First Report of <i>Alternaria tomato</i> Causing Fruit Peduncle Rot on Chili ( <i>Capsicum annuum</i> ) in China

Chili (Capsicum annuum L.) is an important economic crop in China, widely used in food processing, pharmaceuticals, and the spice industry due to stable market demand and high production efficiency (Zhou et al. 2025). Guizhou Province, located in southwestern China, is one of the primary chili-producing regions. In August 2024, fruit peduncle rot symptoms were observed in a chili field in Anshun City, Guizhou Province (26°14′N, 105°55′E). Of the 2,000 fruits surveyed, approximately 34% exhibited sunken, dark brown lesions on the peduncle with slightly water-soaked margins that gradually expanded over time, leading to fruit abscission. Thirty symptomatic fruits were collected from different plants. Lesion margin tissues were surface-sterilized with 75% ethanol for 15–30 seconds, rinsed three times with sterile distilled water, air-dried, and then plated onto potato dextrose agar (PDA). Plates were incubated at 28 °C in the dark for 48 h. Emerging mycelia were subcultured three times to obtain pure cultures and stored in 20% glycerol at -80 °C. Nineteen morphologically similar isolates were recovered; three (GULJ A1, GULJ A6, and GULJ A11) were selected for further study. After 7 days on PDA at 28 °C, colonies of all three isolates reached 60-70 mm in diameter, exhibiting dense, grayish-white, cottony aerial mycelia with slightly raised centers. Colony reverse pigmentation was dark brown centrally, fading to pale gray toward the margins. Conidia were obpyriform to ellipsoidal, brown, with 0-8 transverse septa and evident constrictions at the septa, measuring 10.0-35.0 × 7.0-15.0 μm (mean 22.0 ± 2.0 × 11.0 ± 1.0 μm, n = 50). Genomic DNA was extracted from the three isolates. Five loci, including internal transcribed spacer (ITS), translation elongation factor 1-alpha (EF1-α), large subunit ribosomal DNA (LSU), the second largest subunit of RNA polymerase II (RPB2), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), were amplified using published primers. (White et al. 1990; Berbee et al. 1999; Liu et al. 1999; Rehner and Samuels 1994). Sequences were deposited in GenBank: ITS (PV983285-PV983287), EF1-α (PV932134-PV932140), LSU (PV984034-PV984036), RPB2 (PV932135-PV932141), and GAPDH (PV932136-PV932142). BLASTn analysis revealed 98.86-100% identity with Alternaria tomato CBS 114.35 (KP124446, KP125225, KP124600, KP124916, KP124295). Maximum likelihood phylogenetic analysis (MEGA12) placed the isolates in a strongly supported clade with A. tomato (Woudenberg et al. 2015). Pathogenicity was tested by inoculating peduncles of healthy chili fruits with conidial suspensions (1 × 10⁶ spores/mL) of each isolate (GULJ A1, GULJ A6, and GULJ A11) separately, while control fruits were sprayed with sterile water. All fruits were maintained under natural conditions, and the experiment was repeated three times. Ten days post-inoculation, all inoculated peduncles developed dark brown lesions similar to those observed in the field, whereas controls showed no symptoms. The pathogen was consistently re-isolated from symptomatic tissues and identified as A. tomato based on morphology and sequence data, fulfilling Koch’s postulates. This study reports for the first time A. tomato causing fruit peduncle rot on chili in China. These findings provide a basis for rapid diagnosis and integrated management of this emerging disease.