Key message <p>PMTR1 is essential for melatonin-improved plant broad-spectrum disease resistance to bacterial pathogens and viruses. </p> Abstract <p>Melatonin plays vital roles in various plant stress responses, and the identification of phytomelatonin receptor 1 (PMTR1) in recent decades further extends the phytomelatonin signaling pathway. However, whether and how PMTR1 participates in plant broad-spectrum disease resistance has remained unclear. Here, we confirmed that exogenous melatonin enhanced disease resistance to bacterial blight and mosaic disease, indicating the crucial role of melatonin in cassava broad-spectrum disease resistance. In addition, the transcription of <i>MePMTR1</i> was induced by <i>Xanthomonas axonopodis</i> pv. <i>manihotis</i> (<i>Xam</i>), common mosaic virus (CMV) and exogenous melatonin, suggesting that <i>MePMTR1</i> may be involved in cassava immunity. Further investigation illustrated that <i>MePMTR1</i> was a positive regulator in disease resistance to <i>Xam</i> and CMV, and it was necessary for melatonin-improved broad-spectrum disease resistance to the bacterial pathogen and the virus. Moreover, RNA-sequencing identified many differentially expressed genes (DEGs) in melatonin-improved disease resistance through <i>MePMTR1</i>, including <i>calmodulin-like proteins</i> (<i>CMLs</i>), <i>calmodulin-binding proteins</i> (<i>CaMBPs</i>), <i>transcription factors WRKYs</i> and <i>MYBs</i>, <i>respiratory burst oxidase homolog protein C</i> (<i>RBOHC</i>), <i>heat shock protein 20</i>, <i>pathogenesis-related protein</i> (<i>PR10</i>), etc. In total, this study provides new light on the theoretical foundation of melatonin in modulating broad-spectrum disease resistance in cassava, extending the understanding of the melatonin signaling pathway in plant immunity.</p>