A good zucchini plant can produce enough fresh produce for any family, and because of their vigorous productivity, most gardeners only plant one or two plants to supply all their needs. This can make it especially troubling when a zucchini plant starts to show heavy signs of damage and seem to go from fine one day and dying the next. To care for dying zucchini plants and hopefully save your entire crop you'll need to check over your plant and treat it for one of two possible causes.
Look over the leaves and stalks of the zucchini plant to see if they are wilted and yellow in appearance typically due to a vine borer, or if they have a white substance over them with the worst of infected leaves being shriveled and dried signaling powdery mildew.
Treat powdery mildew by cutting away any dried leaves at their base. Make a solution of one part milk, five parts water and pour into a spray bottle. Spray the tops and undersides of the leaves in the morning until they are dripping.
Respray the leaves after a rain to keep the solution on the leaves and stop the spores of powdery mildew from germinating. As your plant recovers keep a close eye on the leaves and treat as needed if you see any early stages of mildew beginning again.
Inspect the stalk for a vine borer if there is no white substance on the leaves by cutting open a damaged stalk lengthwise and look inside for the borer and/or evidence that the inside of the stalk has been chewed on.
Cover over any damaged areas of the stalk with a few inches of soil and water the base of the plant. Keep watering regularly to ensure the plant doesn't go dry. As the borers leave, the plant may revive and the already yellow leaves should return to normal.
Prevent vine borer problems for the following year by using a pesticide dust on your healthy zucchini plants when they start to grow out from the base. Using the pesticide once a week for five weeks early on should kill off any borers emerging from the soil before they can go into your plants.