Root-Knot Nematode

Mature female root-knot nematodes are pear-shaped and about 0.01 inch long. Root-knot nematodes spend most of their life in galls. Mature females resemble tiny, white pearls; they sometimes can be seen with the use of a hand lens when root galls are cut open.

Root-knot nematodes cause characteristic galls on roots; galls may be up to 1 inch in diameter, but are usually smaller. These galls interfere with the flow of water and nutrients to the plant; infected plants appear less vigorous than healthy plants, may be yellowed, are prone to wilt in hot weather, and respond poorly to fertilizer. Damage areas usually appear as irregular patches and are frequently associated with lighter-textured soils.

Assess the population level and damage potential based on soil sampling or the history of injury in previous crops. Because root-knot nematodes feed and multiply on many weed species, weed control is an important aspect of their management.

Several varieties are resistant to nematodes and should be used where nematodes are present. Rotation with resistant varieties and nonhost crops is as effective as fumigation. Resistant tomato varieties are not effective against the species Meloidogyne hapla, but are effective against M. incognita, M. javanica, and M. arenaria. Cotton is susceptible only to M. incognita and has relatively high tolerance to even that species. Certain varieties of alfalfa and black-eyed peas are resistant to some root-knot species, but M. hapla builds to high numbers on alfalfa.

Soil solarization can provide control of many soilborne diseases, nematodes, and weed pests. For further information, contact your local farm advisor or see UC ANR Publication Soil Solarization: A Nonpesticidal Method for Controlling Diseases, Nematodes, and Weeds.

Cultural control is acceptable for use on an organically certified crop.

Although soil sampling provides more detailed information, checking for root galls is a simple way to confirm that aboveground symptoms are caused by root-knot nematode injury. Check the roots of a few plants in midseason or later, even if the crop appears healthy. Check earlier if you see wilting, poor growth, or other symptoms; galls may appear as soon as a month after planting. Carefully brush or wash soil from roots to look for galls. Be sure to check some plants in the sandiest part of the field, where damage is most likely.

Check the roots of rotation crops as well, but remember that galls will not be present on nonhost crops and may not be as obvious on other susceptible crops as on tomatoes. If the field has been fallowed or planted to a nonhost crop, look for root galls on nightshades and groundcherries. Other weeds may also have galls but are less reliable indicators of root-knot activity.

If you find galls on any host plant, you can assume that susceptible varieties of tomatoes in the same soil would be infected. However, absence of root galls on other plants does not necessarily mean the soil is free of nematodes that could injure tomatoes.

Soil Sampling

Soil sampling provides the best basis for management decisions, especially in coarse textured soils. You can sample whenever the soil is in good working condition, but the best times are in spring before planting and in fall after harvest. Because of the gradual decline in populations over the winter and the gradual increase during summer, samples from fall and spring represent the high and low extremes of the population. Sampling at other times yields immediate results that are more difficult to interpret.

Farm advisors can help you find a laboratory equipped for extracting and identifying nematodes from soil samples. The analysis usually takes about 2 weeks; allow enough time before planting to choose varieties and to treat the soil if necessary. Contact the lab, before you start, to ensure that samples can be processed as soon as they are received. Follow these instructions, but also consult the lab for any special instructions.

• Draw a field map showing areas that differ in soil texture, cropping history, or crop injury.
• Use a grid pattern to divide each area into blocks of 10 acres or less; if conditions are uniform throughout the field, apply the grid to the whole field.
• Use an auger or soil tube to collect soil from at least 20 places in each block. Take moist soil from a depth of at least 18 inches; take deeper samples in dry, fallow ground. Avoid places where soil is too wet or compacted; if surface soil is dry, discard it and include only moist soil. Samples should include roots of any crops or weeds that might be present.
• Thoroughly mix the soil from one block in a bucket or large bag, then transfer it to a plastic bag or other moisture-proof container.
• Label the sample with the block number in pencil on the outside of the container; moisture will ruin labels placed inside.
• Keep the samples cool; do not leave them in the sun or freeze them. The best storage temperature is 50° to 60°F. Seal the containers so they will not dry out. A good way to keep samples in good condition is to put them in an ice chest in the field.
• Send or deliver samples to the lab immediately. Ship them in an ice chest or in a box insulated with newspaper.

Interpreting the Results

The number of root-knot juveniles in soil samples can be a reliable guide to potential yield loss in processing tomatoes; below a certain level, the population has no measurable effect, but yield declines as the number of nematodes increases. If lab results are to be useful, however, you must interpret them carefully.

Labs generally report the number of root-knot juveniles found in a certain weight of soil, usually 100 grams or 1 kilogram. The most common extraction apparatus, the Baermann funnel, extracts only those juveniles already free in the soil; an added mist chamber improves accuracy by promoting the hatching of juveniles from any eggs the samples may contain. All reports should specify the extraction method used.

If your lab reports the estimated total number of juveniles in soil samples, you can use these figures directly in making management decisions. If the lab only reports the number extracted from the samples, then you must also know the recovery rate, or efficiency, of the lab procedure. The recovery rate tells you what percentage of the nematodes in samples were actually extracted; for root-knot juveniles, it is usually from 10 to 30%. To get the total, divide the number extracted by the recovery rate and multiply by 100. For example, if the lab extracted 30 juveniles per kilogram and the recovery rate is 20%, the total would be (30÷20) x 100, or 150 per kilogram. When comparing results from two sets of samples, make sure the same unit of soil weight was used in both cases; otherwise, adjust the figures accordingly before comparison.

The table below is a guide to yield loss that can be expected from a given root-knot nematode population in the San Joaquin Valley. With an estimate of expected yield loss, you can judge whether management options will increase net return. For example, if a lab finds 200 juveniles per kilogram in spring samples of sandy loam soil, the expected yield would drop to about 88% of normal. If normal yield were 30 tons per acre, the yield loss would be 12% of 30, or 3.6 tons per acre. When tomatoes are worth $54 per ton, the value of the loss is $54 x 3.6, or $194.40 per acre. If the cost of soil treatment is less, it will increase net return. The same logic applies to selection of a resistant variety or alternate crop that may cost more to plant or have lower value.

The table is based on samples taken in sandy loam soil; on finer textured soils, such as silt loams or clay loams, the expected yield reduction for a given population would be less than the value in the table. It would also be somewhat lower in areas with cooler springtime soil temperatures. The expected loss would be higher in hot desert soils and in the presence of Fusarium wilt.

There is no formula for predicting yield loss outside the San Joaquin Valley, but analysis of soil samples in other areas can show whether a population is changing from year to year and can identify parts of the field where root-knot nematodes are concentrated. Although the numbers may differ, the general relation between population and yield is probably similar in all areas.

Several kinds of nematodes other than root-knot nematodes are found in tomato soils and may be listed in lab reports. These include stunt nematodes (Tylenchorhynchus spp.), spiral nematodes (Heliotylenchus spp.), pin nematodes, (Paratylenchus spp.), and stubby root nematodes (Trichodorus and Paratrichodorus spp.). None of these affect tomato in California; they generally feed on rotation crops or weeds. Certain root lesion nematodes (Pratylenchus spp.) injure tomatoes in other states, but the most common species in California tomato soils, P. thornei, feeds on grasses and small grains.