First Report of Tomato Sour Rot Caused by Geotrichum candidum in Heilongjiang Province, China.

Tomato sour rot, caused by Geotrichum candidum, is a postharvest disease. It has been detected across major tomato-producing regions in temperate to subtropical zones (e.g., United States, Mediterranean Basin, India), yet the impact in northeastern China remains poorly understood. G. candidum infects various fruits and vegetables, including kiwifruit, strawberry, peaches and so on (Hussain et al., 2016; Lu et al., 2021; Cheng et al., 2021). In September 2023, during the postharvest packing process of tomatoes in Changchan village, Shuangcheng District, Harbin City, Heilongjiang Province, rotten fruits were detected. Asymptomatic tomatoes harvested from the fields developed decay after harvest within two days, resulting in a 9% morbidity rate and 2.5-ton yield loss. Initially, watery spots appeared on the fruits. The skin then ruptured and turned outward, exposing the flesh. The affected area became soft, rotten and producing a sour odor, covered by a white mold layer. Five diseased tomatoes were collected. Tissue pieces (5 × 5 mm) from the margin of lesions were disinfected and cultured on PDA at 28°C. Five isolates were obtained using the single-spore isolation method. Ten uniformly ripened tomatoes were surface-sterilized with 75% ethanol and wound-inoculated via syringe with 30 μL of a 1×10⁶ spores·mL-1 spore suspension for each of 5 isolates. Sterile water was used as the control. Three replicates were conducted, and all samples were incubated at 28°C, 75% RH in humid chambers. After 2 days, 3 isolates showed the same symptoms as the initial, while the control remained healthy. Three same pathogens were re-isolated from the diseased tomatoes and no pathogen was isolated from the control. The diameter of colonies reached 7 cm in 7 days, appearing creamy white, suborbicular, powdery and flat. Microscopy showed the hyphae were colorless and septate. The conidiophores were erect, and the conidia were cylindrical or ellipsoid with rounded ends, single-celled, colorless, and arranged in tandem. Conidia measured 6.80 ± 0.30 μm in length and 5.51 ± 0.59 μm in width (n=30). DNA was extracted from 7-day-old cultures using a modified cetyl trimethyl ammonium bromide method. Internal transcribed spacer (ITS), 18S ribosomal RNA (18S rRNA), and translation elongation factor 1-alpha (TEF1-α) sequences were amplified using primers ITS1/ITS4, NS3/NS8 (White et al., 1990), and EF1-728F/EF1-1567R (Carbone and Kohn, 1999), respectively. The sequences obtained by polymerase chain reaction (PCR) were sequenced and submitted to GenBank. According to BLAST search, the ITS (PQ579186.1), 18S rRNA (PQ579206.1) and TEF1-α (PQ616986.1) sequences showed 99.48%, 98.03%, and 98.31% similarity to G. candidum (PQ836313.1, AB000652.1 and OQ981192.1, respectively), and matched 383 bp/388 bp, 1142 bp/1182 bp and 638 bp/682 bp of each reference sequences, respectively. Phylogenetic analysis by maximum likelihood method generated based on ITS (388 bp), 18S rRNA (1182 bp) and TEF1-α (682 bp) indicated that the isolates formed a well-supported clade to the related G. candidum type sequences. Isolates were found to be most closely related to G. candidum and far from other species. Based on morphological and molecular characteristics, 3 isolates were identified as G. candidum. This is the first report of tomato sour rot caused by G. candidum in Heilongjiang Province, China. This finding provides a foundation for future research on the occurrence, control and management of this disease.