Nematodes While most of the thousands of species of nematodes on Earth are not harmful, some nematodes parasitize and cause diseases in humans and other animals as well as attack and feed on living plants. However, there are ways to deter these pesky pests from disrupting your garden—like planting Marigolds To Protect Your Melons. While most nematodes are beneficial, a few species of these translucent, unsegmented worms are plant parasites. Pest species are only about 1/50 inch long and cause root knots or galls, injured root tips, excessive root branching, leaf galls, lesions or dying tissue, and twisted, distorted leaves. Plants most commonly attacked at the roots include tomatoes, potatoes, peppers, lettuce, corn, and carrots. Plants that sustain leaf and stem injury include chrysanthemums, onions, rye, and alfalfa. What Are They? Often described as wormlike, nematodes are not closely related to true worms. They are multicellular animals with smoot...