Mary from Indiana
These symptoms sound like Verticillium Wilt, a soil-borne fungus that attacks the roots of both cucumbers and zucchini. This disease is often prominent after wet weather. Remove plants immediately to avoid spreading the disease. Because this fungus can over winter in the soil, it's important to keep rotating your plants. Before planting next year, select cultivars that are labeled VFN (Verticillium, Fusarium and nematode) resistant.
About The Author: Ellen Brown is our Green Living and Gardening Expert. Click here to ask Ellen a question! Ellen Brown is an environmental writer and photographer and the owner of Sustainable Media, an environmental media company that specializes in helping businesses and organizations promote eco-friendly products and services. Contact her on the web at http://www.sustainable-media.com
One thing that I know can cause this is that insects get into the stem at the base of the plant. Check and if the stem is cracked open, spray with Malathion or Sevin, then cover the stem with fresh soil and water well. It also sometime helps to choose a few places where the vines are touching the ground to also cover a few inches with soil, and new roots will form at those places.
Harlean from Arkansas.
I had this same problem with both my cucumber (planted in a pot on my deck) and my zucchini (planted in the ground on the other side of the house) I used Sevin on both plants but they both died.