The following is from Bob Mulrooney, Extension Plant Pathologist (ret.) and nematode enthusiast, UD.
Nematode surveys, grower sampling, and troubleshooting samples have demonstrated that varied populations of several plant parasitic nematodes occur on farms in Delaware. When nematode counts are at or above threshold levels, consider implementing control measures for susceptible crops. Many cultural practices (rotation, fallow, resistant and tolerant varieties, etc.) will reduce nematode populations. However, at times, such practices are not possible or feasible. When this is the case, consider using chemicals (nematicides) for nematode control.
*Above threshold levels per 250cc (1 cup) soil sample.
NOTE: Stunt, spiral, stubby-root, lance, dagger, and pin nematodes have also been detected in samples assayed from vegetable fields. At present it is not known if control measures are needed for these nematodes on vegetables in Delaware. Most nematologists would agree that populations above 1,000 would be troublesome. If control is considered advisable, recommendations for control of these nematodes will be indicated on assay report forms.
Nonchemical Management of Nematodes
Prevention of spread. Plant-feeding nematodes move only short distances – a few inches to a few feet – under their own power. Typically, nematodes are spread by the movement of infested soil and/or infected plants. Sanitation and good cultural practices are the best preventive measures against nematodes. Obtain nematode-free transplants from reputable sources. Wash soil from machinery and tools before using them at another location. Nematodes may also be spread by wind, water, soil erosion, and animals.
Crop rotation. Rotation of crops is an effective and widely used cultural practice to reduce nematode populations in the soil. To be most effective, crops that are poor hosts or nonhosts of the target nematodes should be included in the rotation sequence.
Root-knot and lesion nematodes have such a wide host range that a practical rotation plan to reduce these nematode populations for vegetable crops cannot be recommended at the present time. The exception to this is the use of root-knot resistant soybeans in a rotation. A few root-knot resistant cultivars are available.
Cover crops. Some plants commonly used as cover crops are naturally suppressive to certain nematode species, but no single crop is effective against all nematodes. The cover crop plant may be a nonhost and, therefore the nematodes starve, thus reducing their populations as with fallow. Nematodes invade the roots of certain other cover crop plants, but they fail to reproduce. Yet, other “antagonistic” plant species exude chemicals from their roots which are toxic to nematodes, including marigold and asparagus.
Soil amendments and green manures. In general, the incorporation of large amounts of organic matter into the soil reduces populations of plant-feeding nematodes. The decomposition products of some plants kill nematodes. These include butyric acid, which is released during the decomposition of rye and timothy, and isothiocyanates, released during the decomposition of rapeseed and other mustards in the genus Brassica. Maximum benefit of these”natural” nematicides is obtained when the plant material is incorporated into the soil as green manure. Green manure treatments are not equally effective against all plant- parasitic nematodes and therefore it is important to consult with a diagnostic lab or extension agent to make sure the treatment is appropriate for the nematode being controlled. For example, rapeseed is effective against dagger nematodes but not lesion nematodes. Also keep in mind that varieties of the same green manure crop can differ in the amount of toxic chemical components in their cell walls and therefore differ in the amount toxic by-products released during decomposition.
For dagger nematode control, two years of rapeseed green manure is desirable, but it may be possible to get the same benefit by growing two crops of rapeseed within one year. The following timetable is suggested for producing two rotations of rapeseed within one year:
- Prepare seedbed and plant rapeseed by late April or early May. (Plant only recommended winter rapeseed varieties.)
- Turn under green rapeseed by early September. Prepare seedbed and plant second crop by mid-September.
- The second crop should be turned under in late spring after soil temperatures reach 45°F or higher.
- Ideal conditions for incorporating the cover crop are similar to those required for obtaining the maximum benefit from fumigation (i.e., the soil should be above 45°F and moist).
- Alternatively, planting dates may be reversed so that the first planting is in the fall followed by a second crop planted in the spring. This would end the rotation cycle in fall of the following year. Some rapeseed varieties are more effective at suppressing nematode populations than others, and some varieties will not over-winter or they bloom too early in summer to be useful. The winter varieties ‘Dwarf Essex’ and ‘Humus’ work well for both spring and fall planting dates. If planted in the spring, these varieties grow vigorously to crowd out weeds and do not go to seed.
- Rapeseed requires a firm, smooth seedbed that is free of weeds, heavy residue, and large clods.
- Seed may be drilled or broadcast. Seed at a depth of 3/8 inch and avoid planting too deep! If seed is broadcast, a cultipacker may be used to cover seed.
- A seeding rate of 7–8 pounds per acre works well.
- Rapeseed is sensitive to broadleaf herbicide carryover.
- Fall-planted rapeseed should have 8–10 true leaves and a 5- to 6-inch tap root with a 3/8-inch diameter root neck before the ground freezes.
- Sulfur is necessary for rapeseed to produce nematicidal compounds. Some soils may be deficient in sulfur. A soil test for sulfur may be beneficial. Keep in mind that some biofumigant crops like rapeseed and sorghum-sudangrass are hosts for nematodes and it is not until incorporated into the soil as green manure that they will suppress nematode populations.
