Zucchini leaves will turn yellow due to insect infestations or fungi. The insects that attack the zucchini plant turning their leaves yellow are the squash bug, aphids and spider mites. The fungus that targets zucchini is fusarium wilt.
The Squash bug and spider mites dehydrate the plant; aphids congregate on the leaves and emit a sticky substance on the plant. The plant owner should check to see if the soil is too dry or if a sticky residue is present on the leaves. Fusarium wilt injects itself into the plant’s arteries. Prune the leaves of all dead spots and move them to another area of the garden.