Where I live in Michigan, we have lots of hot, humid weather coupled with clay loam soil. I am a HUGE proponent of using a thick layer of spoiled hay/straw for mulch throughout my entire garden (buying hay/straw bales in the fall, letting them sit out in the open, uncovered during the winter, then using it for mulch when I plant in the spring). This has virtually eliminated Septoria and all other fungal disease.
With that said, it does not stop powdery or downy mildew on my squash, melons, and cukes. From a distance, it seems like you might have powdery mildew but I can't tell for sure. If it is powdery mildew, frequent applications of a foliar copper spray like Bonide or Daconil after watering/rain should help stop the spread.
Good luck!
Steve
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