First Report of Powdery Mildew on <i>Rumex longifolius</i> Caused by <i>Erysiphe polygoni</i> in China

The genus <i>Rumex</i>, a member of the Polygonaceae family, includes approximately 200 edible or medicinal species around the world (Vasas et al. 2015). In China, the <i>Rumex</i> species <i>R. longifolius</i> is widely distributed in all regions, except for southern China. In May 2023, typical powdery mildew symptoms were found on <i>R. longifolius</i> on Longwen Hill (26°23'04'' N, 106°37'57'' E) of Guizhou Normal University, China. The incidence was ~90% among 100 <i>R. longifolius</i> plants observed. The mycelia initially showed a sporadic distribution on the leaf surface of <i>R. longifolius</i>. In the late stage of infection, the mycelia coalesced into extensive patches and covered nearly the entire leaf surface. White colonies occurred mainly on the leaf adaxial side, occasionally on the leaf abaxial side and petiole. Hyphae were flexuous to straight, branched, 3.5-6.5 μm wide. Hyphal appressoria were lobed to multilobed, in opposite pairs or solitary. Conidiophores were erect, straight to slightly flexuous, 60-180 µm long (n = 30). Foot cells were subcylindrical, followed by 2-3 cells. Conidia were ellipsoid to ovoid, 32-50 μm long × 14-25 µm wide (n = 50). Conidia formed singly, occasionally two in a chain. Based on these morphological characteristics, the powdery mildew fungus (named as GZRL-1) was provisionally identified as <i>Erysiphe</i> species (Braun and Cook 2012). To confirm the species, the ITS and LSU regions were amplified using primer pairs ITS1/ITS4 (White et al. 1990) and NL1/NL4 (Ziemiecki et al. 1990), respectively. The obtained 647-bp ITS sequence (GenBank accession no. PQ856357) shared 100% identity and 99.38% query coverage with previously reported ITS sequences (AF011308, LC328322, and LC328323) of <i>E. polygoni</i> (Saenz and Taylor 1999; Abasova et al. 2018). The obtained 618-bp LSU sequence (PQ857798) shared 100% identity and 100% query coverage with LSU sequences (LC328322 and LC328323) of E. polygoni (Abasova et al. 2018). Based on the combined ITS and LSU sequences of <i>Erysiphe</i> taxa (Takamatsu et al. 2015), phylogenetic tree was constructed using MEGA 12. The isolate GZRL-1 was grouped in the same clade as <i>E. polygoni</i>. To perform pathogenicity tests, leaves of three healthy potted <i>R. longifolius</i> plants were inoculated by gently pressing with diseased leaves. Three non-inoculated plants were used as controls. All plants were incubated in a greenhouse 25 ± 2°C, 80% relative humidity. After 15 days, the inoculated plants developed typical powdery mildew symptoms, whereas control plants remained symptomless. The fungus from inoculated <i>R. longifolius</i> plants was morphologically identical to that on originally diseased plants. Similar results were obtained from three independent pathogenicity tests. Based on the morphological and molecular characterization, the powdery mildew fungus was identified as <i>E. polygoni</i>. Although powdery mildew caused by <i>E. polygoni</i> has been reported on <i>R. longifolius</i> (synonym <i>R. domesticus</i>) in Finland, Romania, South Korea, and Sweden (Braun 1995; Cho and Shin 2004), this is the first report of <i>E. polygoni</i> on <i>R. longifolius</i> in China. This work expands the geographical range of <i>E. polygoni</i> on <i>R. longifolius</i> plants.