I always have a problem with Zucchini and Cucumber plants. The plants grow very well and start to produce then overnight they wilt and die. I spray regularly with Seven but still lose the plants. Thank you.
Zucchini and Cucumber
Certified GKH Gardening Expert
Make sure you are using crop rotation to help control any disease or fungus that is living in the soils.
I would recommend Neem Oil as is does work as both an insecticide and fungicide and does not hurt bees that you need for pollination.
Here are some links for you.