First Report of Diaporthe melonis Causing Stem blight of Eggplant in China

Abstract

In recent years, widespread epidemics of eggplant stem blight were observed in eggplant fields of Tongdao County, Hunan Province, China (26°24′ N, 109°54′ E), with a mean disease incidence of approximately 60%. Infected plants initially develop water-soaked brown lesions on stems, which gradually expand to girdle the stems, ultimately resulting in plant wilting. Tissue pieces from lesion margins were surface-sterilized 90 s in 2% NaClO, 30 s in 75% ethanol, then rinsed three times with sterile distilled water. Tissues were dried with sterile filter paper, plated on PDA, and incubated at 28℃ for 4 days in the dark. A fungus was consistently recovered from diseased samples, and six isolates exhibiting identical morphological characteristics were obtained. Hyphal tips of these isolates were transferred onto fresh PDA plates and incubated for 30 days to produce spores. Single spore isolates were then obtained by dilution plating. After 7 day incubation on PDA, the colony produced sparse white mycelium that expanded outward, forming concentric rings. When the fungus was cultured on sterilized eggplant stems, black stromata were formed. Pycnidia were borne on the surface of stromata, and numerous β-conidia and a small number of α-conidia were observed. The unicellular α-conidia are fusiform to elliptical with rounded apices and bases, usually containing two oil globules, and measure 5.52 to 12.48 × 1.73 to 3.32 µm (n = 50). The filiform and aseptate β-conidia are curved at the apex, with dimensions of 20.62 to 34.48 × 1.06 to 1.74 µm (n = 50). Based on the above morphological characteristics, the fungus was preliminarily identified as Diaporthe (Phukhamsakda et al., 2025). Two representative isolates (TDQZ01 and TDQZ02) were subjected to molecular identification based on ITS, TEF1, HIS3, CAL and TUB2 sequences amplified with primers ITS4/ITS5(White et al., 1990), EF1-728F/EF1-986R (Carbone and Kohn, 1999), CYLH3F/H3-1b (Crous et al., 2006; Glass and Donaldson, 1995), CAL228F/CAL737R (Carbone and Kohn, 1999), and T1/Bt-2b (Glass and Donaldson, 1995) respectively. Sequences were deposited in GenBank under accession numbers ITS (PZ159274,PZ798684), TEF1 (PZ278281, PZ822606), HIS3 (PZ278283, PZ822605), CAL (PZ278282, PZ815256) and TUB2 (PZ278284, PZ815257), and showed >99%, >94%, >99%, >98% and >99% sequence identity, respectively, to Diaporthe melonis strain CBS 435.87 (accession numbers KC343141, KC343867, KC344109, KC343383, KC343625) based on BLASTn analysis. Furthermore, Maximum likelihood analyses based on concatenated multilocus sequences placed isolates TDQZ01 and TDQZ02 within the clade of Diaporthe melonis. Eggplants were grown at 28 °C with a 12 h light/12 h dark photoperiod to the six true leaf stage for inoculation assays. Mycelial discs (6 mm in diameter) of TDQZ01 and TDQZ02 were inoculated onto the stem wounding eggplant (5*5 mm), whereas PDA discs served as the control. All treated stems were wrapped with parafilm for moisturized incubation. Six days after inoculation, dark-brown lesions formed on the stems inoculated with the pathogen. The inoculated plants gradually wilted and died by 15 days after inoculation, followed by the formation of black stromata on stem tissues, whereas PDA controls remained healthy. D. melonis was reisolated only from symptomatic tissues and confirmed by morphology and sequencing, fulfilling Koch’s postulates. Collectively, these results demonstrate that Diaporthe melonis is the causal pathogen of eggplant stem blight. To our knowledge, this is the first report of eggplant stem blight caused by Diaporthe melonis in China. The present findings provide references for breeding resistant cultivars and integrated management of eggplant stem blight.