First Report of Erysiphe adunca Causing Foliar Powdery Mildew on Poplar in China

Abstract

Hybrid poplars are important economic plants and sources of biofuel all over the world. Poplars are subjected to a wide range of diseases (Cellerino 1999). In September 2019, powdery mildew was observed on the hybrid poplar Populus davidiana × P. alba var. pyramidalis, a fast-growing elite poplar cultivar widely planted in the northeast of China, in one poplar nursery room in the Northeast Forestry University in China (45.7266°N, 126.6380°E). Disease incidence during September and October was only 5 to 15% but gradually increased to 100% during November and December. The symptom of white circular powdery spots with diameters of up to 12 mm appeared on the upper surface of the mature leaves of poplar plants over 40 days old. Under nursery conditions (50 to 65% humidity, 20 to 25°C, and 16-h/8-h day/night cycles), the diameter of a mildew spot could develop from merely visible to 10 mm in 3 days. The mildew site was observed with a stereomicroscope (Leica DVM6S, Germany). A single fungal species demonstrating characteristics of the obligate pathogens Erysiphe spp. was observed. The conidiophores grew straight upward on the leaf surface until reaching 102 to 165 μm in length (n = 30). Then, even-looking, ellipse-shaped conidia were developed, forming a singular, curved string of up to tens of conidia, on top of each conidiophore. Conidia (n = 30) measured 29.4 to 35.1 × 18.1 to 21.0 μm. Appressoria were lobed or nipple-shaped and appeared in pairs or singular. The germ tubes were slightly curly unbranched tubes pointing at one direction near the long axis of the conidium. We carefully collected the mildew by scraping it off the upper surfaces of fresh diseased leaves with a scalpel and extracted the genomic DNA. The internal transcribed spacer rDNA region was amplified according to the method of Siahmard et al. (2017). The resulting nucleotide sequence (deposited in GenBank as MN998572) displayed 99.84% identity with that of Erysiphe adunca (KY653179.1). Hence, we identified the causal pathogen of this poplar powdery mildew as an E. adunca strain. Furthermore, an Mcm7 sequence (deposited as MN998573) and a Chs sequence (deposited as MN998574) were cloned according to the method of Ellingham et al. (2019). A voucher specimen and the sequence information have been deposited in the Northeast Forestry University Culture Collection (NECC) and named as NECCFP001. Pathogenicity was tested in the nursery, by merely placing healthy 40-day-old plants next to diseased plants (n = 10, with a 15 cm distance between plants), and by pressing the mildew sites of diseased leaves onto each mature leaf of the healthy plants (n = 10, also with 15 cm distance). The same symptoms appeared on heathy plants after 10 to 15 days, and 6 to 8 days, respectively. The induced mildew was confirmed morphologically to be NECCFP001 by microscopic examination. E. adunca occurs on aspen, willow, and Populus spp. throughout temperate regions of the Northern Hemisphere and forms fruiting bodies (chasmothecia) to overwinter (Horst 2001; Sinclair and Lyon 2005). However, chasmothecia were not observed in our case, suggesting no sexual morph under the nursery conditions. Incidents of powdery mildew on hybrid poplars caused by Phyllactinia populi had been reported in Jiangsu and Shandong provinces of China (Zhao et al. 2013). This is the first report of E. adunca causing poplar powdery mildew in Heilongjiang, China.