First Report of Erysiphe magnoliicola Causing Powdery Mildew of Magnolia × soulangeana in China.

Magnolia × soulangeana Soul.-Bod., the saucer magnolia is an important woody ornamental plant cultivated widely in China, UK and USA. In August 2021, symptoms and signs of powdery mildew appeared on leaves of M. × soulangeana at the campus of Nanjing Forestry University (NJFU). The powdery mildew mainly infected young seedlings, with an incidence of 96.8% (436/450 seedlings), and some adult trees also been infected (5/30 trees). The mycelium was amphigenous, thinly effused or conspicuous, forming circular to irregular white patches. Noticeable brown lesions and necrosis occurred in the later stage of infection. Chasmothecia started to develop in October, 2021 and fully matured in early November, 2021. Ten fresh specimens were collected and examined to identify of the pathogen. Photos were taken with a ZEISS Axio Imager A2m microscope, a Zeiss stereo microscope (SteRo Discovery v20), and a scanning electronic microscope (JSM-7600F). Conidiophores arose from the upper part of mother cells, 78.5 ± 11.2× 10.9 ± 1.7 μm (n=30). Foot cells in conidiophores are straight and cylindrical with a constricted basal septum close to hyphal mother cell, 33.6 ± 4.3 × 10.3 ± 1.2 μm (n=30). Conidia were hyaline, ellipsoid to oval, solitary or in chains of two to four, 38.5 ± 3.3 × 18.4 ± 1.0 μm (n=30). Chasmothecia were amphigenous, scattered or aggregated, blackish brown, oblate, 101.1 ± 11.4μm diam. (n=30), with 6-10 appendages. Appendages were aseptate, rarely 1-septate, 5-6 times frequently dichotomously branched; tips were noticeably recurved, brown at the base, 105.1 ± 10.7 × 8.5 ± 1.4 μm (n=30). Asci were 6 to 8 per chasmothecium (n=30), ellipsoid to obovoid or saccate with a short stalk or sessile, 64.2 ± 6.5× 46.1 ± 5.7 um (n=30) in length, 4 to 6 spored. Ascospores were oblong-ovoid, 26.2 ± 1.4 × 13.8 ± 0.7 μm (n=30). Based on the morphological characteristics, the fungus was identified as Erysiphe magnoliicola S.E. Cho, S. Takam. & H.D. Shin. To confirm the causal fungus identity, a representative voucher specimen collected and deposited in herbarium of NJFU (NF50000008) was used for a phylogenetic analysis. Mycelia and conidia were collected from diseased leaves and genomic DNA of the pathogen was extracted. The internal transcribed spacer region (ITS) and large subunit (LSU) loci were amplified with primers ITS1/ITS4 and LR0R/LR05. The resulting sequences were deposited in GenBank (OL454094 for ITS, OM758416 for LSU). BLAST results showed that the ITS sequence was highly similar with a sequence of E. magnoliicola (type) [KJ567072, 614/619 (99.2%)], while LSU sequence was highly similar with E. magnoliicola [KJ567068, 889/891 (99.8%)] and E. magnoliae [JX235969, 903/909 (99.3%)]. Phylogenetic analyses using ITS and LSU sequences with maximum likelihood and Bayesian posterior probability using IQ-TREE v. 1.6.8 and MrBayes v. 3.2.6 placed this fungus in the E. magnoliicola clade. Based on the morphology and phylogeny, the fungus was identified as E. magnoliicola . Pathogenicity tests were carried out on six potted plants of M. × soulangeana . Three seedlings were inoculated by gently pressing the naturally infected leaves onto healthy leaves. Healthy leaves from three other seedlings served as control. Inoculated and control seedlings were placed in separate growth chambers at 23 ± 2°C/16 ± 2°C, 70% relative humidity, with a 16 h/8 h light/dark period. Symptoms developed 10 days after inoculation. The powdery mildew developing on the inoculated seedlings was examined, sequenced and confirmed as E. magnoliicola . The control leaves did not develop powdery mildew. Magnolia × soulangeana is a hybrid of Magnolia denudata × Magnolia liliiflora , both species, as well as M. sieboldii were already known as host plants of E. magnoliicola . This is the first report of powdery mildew caused by E. magnoliicola on M. × soulangeana . This finding provides crucial information for developing effective strategies to monitor and manage this disease.