BACKGROUND Aphis gossypii Glover is the main pest found in most cucumber‐producing areas. Melatonin (MT) has been widely studied in protecting plants from environmental stresses and pathogens. However, little knowledge is available on the impact of MT on insect resistance. RESULTS The fecundity of aphids on MT‐treated cucumber leaves was inhibited. Interestingly, MT‐treated plants were more attractive to aphids, which would prevent the large‐scale transmission of viruses caused by the random movement of aphids. Meanwhile, MT caused varying degrees of change in enzyme activities related to methylesterified HG degradation, antioxidants, defense systems and membrane lipid peroxidation. Furthermore, transcriptomic analysis showed that MT induced 2360 differentially expressed genes (DEGs) compared with the control before aphid infection. These DEGs mainly were enriched in hormone signal transduction, MAPK signaling pathway, and plant‐pathogen interaction, revealing that MT can help plants acquire inducible resistance and enhance plant immunity. Subsequently, 2397 DEGs were identified after aphid infection. Further analysis showed that MT‐treated plants possessed stronger JA signal, reactive oxygen species stability, and the ability of flavonoid synthesis under aphid infection, while mediating plant growth and sucrose metabolism. CONCLUSION In summary, MT as an environmentally friendly substance mitigated aphid damage to cucumbers by affecting the aphids themselves and enhancing plant resistance. This will facilitate exploring sustainable MT‐based strategies for cucumber aphid control. © 2022 Society of Chemical Industry.

Screens for Fusarium wilt antagonistic bacteria have generally focused on screening for antagonistic strains from healthy plants, with few reports of screening for antagonists from leaves of diseased plants. In this study, we used cucumber leaves infected with Fusarium wilt disease for the isolation of antagonists and a total of four endophytes with obvious antagonistic effects were screened. Based on the results of physiological and biochemical experiments and 16S rDNA gene sequencing, four strains were identified as Paenibacillus polymyxa (Y-4, Y-6, Y-14) and Bacillus Zanthoxylum (Y-9), and all antagonistic strains exhibited the ability to produce amylase, pectinase and hydrogen peroxide (H2O2). In pot test, antagonistic bacteria were able to significantly reduce the disease index in cucumber seedlings challenged with the pathogen. Four strains of antagonistic bacteria can alleviate the damage caused by Fusarium oxysporum f. sp. cucumerinum (FOC) invasion by increasing the photosynthetic efficiency, antioxidant capacity and root vigor of cucumber seedlings. In conclusion, the leaves of cucumber diseased strain may also be a potential source for screening novel antagonistic agents against cucumber Fusarium wilt. The screened four strains of antagonists had a certain biocontrol potential against FOC and could play a certain role in environmental-friendly agriculture in the future.

As an important habitat for microorganisms, the phyllosphere has an important impact on plant health and growth. The changes of the phyllosphere microorganisms are closely related to the occurrence of leaf diseases. However, little is known about how the microbial community changes in the phyllosphere under the infection of pathogen. Here, we analyzed the microbial communities in the phyllosphere at different disease levels (L1 ≤ 10%, 30% ≤ L2 < 50%, and L3 ≥ 50%) to explore the dynamic changes of the phyllosphere microorganisms. The analysis showed that bacterial microbial community diversity increased with disease severity and fungal diversity gradually decreased. There were no significant differences in the community structure and composition of phyllosphere microorganisms at different levels of incidence, the relative abundance of species changed significantly. The relative abundance of rosenbergiella, rickettsia and cladosporium was negatively correlated with the severity of the disease, while bacillus, pantoea, kocuria and podosphaera were positively correlated with the severity of the disease. Co-occurrence network showed that both fungal and bacterial communities L1 have relatively stable and complex ecological networks. The bacterial network tends to be unstable with the increase in disease degree, and the fungal network gradually stabilizes. These studies showed that the severity of the disease will have a certain impact on the phyllosphere microbial community, which provides a new reference for cucumber powdery mildew control from a microbial perspective.

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