First Report of Powdery Mildew on Arabis paniculata Caused by Erysiphe cruciferarum in China

The perennial Arabis paniculata is a member of the Brassicaceae widely used as a model species for perennial flowering (Wang et al. 2009). In April 2023, typical powdery mildew symptoms were observed on A. paniculata at Longwen Hill (26°23' N, 106°37' E), Guizhou Normal University, China. Disease incidence reached ~100% among 60 wild A. paniculata plants examined. Mycelia appeared sporadically distributed on leaf surfaces. Hyphal appressoria were lobed, solitary or in opposite pairs. Hyphae measured 5.5–7.5 μm wide, exhibiting branched, straight to flexuous morphology. Conidiophores were erect, straight to slightly flexuous, and 50–125 µm long (n = 30). Foot cells were followed by 2 or 3 cells. Conidia formed singly, ellipsoid, occasionally in chains of two, and measured 30–45 μm long × 14–18 µm wide when shed (n = 50). Based on these morphological characteristics, the powdery mildew fungus (designated GZAA-1) was provisionally identified as an Erysiphe species (Braun and Cook 2012). To confirm the species, the internal transcribed spacer (ITS) and large subunit (LSU) regions were respectively amplified using primer pairs ITS1/ITS4 (White et al. 1990) and NL1/NL4 (Ziemiecki et al. 1990). The obtained 649-bp ITS sequence (GenBank accession no. PV682595) showed 100% similarity (649/649-bp) to previously reported ITS sequences (such as AB522714 and OQ195144) of E. cruciferarum. The obtained 617-bp LSU sequence (PV682641) had 100% similarity (616/616-bp) with LSU sequences (LC009943 and OQ195144) of E. cruciferarum. Sequencing of a second isolate (GZAA-2) confirmed 100% ITS and LSU sequence identity with GZAA-1. Phylogenetic analysis of concatenated ITS (649-bp) and LSU (616-bp) sequences further clustered GZAA-1 within the E. cruciferarum clade alongside reference strains. Pathogenicity tests were conducted by spray-inoculating three healthy potted A. paniculata plants with a suspension of fresh conidia (1 × 10⁵ spores/mL). Three control plants were treated with distilled water. All plants were maintained in a greenhouse at 25 ± 2°C and 80% relative humidity. Typical powdery mildew symptoms developed on inoculated plants after 12–15 days, whereas controls remained asymptomatic. The pathogen reisolated from inoculated plants was morphologically identical to the original field isolate. These tests were repeated three times with consistent results. Collectively, morphological, molecular, and pathogenicity data confirmed the pathogen as E. cruciferarum. While E. cruciferarum has been reported to infect A. paniculata in Romania (Braun 1995), this study represents the first report of E. cruciferarum-induced powdery mildew on A. paniculata in China. These findings expand the known geographical distribution of E. cruciferarum on this host.