on its morphological characteristics and ITS sequence. Powdery mildew caused by E. cruciferarum has been reported on Cleome hassleriana in Italy and B. juncea in Australia (Garibaldi et al. 2009; Kaur et al. 2008). To our knowledge, this is the first time that powdery mildew caused by E. cruciferarum have been reported on O. violaceus in China. This disease poses a potential threat to the quality and yield of O. violaceus plants, which may warrant the development of preventative and management strategies in the future.
Keywords: Erysiphe cruciferarum; Orychophragmus violaceus; Powdery Mildew.