Okay, so what is the pest? It took me a few years to identify, I thought it was more an environmental problem I was dealing with when one day my Zucchini would be bountifully producing and vigorously growing and then seemingly overnight it would yellow, droop and fall over dead. Have you ever had that happen? It is quite disheartening because I for one, LOVE my zucchini! After a few years of this happening (I think I must be slow to catch on) I realized it was not some kind of soil borne problem causing the plants to die but rather, a nasty little creature that liked Zucchini and Yellow squash (as well as all gourd plants, pumpkins and squash) even more than I did. THE SQUASH VINE BORER. This is a nasty pest that is the larvae of a flying insect. Some say that you can get rid of the problem by picking off the flying creature at daylight or dusk, I have never had much luck with this.
If you have any tried and true methods that have helped you, please do share. I have not had luck with Diatomaceous earth, crop rotation, or even soap shields but if there is something else out there, please do let me know!