Nematodes may be microscopic, but they cause significant crop losses worldwide. Researchers calculate the damage from plant-parasitic nematodes at $12 billion each year in the U.S. Dr. J. Ole Becker, cooperative extension specialist and nematologist at the University of California, Riverside, estimates $1 billion in annual losses in California alone.
Just a fraction of the numerous species of nematodes are agricultural parasites. The troublesome nematodes lurk in the soil, feeding on roots and other plant tissues.
Root-knot, root lesion, stem and bulb, citrus, ring, pin, cyst and stubby-root nematodes are the most damaging for vegetables and fruit and nut trees.
Root-knot nematodes feed on plant roots, causing swellings called galls or knots. While each crop responds differently, root-knot nematodes can infest beans, beets, cantaloupe, carrots, celery, cole crops, cucumbers, eggplant, greens, herbs, leeks, lettuce, melons, okra, parsnips, peas, peppers, potatoes, pumpkins, radishes, spinach, squash, sweet potatoes, tomatoes, turnips and strawberries, as well as apple, apricot, peach, nectarine and walnut trees. The galls restrict the water and nutrient-conducting abilities of the roots. They also allow soilborne pathogens to enter roots. Aboveground symptoms are wilting, yellowing leaves, loss of vigor and low fruit set. Nematode feeding on root and tuber crops renders the vegetables unmarketable.
In general, vegetable research has focused on the northern root-knot nematode, Meloidogyne hapla, and the southern root-knot nematode, M. incognita. The latter typically produces larger galls, but both injure plants.
Dr. George Abawi, professor of plant pathology and international agriculture at Cornell University, observed that the extent of root galling depends on the nematode population density, the Meloidogyne species and race, the host plant species and its cultivar. Carrots typically undergo severe forking, and lettuce galls are like beads. With some crops, such as grasses and onions, the galls usually appear as slight swellings. Tomato roots can display bumpy, pronounced galls.
Michigan State University Department of Entomology professor emeritus George Bird and crop consultant Loren Wernette noted four species of root-knot nematodes known to be problematic in U.S. potato production. The Columbia root-knot nematode is the most important; it appears to be found only in the Pacific West, Mountain West and West South Central regions. The southern, Javanese and northern root-knot nematodes are more widely distributed.
All four species cause galls on roots and tubers, reducing yield and tuber quality. In addition, the root lesion, potato cyst, potato rot, stubby-root and other nematodes impact potato production. Potato early dying, caused by an interaction between the Pratylenchus penetrans root lesion nematode and the pathogen Verticillium dahliae, is a problem in half of Michigan's potato acreage. Without management, yield losses can reach 50 percent.
Joseph Noling, nematology professor at the University of Florida, has documented the synergistic effect of various species of root-knot nematodes. The simultaneous occurrence of Meloidogyne spp. and fusarium wilt magnifies the yield losses of tomatoes.
The stem and bulb nematode (Ditylenchus dipsaci), also called bloat nematode, re-emerged four years ago as a serious pest of onion and garlic in a New York garlic field. Since then, it has been confirmed in Canada, Maine, Massachusetts, Pennsylvania, Vermont and several other states. Abawi noted that it attacks celery, chives, garlic, leeks, onions, and certain varieties of peas and lettuce.
The extremely wide range of hosts makes nematode control difficult. As with other agricultural pests, an integrated approach is most effective. It's essential to employ area-specific controls and target the specific nematode pest, so consult your local cooperative extension.
Nematodes prefer warm soil, so it follows that California, Florida, and the Southwestern and Southeastern growing regions can experience problems, but other growing regions cannot claim immunity. Sandy, irrigated soils and soils rich in organic matter can attract considerable nematode populations. However, some areas with heavily infested soils have focused on early and late crops, when nematodes are less active.
Planting resistant fruit rootstocks and vegetable varieties is frequently recommended. The VFN (verticillium wilt, fusarium wilt, nematodes) designation for tomatoes, however, can be misleading, because the tomato gene usually used for nematode control resists southern root-knot nematode, but not northern. Dr. James LaMondia, chief scientist at the Connecticut Agricultural Experiment Station, confirmed this finding. Also, while the northern root-knot nematode is more prevalent in the North, areas such as Maryland have both northern and southern root-knot nematodes. To further complicate matters, Becker noted that southern root-knot nematode has broken the tomato resistance in Bakersfield, California, which has more than adequately warm soil.
Correct identification of the nematode species requires a soil test. In addition, the nematode count must be determined in order to develop an effective control program. The National Plant Diagnostic Network website (http://www.npdn.org) lists diagnostic laboratories by state.
Cereal rye, cowpea, brassicas, Sudan grass and sorghum, sunn hemp and some clovers have suppressed root-knot nematodes in some regions. Cover crops and amendments are beneficial for the soil, but they must be crop and nematode-specific. In some research, grain crops did not diminish lesion nematodes. Brambles, avocado, cherry, pear, plum and corn are affected by lesion nematodes. According to Bird and Wernette, "[The] use of cover crops for nematode control is tricky and should only be undertaken with advice from a local expert or as an on-farm research project."
Another practice, permitting the soil to lie fallow for one or more years, has not been very effective in LaMondia's experience. In addition, he cautioned that weed control for the fallow fields must be monitored, since many broadleaf and other weeds are nematode hosts.
Solarization may temporarily reduce the nematode population in the top 12 inches of soil. This can benefit annual crops and help woody crops become established, but the University of California noted that long-term protection for fruit trees will not be provided.
Good agricultural practices of prevention and sanitation can reduce populations. These include planting certified nematode-free seed and transplants, using seed coated with a nematicidal compound or treating certain seeds with hot water, cleaning and disinfecting handling equipment, and destroying the vegetable plants after harvest (including the roots).
Biofumigation involves incorporating plants, often brassicas such as yellow mustard, rapeseed and oilseed radish, into the soil. These release compounds that suppress certain nematodes. However, Bird noted that biofumigation has had mixed results in Michigan.
Chemical fumigation of the soil can be done under specific weather conditions, with an appropriate fumigation management plan, in accordance with the specific label, and with a buffer zone and trained applicators using approved equipment.
Promising biocontrols are under development for nematodes. Becker expects the introduction of new products within one or two years. Plus, he reported even more good news: "The new generation of controls will carry only a caution label, which has much lower toxicity than [the] chemical controls now used."
The author is a writer-researcher specializing in agriculture.