Plant nutrition and general care of the plant. The harmful effects of nematodes on plants can be reduced by providing plants with adequate nutrition, moisture, and protection from stress. These tactics sometimes may be of limited usefulness, because, if susceptible crops are grown continuously, the nematode population may increase to levels that cause serious damage.
Resistance. Some vegetable varieties are available with resistance to root-knot nematode; e.g., tomato, pepper, and sweet potato. Some of these resistant varieties have limited horticultural use in our area. Check with seed company and Extension horticulturists for current variety suggestions.
Integrated management practices. Each of the practices mentioned above reduces the soil population of plant-feeding nematodes to varying degrees. Each practice has limitations. The degree of nematode control achieved depends on environmental factors, as well as the particular nematode and crop being considered. Maximum benefit is realized when several of these practices are employed in an integrated crop management program. Because the host range of different nematodes varies, the selection of cover crops, rotation crops, and green manures will be determined by the kinds of nematodes present. No single practice is a cure-all for all nematode problems.
Chemical Control Measures – Nematicides
Fumigants. Most pre-plant fumigants – dichloropropene (Telone II), chloropicrin, or metam-sodium (Busan, Nemasol, Vapam) – can be used for nematode control. Dosage, restrictions, and crop specificity are listed on the label. Follow the label instructions carefully to ensure satisfactory results. The ideal time to fumigate is when soil temperatures at a 6-inch depth range between 50-80 degrees F and the soil has moderate moisture. Fall months are generally best for fumigation. Pre-plant fumigation in the spring should be done with CAUTION. Some fumigants may linger in cool, wet soils and increase the possibility of injury to young plants.
Fields to be treated with soil fumigation must be prepared sufficiently to seed a vegetable crop. The soil should contain little or no crop debris, be free of clods, and soil moisture must be adequate to support seed germination. If soil moisture levels are low, fields should be irrigated to bring the moisture to a satisfactory level. If fields are not properly prepared, soil fumigation will be not effective due to lack of penetration of all soil particles by the gaseous fumigant.
Soil temperature at the 6-inch depth should be in the range of 50o to 80oF ( 10o to 26.7oC). Fall months are ideal for fumigation. Fumigation in the spring is less desirable because some fumigants may linger in cool, wet soils and increase the likelihood of reducing seed germination or injuring young plants.
The following multipurpose soil fumigants should be used to provide disease and nematode control:
chloropicrin – 50 gal/A, or
metam-sodium (Busan, Nemasol, Vapam HL) –37.5-75 gal/A
For nematodes only use one of the following:
dichloropropene (Telone II)—9-12 gal/A, or
dichloropropene + chloropicrin (Telone C-17)– 11-17 gal/A, or
dichloropropene + chloropicrin (Telone C-35)–13-20.5 gal/A
Soil fumigants are injected to a depth of 6 to 8 inches. Immediately after application, soil should be dragged, rolled, or cultipacked to delay loss of fumigant. Metam-sodium is water soluble and can be injected and applied via irrigation systems (solid set sprinkler or drip/trickle). Metam-sodium must be injected for the entire time that the field is irrigated (apply an acre inch of water). Rinse the irrigation system with clean water only long enough to clear the system. Too much rinsing or a heavy rainfall within 24 hours of application will reduce the efficacy of the treatment.
At least 2 to 3 weeks should intervene between the application of most soil fumigants and the time a crop is planted. See manufacturer’s label recommendations for specific crops and fumigants.
One week after application, work soil to a depth of several inches so that gases may escape. Severe injury or killing of sensitive plants may occur if the fumigant has not sufficiently dissipated.
To determine if it is safe to plant into fumigated soil, collect a soil sample from the treated field (do not go below the treated depth). Place the sample in a glass jar with a screw top lid. Firmly press numerous seeds of a small seeded vegetable crop (lettuce, radish, etc.) on top of the soil and tighten the lid securely. Repeat the process in another jar with nonfumigated soil to serve as a check. Observe the jars within 1 to 2 days. If seeds have germinated, it is safe to plant in the field. If seeds have not germinated in the fumigated sample and have germinated in the nontreated sample, then the field is not safe to plant. Rework the field and repeat the process in a few days.
Note: Since nitrifying bacteria are reduced by the fumigants, at least 50 percent of the nitrogen in the initial fertilizer application should be in the nitrate form.
Nonfumigants. Several nonfumigant nematicides can be used on selected crops. These nematicides do not volatilize in the soil as the fumigants do. Therefore, soil temperatures and moisture requirements are not as critical for these chemicals. Be sure to check the label before using any of these materials.
Nematicide Vegetable Cleared for Use
Counter 20CR Sweet corn
Mocap 10G, 15G and 6EC Cabbage, Snap bean, Lima bean, Sweet corn, Cucumber, Irish potato, Sweet Potato.
Do not use Mocap as a seed furrow treatment because crop injury may occur.
Vydate L Carrots, Cucumber, Cantaloupe, Eggplant, Honeydew Melon, Pepper (Bell & non-Bell), Sweet Potatoes, Squash, Tomatoes, Watermelon. Can be injected through trickle irrigation systems.