| Abstract |
Echeveria sunyan (Crassulaceae) is a popular succulent plant for balconies and gardens in Asia. During June and September 2019, powdery mildew was observed on the leaves of pot-grown E. sunyan, with an incidence up to 90% in nurseries from two locations: Qingdao Agricultural University (36.31° N; 120.40° E), Qingdao, and Tushan Town (37.06° N; 119.69° E), Laizhou, China. Voucher specimens from Qingdao (HMQAU190622, 24, 25, and 26) and Laizhou (HMQAU190726 and 27) were deposited in the Mycological Herbarium of Qingdao Agricultural University. The infected leaves were initially covered with sparse white mycelium on the upper surface; as the disease progressed, the whole leaf surface was covered with white spores, and the entire leaf gradually turned yellow and underwent premature senescence in later stages, resulting in withering of the whole plants. The symptoms of infected plants from Laizhou were identical to those from Qingdao. Samples from infected plants in both locations were used for morphological characterization and molecular analysis. The hyphal appressoria were well-developed and lobed, produced singly or in opposite pairs, with diameter between 3.5 and 5.7 μm (n = 37). The hyphae were septate, flexuous to straight, branched, and 3.0 to 7.9 wide (n = 40). Conidiophores were straight with a length of 90.8 to 110.2 μm (n = 40). The foot cells were straight, cylindrical, 36 to 46 × 10 to 12 μm (n = 40), followed by one to three cells. Conidia were usually oblong-elliptical, subcylindrical, or oval, 32 to 52 × 13 to 18 μm (n = 50), formed singly and without fibrosin bodies. Simple germ tubes were produced in perihilar position of the conidia, and chasmothecia were not observed. These structures are consistent with the powdery mildew Pseudoidium anamorph of the genus Erysiphe (Braun and Cook 2012), and we tentatively determined the isolates as Erysiphe sp. To confirm the initial identification of the causal fungus, the internal transcribed spacer (ITS) regions and 28S rDNA of samples were amplified with universal primers ITS1/ITS4 and NLS2/TW14, respectively. The resulting ITS and 28S sequences were deposited in GenBank (MN147817, MN512449, MN511727, MN511726, MN493831, MN493832; MN314843, MN512448, MN511728, MN512451, MN508610, MN508611). Six ITS and 28S sequences were identical to each other. Phylogenetic trees were constructed in MEGA 7 using the neighbor-joining (NJ) method with 38 ITS sequences and 28 28S rDNA sequences, respectively. The NJ trees showed that the isolates from E. sunyan belong to the E. aquilegiae clade, as defined by Takamatsu et al. (2015) on the basis of ITS and 28S rDNA datasets, with 99% bootstrap value, respectively. The phylogenetic position supports the morphological observation (genus Erysiphe). Inoculation testing was performed to confirm the pathogenicity by pressing a diseased leaf onto young leaves of three healthy plants. Three noninoculated plants were used as controls. Plants were maintained in a greenhouse at 25 to 28°C. White powdery mildew colonies appeared 8 days after inoculation, and no symptoms developed on the control leaves. Leaves of inoculated plants began to wither 20 days after inoculation. The symptoms were similar to those observed on the original diseased leaves. The pathogenicity tests were performed twice with similar results. Records of powdery mildews on Echeveria spp. have been found in India and the United Kingdom (Baiswar et al. 2009; Farr and Rossman 2019; Henricot 2008), and the ITS sequence of Chinese isolates from E. sunyan showed 99% sequence identity with Erysiphe sp. isolates affecting Echeveria spp. in the United Kingdom. This is the first report of powdery mildew caused by Erysiphe sp. on E. sunyan in China. Because this powdery mildew may cause losses of the ornamental value of E. sunyan, thereby causing losses in selling of nursery plants and commercial value, control strategies should be developed to mitigate possible outbreaks of this disease in nurseries and gardens.
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