Nematodes are tiny, microscopic roundworms. Most are soil-dwelling where they live off fungi and bacteria and are not harmful to plants but a few are plant pathogens. Others are beneficial as insect predators, used to help control insect pests such as Japanese beetles and other soil-inhabiting grubs. About 40 species of plant-parasite nematodes have been reported in Missouri where they infest a wide range of plants. In addition to being plant pathogens in their own right, root-feeding nematodes can provide entrances to root rots and wilts. Some also transmit viral diseases.
Nematodes are grouped into several categories but those which are most often recognized by gardeners are the root-knot nematodes that infest plant roots and foliar nematodes that live in leaf tissue. Both often go undetected because of the nematodes' microscopic size and the general nature of the symptoms they cause.
Root-knot nematodes infest plant roots either by feeding from the outside or entering the root tissue where they cause small growths, galls or knots on the roots. Above ground symptoms are generally characterized as poor growth and stunting which results from the plant's reduced root system and its reduced ability to take up water and nutrients. Wilting may occur during times of stress. Symptoms of nutrient deficiencies may also occur. Unless plants are removed from the ground and roots examined, poor growth due to nematodes may easily evade detection. Furthermore, galls on the roots of legumes (plants in the bean family) may be confused with nitrogen fixing nodules and vise versa. (To distinguish the two, nodules caused by nitrogen fixing bacteria are loosely attached and can be easily rubbed off whereas galls or knots caused by nematodes are actually part of the root and cannot be removed.)
Many trees, ornamentals, grasses and vegetables are susceptible to root-knot nematodes. According to the University of Missouri, some vegetable crops in Missouri that are most susceptible to root-knot nematodes are tomatoes, okra, beans, squash, peppers, carrots, cucumbers, muskmelons, eggplant and watermelons. Nematode damage also has been observed on Swiss chard, peas, parsnips, Irish potatoes, New Zealand spinach and fall-grown turnips and spinach. Plants that are fairly resistant include: broccoli, Brussels sprouts, mustard, chives, cress, garlic, leek, groundcherry and rutabaga. Other plants that grow well even when root-knot nematodes are present are globe artichoke, Jerusalem artichoke, asparagus, sweet corn, horseradish, some lima bean varieties, onion and rhubarb.
Established trees and ornamentals may live with root-knot nematode damage for many years with few signs of damage. Even if the plant dies from the nematodes during a period of stress the nematodes may escape detection unless the root system of the plant is dug up and examined.
Foliar nematodes can infest leaves, stems and buds of plants where they can cause deformed leaves or yellow to dark brown spots between major leaf veins. Many ornamentals are known to be attacked by foliar nematodes; some common plants include: hosta, Japanese anemone, chrysanthemum, azalea, and ferns. Positive identification of the presence of foliar nematodes can be made with the aid of a microscope after the nematodes have been "teased" out of living tissue and placed in a petri dish of water.
A very serious pest of many conifers in Missouri, especially pines, is the pinewood nematode. For detailed information on this pest see the IPM sheet entitled "Pinewood Nematode Wilt of Needled Evergreen".