High Temperature Triggers Systemic Necrosis and Reveals PRSV as the Primary Cause of “‘Mazi’ Watermelon”

Huaping County, Lijiang City, Yunnan Province, is a key production area for early-season winter watermelons. In recent years, watermelon fruits in this region have exhibited abnormal symptoms, such as protrusions and necrotic lesions on the surface, which has significantly reduced their commercial value; such fruits are known locally as “’Mazi’ watermelon”. Field symptoms included leaf mosaic, necrosis, malformation, and stem cracking, as well as epidermal protrusions and clustered necrosis. To identify the causal agents, typical diseased samples were collected from Xinzhuang Village, Huaping County, in April 2026. Systematic pathogen identification was conducted using tissue isolation, plate coating, and small RNA-seq techniques to detect fungal, bacterial, and viral pathogens. No pathogenic fungi or bacteria were isolated; however, small RNA sequencing identified the presence of papaya ringspot virus (PRSV) and zucchini tigré mosaic virus (ZTMV) in diseased plants. RT-PCR further confirmed mixed infection of PRSV and ZTMV in one sample and PRSV alone in another. Artificial inoculation experiments demonstrated that both viruses could induce mosaic and malformation symptoms in watermelon leaves, but disease manifestation was temperature-dependent. PRSV caused systemic necrosis under high temperatures (>28 °C), while ZTMV triggered mild necrosis at low temperatures (<25 °C). In greenhouse trials (22~35 °C), PRSV-inoculated plants developed stem cracking, fruit protrusions, and necrotic lesions, matching field “Mazi watermelon” symptoms. In conclusion, PRSV has been identified as the primary pathogen responsible for the “Mazi watermelon” symptoms in Huaping, and high temperature is a critical environmental factor that triggers systemic necrosis and severe fruit symptoms.