First Report of Powdery Mildew Caused by Erysiphe sedi on Crassula arborescens in China

Crassula arborescens (Mill.) Willd. (Crassulaceae) originated from South Africa and is grown as an ornamental plant in Asia. However, it is widely used for the treatment of diarrhea, corns, and epilepsy (Amabeoku et al. 2014). During 2018 to 2019, between October and December, the leaves of C. arborescens were found to be infected with a powdery mildew in a nursery in Qingdao Agricultural University (36.31° N; 120.40° E), Qingdao, China. Two voucher specimens were deposited in the Mycological Herbarium of Qingdao Agricultural University (HMQAU191210 and HMQAU191211). The disease incidence was about 60%. Circular to irregular white powdery colonies were observed on both sides of leaves; as the disease progressed, affected leaves became wrinkled or even fell off the plant. Hyphae were septate, branched, and 3.1 to 7.2 μm wide (n = 50). Hyphal appressoria were well developed and lobed, solitary or in opposite pairs, with diameter of 3.2 to 6.1 μm (n = 50). Conidiophores were cylindrical, with a length of 90 to 110 μm (n = 50). Foot cells of conidiophores were straight, cylindrical, 29 to 47 × 8 to 10 μm (n = 50), followed by one to three cells. Singly produced conidia were variable in shape, oblong-elliptical, subcylindrical or oval, 28 to 50 × 12 to 21 μm (n = 50), devoid of fibrosin bodies. Germ tubes were produced in perihilar position of the conidia, and no chasmothecia were observed. The morphological characteristics showed this fungus was consistent with previous records of Erysiphe sedi (Braun and Cook 2012). To confirm the initial identification of the causal fungus, the internal transcribed spacer (ITS) regions of HMQAU191210 and HMQAU191211 were amplified with universal primers ITS1/ITS4. The resulting ITS sequences were deposited in GenBank (accession nos. MN796072 and MN796070). BLASTn analysis revealed >99% sequence identity with those of E. sedi (accession nos. JX173288, KT748731, and MK411006). A phylogenetic tree was constructed in MEGA7 with 32 ITS sequences using the neighbor-joining (NJ) method, and the NJ tree showed that the isolates from C. arborescens clustered into the E. aquilegiae clade (Takamatsu et al. 2015), closely with E. sedi from several Crassulaceae hosts. Both the morphological characteristics and phylogenetic analysis support the identification of this powdery mildew on C. arborescens as E. sedi. Koch’s postulates were performed by pressing a diseased leaf onto young leaves of three healthy plants; three noninoculated plants served as controls. Plants were maintained in a greenhouse at 25 to 28°C. Symptoms developed 15 days after inoculation, whereas control leaves remained symptomless. White colonies developed similar to those observed on the original diseased plants. E. sedi can infect many crassulaceous plants such as Kalanchoe blossfeldiana, Sedum spectabile, and Bryophyllum pinnatum (Cho et al. 2012; Götz et al. 2019; Kiss and Daughtrey 2001; Tang et al. 2016). To our knowledge, this is the first report of powdery mildew caused by E. sedi on C. arborescens in China. The findings extend the host range of E. sedi. Because this powdery mildew poses a serious threat to the production of C. arborescens, control strategies should be followed.