First Report of Golovinomyces orontii Causing Hydrangea macrophylla Powdery Mildew in Qinghai-Tibet Plateau, China

Hydrangea macrophylla (Thunb.) Ser. (Hydrangeaceae) is the most popular hydrangea species grown in home gardens and landscapes in China. Plants of H. macrophylla with symptoms of powdery mildew were found in a commercial wholesale nursery in Huzhu, Haidong (36°49′11.87′′N, 101°57′03.36′′E, altitude 2,490 m), in May 2020, with disease incidence reaching 80%. Symptoms included yellowing and necrosis of leaves. Upon microscopic observation, masses of conidia and mycelium were observed covering the symptomatic tissues. Fungal isolates displayed nipple-shaped hyphal appressoria, often poorly developed, conidiophores erect, arising laterally or from the upper surface of hyphal mother cells, and positioned almost centrally or toward one end of the cells, up to about 160 μm long (n = 30), with foot cells straight or flexuous, 32 to 86 × 8 to 13 μm (n = 50), followed by one to three shorter cells about 11 to 24 × 10 to 15 μm (n = 50), forming catenescent conidia in usually predominantly chains, conidia doliiform to limoniform, hyaline, and 24 to 35 × 13 to 25 μm (n = 50). Conidial germination was of the Euoidium type. Chasmothecia were not observed. To confirm fungal classification, single spores were isolated and cultured on a detached leaf bioassay following the protocol described in Farinas et al. (2019). Total DNA was extracted directly from single-spore cultures using a Chelex extraction method (Walsh et al. 1991). The rDNA internal transcribed spacer (ITS) regions were amplified by polymerase chain reaction (PCR) utilizing the universal primer pairs ITS1/ITS4 (White et al. 1990). The sequences (726 to 727 bp) were deposited in GenBank (accession nos. MT568633, MT757924, and MT757925). The ITS sequences showed 99.9 to 100% identity with a sequence of Golovinomyces orontii reported on Papaver rhoeas (AB769466) in Switzerland. Based on the ITS rDNA phylogenetic tree, the sequences retrieved from the specimen clustered within a strongly supported clade with G. orontii (AB769466), confirming the identity of the pathogen (Takamatsu 2013). Cladistic trees were constructed using the neighbor-joining method with the Kimura two-parameter substitution model in MEGA 6.0. Branch robustness was assessed via bootstrap analysis with 1,000 replicates. To confirm pathogenicity, eight H. macrophylla plants were sprayed until run-off with a suspension containing 1 × 10⁵ conidia/ml. Four plants were used for fulfilling Koch’s postulates, and four plants were used as mock-inoculated controls sprayed only with sterile distilled water. Inoculated and noninoculated plants were covered with plastic bags separately and maintained overnight in a greenhouse at 25 ± 2°C and 50 to 60% relative humidity. Typical powdery mildew colonies developed on inoculated plants 10 to 15 days after inoculation, which were morphologically identical to those originally observed on the diseased plants, whereas the control plants remained symptomless. To our knowledge, this is the first report of powdery mildew caused by G. orontii on H. macrophylla in the Qinghai-Tibet Plateau, China (Braun and Cook 2012).