First Report of <i>Botryosphaeria dothidea</i> Causing Leaf Blight on <i>Photinia</i> × <i>fraseri</i> in China

Photinia × fraseri (Rosaceae) is an evergreen ornamental shrub valued in horticulture for its striking young red foliage, widely employed in urban landscaping and garden design. In October 2024, a leaf blight disease was observed on P. × fraseri in a seedling nursery in Shouguang City, Shandong Province (36°53′42.16″N, 118°47′22.4″E), with a 20-60% disease severity per affected plant (n = 120). Symptoms first appeared as irregular brown to reddish-brown lesions with dark-brown margins, originating at the leaf tips or margins. The lesions gradually expanded to complete leaf necrosis. Fifteen symptomatic leaves were randomly collected from 10 trees. Twenty tissue samples (4 × 4 mm) were excised from lesion margins, surface-disinfected with 75% ethanol (30 s), followed by 1% NaClO (30 s), and rinsed three times with sterile water. The samples were plated onto potato dextrose agar (PDA) and incubated at 25°C for 5 days. Fungal colonies exhibiting consistent morphology were subcultured via hyphal-tip purification. Two representative isolates (HSN1, HSN2) were selected for morphological and molecular characterization. Colonies on PDA attained 8.5 cm in diameter, initially exhibited white, transitioning to dark gray after 5 days. Conidiophores were cylindrical, hyaline, and smooth. Conidia were hyaline, aseptate, fusiform, 21–29 × 4–8 μm (av. 26.6 × 6.8 μm, n=50). Morphological features matched descriptions of B. dothidea (Crous et al. 2006; Slippers et al. 2014). Genomic DNA of the two isolates was extracted. Three genomic loci—the internal transcribed spacer (ITS; GenBank accession nos. PQ901792, PQ901793), β-tubulin (TUB2; PV007810, PV007811), and translation elongation factor 1-alpha (TEF1-α; PV007812, PV007813)—were amplified using universal primers (Jia et al. 2019). BLASTn analysis of the sequences showed 99–100% identity to corresponding loci from the ex-type strain B. dothidea CMW 8000 (ITS: AY236949; TUB2: AY236927; TEF1-α: AY236898). A maximum likelihood phylogenetic analysis was conducted in MEGA 11.0 using concatenated sequences. Both isolates HSN1 and HSN2 clustered within the B. dothidea clade, confirming taxonomic placement. To fulfill Koch’s postulates, pathogenicity tests were performed on 2-year-old seedlings in the greenhouse. Ten leaves per plant were wounded using a sterile needle and inoculated with 5 mm mycelial plugs from a 3-day-old isolate on PDA cultures. Control leaves were wounded and inoculated with sterile PDA plugs. Five plants were used per treatment group. All plants were potted at 25°C with 75% relative humidity and a 12-h photoperiod. Initial leaf blight symptoms consistent with field observations developed on 90% of inoculated leaves after 7 days, while control leaves remained asymptomatic. The experiment was repeated three times independently. B. dothidea was successfully reisolated from the symptomatic tissues (100% recovery rate) and confirmed through morphological characteristics and multilocus sequence analysis. B. dothidea has been previously documented to cause leaf blight on diverse hosts, including Kadsura coccinea (Su et al. 2021), Camellia oleifera (Hao et al. 2023), and Parrotia subaequalis (Yang et al. 2024). To our knowledge, this is the first report of B. dothidea infecting P. × fraseri in China. This identification will be helpful for subsequent disease control and field management of ornamental shrubs.