First Report of Root Rot of Melon Caused by Aspergillus tubingensis in China

Melon (Cucumis melo L.) holds substantial nutritional and economic importance, with an annual planting area maintained at roughly 400,000 hm2 in China. From April to May 2025, Aspergillus root rot was observed on greenhouse-grown melon (cv. Huangguan) in Zengcheng, Guangdong. Five randomly selected rows per greenhouse were surveyed (500–600 plants). Disease incidence across the eight greenhouses ranged from 6.4% to 32.5%. Initially, the lower leaves showed chlorosis; then, all foliage turned yellow, wilted and desiccated, while roots developed water-soaked lesions, followed by tissue rot and even left only a few residual roots. Ultimately, entire plants dried out. Nine symptomatic root tissues were surface-sterilized sequentially with 75% ethanol for 30 s and 2% NaOCl for 5 min, rinsed three times with sterile water, placed onto PDA plates, and incubated at 26°C for 3 days; fungal colonies were then subcultured onto PDA and grown for 5 days. 27 isolates with similar morphological characteristics were obtained. On PDA, colonies were initially white, gradually turning black. On Czapek’s agar, morphological examination revealed brownish-black conidial heads (66.20 to 122.85 µm in diameter), metulae (34.60 to 52.84 µm in length), phialides (5.66 to 9.40 µm in length), and smooth, globose conidia (3.18 to 4.66 µm in diameter). Such characteristics are typical of the genus Aspergillus (Samson et al. 2014). The ITS, β-tubulin and calmodulin genes were amplified using primer pairs ITS1/ITS4 (White et al. 1990), Bt2a/Bt2b (Glass and Donaldson. 1995) and CMD5/CMD6 (Hong et al. 2005), respectively. All generated sequences were deposited in GenBank (ITS: PZ251174, PZ255421, PZ255422; β-tubulin: PZ260849, PZ260850, PZ260851; calmodulin: PZ260846, PZ260847, PZ260848). BLAST against GenBank showed 99.66%, 100% and 100% identity for the ITS, β-tubulin, and calmodulin sequences, respectively, with A. tubingensis strain CBS 116417 (FJ629363, FJ629314, PV012869). Phylogenetic analysis of a concatenated three-gene alignment placed isolates QGg-4, QGg-12, and QGg-21 in a well-supported clade with A. tubingensis. For pathogenicity assays, QGg-4, QGg-12, and QGg-21 were inoculated onto sterilized wheat grains and incubated at 26°C for 7 days. Thirty fungal-colonized grains were added to 200 g of sterilized soil or natural nonsterile soil, thoroughly mixed, and transferred to a pot; then, a 20-day-old melon seedling with wounded roots was planted in the pot. Each isolate was tested on six seedlings, while control plants were treated with fungal-free grains. All plants were maintained in a greenhouse (26°C, 90% relative humidity). Seedlings in sterilized soil exhibited leaf chlorosis and root rot 8 days post-inoculation, whereas those in natural soil showed the same symptoms at 13 days and developed water-soaked lesions on roots. The control groups showed no symptoms. The pathogens were reisolated from symptomatic root tissues and confirmed as A. tubingensis via ITS sequencing. The greenhouse soil was used continuously for melon cultivation over six years; root rot emerged in year three and worsened annually. Due to the lack of a control system, greenhouse temperatures exceeded 30°C from April to May. Notably, another melon variety (cv: Yuelu) grown concurrently in the same greenhouse had a markedly lower disease incidence. Aspergillus species are typically saprophytic or opportunistic pathogens. Therefore, the outbreak of root rot disease can be attributed to three factors: continuous monocropping, sustained high temperatures during the critical growth period and variety susceptibility. To our knowledge, this is the first report of A. tubingensis causing melon root rot in China.