Nematodes, or eelworms, differ from most of the other organisms that cause plant diseases in that they themselves are animals, not plants. A great number live in the soil and, though most can be classed as harmless, several hundred species are known to feed on living plants as parasites and to be the causes of a variety of plant diseases. Information accumulated during the past century indicates that all of the crop and ornamental plants grown in the world can be attacked by plant parasitic nematodes.
The golden nematode of potatoes Heterodera rostochiensis is a leading menace to the potato industry of Europe. A great effort is being made to control it. A related species, the sugar beet nematode H schachtii, is a critical pest that has restricted the acreage of sugar beets in Europe, Asia and America. The citrus nematode Tylenchulus semipenetrans occurs throughout the citrus-producing regions and exacts a heavy toll, symptoms of which are the common slow decline and dieback visible in many groves over 15 years old. The burrowing nematode Radopholus similis is a serious parasite in the tropics, where it attacks citrus, banana, pepper, abaca and other important crops, causing severe losses. The Xiphenema americanum group of nematodes are of phytosanitary significance as they are efficient vectors of four important plant viruses presenting the risk of serious economic impact in the EU : cherry rasp leaf nepovirus, peach rosette mosaic nepovirus; tobacco ringspot nepovirus, potato calico strain (potato black ringspot virus) and tomato ringspot nepovirus.
There are no statistics for the world wide damage caused by nematodes, but it has been estimated that in the USA alone the damage done to crops is at least several hundred million dollars per year. The use of soil fumigants for nematode control during the past several years has often produced dramatic proof that nematodes in the soil can make the difference between a good crop and one not worth harvesting. Yield increases of 25% to 50% after soil fumigation are common. Experiments with soil fumigation have also made it evident that severe nematode damage can occur on a great variety of crops, including tree crops. The underground parts of plants, roots, tubers, corns, and rhizomes are more apt to be infected than above ground parts. Damage to plants attacked by nematodes is due primarily to the feeding of the nematodes on the plant tissues. The most common types of nematode damage are manifested as rotting of the attacked parts and adjacent tissue or the development of galls and other abnormal growths. Either can interfere with the orderly development of the plant and cause shortening of stems or roots, twisting, crinkling or death of parts of stems and leaves, and other abnormalities. The various kinds of nematode damage interfere with the growth of plants. Reduction in the size of the root system by rotting or galling restricts its efficiency in obtaining the food and water the plant must get from the soil. Root knot galls distort the tissue that has the function of conducting food and nutrients to the upper part of the plant. Damage to stems and leaves also interferes with normal growth. Consequently the yield of crop plants is reduced. Crippled plants cannot produce a high-quality crop. With some crops such as carrots and white potatoes, galls and rot caused by nematodes can make culls out of what would otherwise be saleable produce.