First Report of Botryosphaeria dothidea Causing Idesia polycarpa Fruit Rot in China

Idesia polycarpa is ecologically and economically valuable for conservation and landscaping. Its fruits yield 20%–30% oil, of which over 80% consists of unsaturated fatty acids, making it a high-quality woody oil source (Wu et al., 2010; Kuang et al., 2024). In 2025, fruit rot was first observed in a plantation in Hema Town, Renhuai City, Guizhou Province (106.31°E, 28.00°N). Early symptoms: sporadic black spots. Mid-stage: lesions enlarge into irregular, sunken, black necrotic areas. Late stage: lesions coalesce, causing extensive dark brown necrosis, shriveling, and rot, which eventually cover the entire fruit surface with blackening, drying, and sunken rot. The disease occurs in autumn and spreads over large contiguous areas, affecting over 200 hectares. A survey of 100 trees revealed that the incidence of fruit disease ranged from 30% to 35%, with a disease index of 16% to 16.25%. Symptomatic fruits were collected from 10 randomly selected trees (ten diseased fruits per tree). Tissue blocks (5 × 5 mm) from the disease-health junction were surface-sterilized with 3% sodium hypochlorite for 30 s and 75% ethanol for 1 min, rinsed four times with sterile water, dried, and incubated on potato dextrose agar (PDA) at 28 ℃ in darkness for seven days. After purification, three morphologically similar fungal strains (GF-1, GF-2, GF-3) were obtained. On PDA after seven days, The colony center was slightly raised; aerial mycelium was dense and nearly grayish-white. The reverse side was light yellow, with grayish-black pigment deposits concentrated at the center, diminishing outward and appearing misty and mottled. Conidia were unicellular, subelliptical, hyaline, measuring (mean ± SD) 22.9 to 26.5 × 4.9 to 5.8 µm (n=50). For molecular identification, only GF-1 was molecularly identified by DNA sequencing. For amplification of ITS, EF1-α, and TUB2, primers ITS1/ITS4, EF1-728F/EF-986R, and Bt2a/Bt2b were used, respectively (Carbone and Kohn 1999; Zheng et al. 2020). The sequences were deposited in GenBank under accession numbers PZ282479.1 (ITS), PZ293717.1 (EF1-α), and PZ293716.1 (TUB2). A phylogenetic tree was constructed from the concatenated sequence dataset, and isolate GF-1 clustered with B. dothidea with 99% bootstrap support. Based on morphological and molecular analyses, the causal agent was identified as B. dothidea. In the pathogenicity test, surface-sterilized I. polycarpa fruits were wounded (1 mm depth) and inoculated with 5 mm mycelial plugs of strain GF-1; sterile PDA plugs served as controls. Three biological replicates were performed, with 10 fruits in the inoculated group and 10 in the control group per replicate. After seven days at 28 °C and 80% RH, inoculated fruits developed dark brown lesions covering the entire fruit (100% incidence), while controls remained symptomless. The fungus was re-isolated from all symptomatic fruits and confirmed as B. dothidea by morphology and multi-locus sequencing (ITS, TUB2, EF1-α). B. dothidea is widely distributed and causes various postharvest fruit diseases, including fruit rot of Nai plum (Yuan et al., 2024). Theoretically, diseased fruits may be unsuitable for oil processing, which could potentially affect yield and economic returns. The pathogen may potentially spread via windborne spores, leading to new infections. Pathogen identification may facilitates disease management and reduces economic losses. This is the first report of B. dothidea causing I. polycarpa fruit rot in China.