First report of powdery mildew caused by Erysiphe buhrii on Gypsophila capituliflora in China

Gypsophila capituliflora Rupr. (purplehead baby’s breath), a perennial herb, is native to the desert region of northwestern China and Central Asia. It is valuable for ornamental use and preventing land erosion (Mao 1996). In August 2017, during a survey of diseases of desert plants, severe outbreaks of powdery mildew infections with approximately 90% disease incidence on G. capituliflora were observed in the desert region of Alashankoy City, Xinjiang, China. Symptoms first appeared as thin white patches on needle-like leaves, which subsequently turned yellow and necrotic, and resulted in permanent death of plants. Three representative voucher specimens were deposited in the Mycological Herbarium of Tarim University (HMUT6449.1 to 6449.3). Appressoria were well-developed, multilobed or slightly lobed, solitary or opposite in pairs. Conidiophores arose from the top of the mother cell and produced single conidia. Foot cells were cylindrical, straight, or slightly flexuous at the base, 27 to 58 × 5 to 12 μm (n = 30), and were followed by one to three mostly short cells. Conidia were cylindrical to oblong-ellipsoid, 35 to 56 × 12.5 to 18 μm (n = 50), without fibrosin bodies, and showed angular/rectangular wrinkling of outer walls. Germ tubes were produced on the terminal or subterminal position of conidia with lobed appressoria. Chasmothecia were gregarious or scattered, dark brown, and 85 to 195 μm in diameter (n = 30). Appendages were septate, usually irregularly branched, arising from the lower half of the chasmothecia, 35 to 210 μm in length (n = 30). Asci were sessile or short-stalked in shape, 45 to 87 × 23 to 48 μm, and mostly contained two to six colorless ascospores, ellipsoid-ovoid to subglobose, with dimensions of 15 to 30 × 9 to 21 μm (n = 30). The specific measurements and morphological characteristics were consistent with those of Erysiphe buhrii U. Braun (Braun and Cook 2012; Choi et al. 2016). To confirm the identification, the internal transcribed spacer (ITS) and 28S rRNA gene of the three deposited isolates (HMUT6449.1 to 6449.3) were amplified and sequenced following the method of Meeboon and Takamatsu (2015) and deposited in GenBank (accession nos. MH256036 to MH256038). An NCBI-BLAST search indicated that these sequences showed 100% identity of the ITS (559/559 nucleotides) and LSU (805/805 nucleotides) with E. buhrii on G. paniculata (LC009959) and 99.3% identity of the ITS (558/562 nucleotides) with E. buhrii on G. oldhamiana (KU945278). Based on phylogenetic trees generated from maximum likelihood and Bayesian analysis of the combined alignment of ITS and 28S rRNA, the three strains clustered with E. buhrii (MUMH 0787) in an independent clade with 100% bootstrap support value and posterior probability of 1. The pathogenicity test was confirmed by Koch’s postulates. Leaves of six healthy potted plants were gently pressed against diseased leaves for inoculation. Three noninoculated plants served as controls. Each plant was kept in a separate plastic humid chamber in a greenhouse at 25 to 30°C for 7 days. Typical symptoms of powdery mildew, morphologically identical to the original observations on diseased plants, developed on the inoculated plants. No symptoms were observed on the control plants. E. buhrii has been reported on Gypsophila spp. in Hungary, Romania, Ukraine, Turkey, USSR, Argentina, Korea, and Mongolia (Farr and Rossman 2020). To our knowledge, this is the first record of E. buhrii on G. capituliflora. Considering G. capituliflora is one of the major plant species in the local ecosystem, the occurrence of the powdery mildew disease poses a serious threat to the fragile ecological systems of the desert area, and it may continue to accelerate the process of desertification in Xinjiang, China.