What is causing the edges of the leaves on my zucchini plants to turn black?
To whom it may concern,
It is difficult to diagnose your problem based on this image alone, but you should consider powdery mildew and downy mildew as possibilities. Please visit these UC IPM webpages to confirm and learn about management:
You also could submit a sample to your county's UCCE Master Gardener Help Desk program for free problem diagnosis and management advice: http://www.mastergardeners.org/ask-a-question