First Report of <i>Cytospora leucostoma</i> Causing Leaf Blight on <i>Photinia serratifolia</i> in China

Photinia serratifolia (syn. P. serrulata), commonly called Chinese photinia, is an evergreen ornamental plant, and widely cultivated in gardens and urban landscapes. In December 2024, leaf blight symptoms were discovered on P. serratifolia in a park (36°53′7″N, 118°46′24″E), Weifang City, Shandong Province, China, reaching 30% incidence rate among surveyed plants (n = 120). Light brown spots initially appeared on the infected leaf margins or tips. The lesions gradually formed irregular greyish-white spots, fused and expanded, resulting in wilting of the leaves. Fifteen symptomatic samples (3 × 3 mm) from 15 leaves were collected from the margins of the lesions, surface sterilized in 0.5% NaClO for 1 min and 75% ethanol for 30 s, rinsed three times with sterile water. The samples were plated on potato dextrose agar (PDA), and incubated at 25°C for 3 days in darkness. Ten fungal strains with similar colony morphology were isolated via single-spore. Two representative isolates (TYSN-a1, TYSN-a2) were selected for morphological and molecular characterization. Colonies grown on PDA attained 82 mm in diameter after 5 days at 25°C in darkness. Colonies were thick, aerial mycelium lacked, initially white, turning dark green to dark after 5 days. Conidiogenous cells were phialidic, sub-cylindrical to cylindrical, 8–14 × 2–2.3 μm. Conidia were hyaline, allantoid, aseptate, 4–5 × 1–1.4 μm (av. 4.6 × 1.2 μm, n = 50). Based on the morphology features, the isolate was identified as Cytospora leucostoma (Fan et al. 2020). For molecular analysis, four loci–ITS (GenBank accession nos. PV454998, PV454999), ACT (PV472615, PV472616), TEF1-α (PV472623, PV472624) and TUB2 (PV472619, PV472620) were amplified using universal primers (Gao et al. 2021) and sequenced. BLAST results showed that the sequences were exhibited >99% nucleotide identity to C. leucostoma CFCC53141 (ITS：MN854446, 478/480 bp, 479/480 bp; ACT: MN850761, 244/244 bp; TEF1-α: MN850754, 596/602 bp; TUB2: MN861116, 373/378 bp, 374/379 bp). A multilocus maximum-likelihood phylogenetic tree was constructed in MEGA 11.0 based on the concatenated sequences of the four genes. It showed that the isolates TYSN-a1 and TYSN-a2 were clustered within the C. leucostoma clade with 100% bootstrap support. To fulfill Koch's postulates, pathogenicity was tested on ten 2-year-old potted plants of P. serratifolia in greenhouse. Five disinfected leaves per tree were wounded with a sterile needled and inoculated with 10 ul of conidial suspension (1 x 106 conidia/ml) of the isolate TYSN-a1. The control leaves were inoculated with 10 ul of sterile water. All plants were potted at 25 °C with 80% relative humidity. Experiments were repeated three times. After 7 days, the inoculated leaves showed necrotic leaf spots consistent with field observations, while controls remained asymptomatic. C. leucostoma was reisolated (100% recovery rate) from lesions and confirmed through morphology and multilocus sequencing. C. leucostoma commonly causes branch canker on diverse hosts in China, including Betula platyphylla (Yang et al. 2018), Castanea mollissima (Jiang et al. 2020) and Corylus heterophylla (Gao et al. 2021). This is the first report of C. leucostoma causing leaf blight on P. serratifolia in China. The report further confirms that C. leucostoma has a wide range of hosts in nature. Leaf blight caused by C. leucostoma has seriously affected its ornamental value and appropriate control strategies should be formulated.