First Report of Powdery Mildew on Berchemia floribunda Caused by Erysiphe berchemiae in China.

Abstract

The genus Berchemia (family Rhamnaceae), a group of climbing plants, is mainly distributed in Asia, Africa, and South America. Berchemia plants are widely used in traditional medicine in some Asian countries (Inoshiri et al. 1987). For example, in Japan, B. racemosa (synonym B. floribunda) is used for the treatment of gallstones, liver diseases, neuralgia, and stomach cramps, and in China, B. floribunda is used for the treatment of rheumatism and lumbago. In August 2022, typical powdery mildew symptoms were observed on wild B. floribunda plants in Huaxi District, Guiyang, Guizhou Province, China. The incidence was approximately 60% among 100 B. floribunda plants observed outdoors. White colonies almost entirely covered on both adaxial and abaxial surfaces of all leaves on symptomatic plants. Infected leaves appeared curled or chlorotic, infection occasionally leading to defoliation. To describe the pathogen morphologically, fungal samples were collected from two individual B. floribunda plants and microscopically characterized. In these samples, hyphae were flexuous to straight, branched, septate, 3-6 μm wide, with lobed hyphal appressoria. Conidiophores were erect, flexuous to straight, and 50-160 µm long (n = 30). Foot cells were subcylindrical to slightly curved-sinuous at the base, 20-40 µm long (n = 30), followed by 2-3 shorter cells. Conidia formed singly, occasionally 2-3 in a chain. Conidia were ellipsoid to ovoid, 20-42 × 12-18 µm (n = 50), without fibrosin bodies. Chasmothecia were not found. For molecular identification, the ribosomal DNA internal transcribed spacers (ITSs) of the two fungal samples were amplified and sequenced using the ITS1/ITS4 primer pair (White et al. 1990). The obtained 649-bp ITS sequences (GenBank accession nos. OR414364 and OR414365, respectively) shared 100% identity, and they showed 99.52% identity with the ITS sequence (GenBank accession no. LC009934) of Erysiphe berchemiae. Phylogenetic analysis grouped OR414364 and OR414365 in a clade with LC009934. Based on morphological and molecular characteristics, the powdery mildew fungus from B. floribunda was identified as E. berchemiae (Braun and Cook 2012). The voucher specimen (accession no. GZNU-BFEE/0820/2022) were deposited at the School of Life Sciences, Guizhou Normal University. Pathogenicity was assessed by gently pressing naturally diseased leaves of B. floribunda onto leaves of three healthy potted 1-year-old B. floribunda plants. Three non-inoculated healthy plants were used as controls. The plants were incubated in a greenhouse at 25 ± 2°C, 80% relative humidity. Similar powdery mildew symptoms were observed on the inoculated plants 9 days after inoculation, whereas the control plants remained symptomless. The reisolated fungus from inoculated leaves was morphologically identical to that observed on the original diseased leaves, and the ITS sequence of the reisolated fungus shared 100% identity with OR414364 and OR414365, fulfilling Koch's postulates. E. berchemiae has previously been described as a powdery mildew pathogen on B. yunnanensis (Chen et al. 1987) and B. kulingensis (Chen 1993) in China and B. racemosa (synonym B. floribunda) in Japan (Braun and Cook 2012; Takamatsu et al. 2015), but this is the first report of E. berchemiae causing disease of B. floribunda in China. This work suggests that E. berchemiae is an important pathogen of Berchemia plants, at least for some species in the genus Berchemia.