| Summary: | Melon pest management relies on the excessive application of pesticides. Reducing pesticide spraying has become a global issue for environmental sustainability and human health. Therefore, developing a new cropping system that is sustainable and eco-friendly is important. This study found that melon seedlings irrigated with ultrafine water containing H<sub>2</sub> and O<sub>2</sub> (UFW) produced more root hairs, increased shoot height, and produced more flowers than the control irrigated with reverse osmosis (RO) water. Surprisingly, we also discovered that UFW irrigation significantly reduced aphid infestation in melons. Based on cryo-scanning electron microscope (cryo-SEM) observations, UFW treatment enhanced trichome development and prevented aphid infestation. To investigate whether it was H<sub>2</sub> or O<sub>2</sub> that helped to deter insect infestation, we prepared UF water enrichment of H<sub>2</sub> (UF+H<sub>2</sub>) and O<sub>2</sub> (UF+O<sub>2</sub>) separately and irrigated melons. Cryo-SEM results indicated that both UF+H<sub>2</sub> and UF+O<sub>2</sub> can increase the density of trichomes in melon leaves and petioles. RT-qPCR showed that UF+H<sub>2</sub> significantly increased the gene expression level of the trichome-related gene <i>GLABRA2</i> (<i>GL2</i>). We planted melons in a plastic greenhouse and irrigated them with ultrafine water enrichment of hydrogen (UF+H<sub>2</sub>) and oxygen (UF+O<sub>2</sub>). The SPAD value, photosynthetic parameters, root weight, fruit weight, and fruit sweetness were all better than the control without ultrafine water irrigation. UFW significantly increased trichome development, enhanced insect resistance, and improved fruit traits. This system thus provides useful water management for pest control and sustainable agricultural production.
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