Nematodes

Plant-parasitic nematodes are microscopic roundworms that feed on plant tissues. On walnut, the most notorious species live in soil and roots. In California, the species of nematode most commonly found in orchard soils causing problems on walnuts is the lesion nematode, Pratylenchus vulnus. Ring nematode (Mesocriconema xenoplax) is also damaging to walnuts, and root-knot nematodes (Meloidogyne spp.) cause problems on Paradox and English walnut rootstocks.

The severity of nematode damage depends on the age of the tree and density of the nematode population. Do not replant young trees in a heavily infested site. The roots will be severely damaged and the trees stunted and weak. Mature trees can tolerate somewhat higher numbers of nematodes. Even more mature trees show a lack of vigor, poor growth, and reduced cropping when nematode numbers are very high; trees cannot regenerate new roots.

Symptoms described below are indicative of a nematode problem, but are not diagnostic since they could result from other causes as well.

• Damage to roots will restrict their ability to take up water and nutrients. Aboveground symptoms of nematode damage include lack of vigor, and a decline in growth and yield that cannot be corrected by management practices. The decline in vigor predisposes the tree to sunburn, branch wilt, flatheaded borer, and deep bark canker.
• Lesion nematodes feed and migrate inside roots causing black lesions. These lesions can sometimes be seen in large roots by scraping off a thin layer of the outer covering.
• Ring nematode infestation will result in stunted roots, which sometimes proliferate and form dense mats.
• Root-knot nematodes cause swelling of roots, called galls, mainly on Paradox and English walnut rootstocks.

To make management decisions, it is important to know the nematode species present and to have an estimate of their numbers. If a previous orchard or crop had problems caused by nematodes that infect walnut trees, numbers may be high enough to damage young trees. If nematode species have not previously been identified, soil and root samples should be taken and sent to a diagnostic laboratory for identification.

To sample for nematodes:

1. Divide the field into sampling blocks that are representative of cropping history, crop injury, or soil texture. Blocks should be no smaller than 5 acres for smaller parcels (less than 100 acres), and no larger than 20 acres for larger parcels (greater than 100 acres). For parcels less than 20 acres, a minimum of 4 samples should be taken.
2. Take soil and small root subsamples from within the root zone where there is soil moisture, the area of the root system where the highest nematode activity is expected.
3. Take several subsamples randomly from each sampling block and mix them thoroughly in a container to make a composite sample. About one quart of soil is needed for each sample.
4. Place the samples in separate plastic bags, seal them, and place a label on the outside with your name, address, location, and the current or previous crop and the crop you intend to grow. (See IPM for Walnuts, UC ANR Publication 3270, for more details.)
5. Keep the samples cool (do not freeze) and out of direct sunlight, as drying out and heat exposure will make it difficult to extract the nematodes from soil. Transport the samples to a diagnostic laboratory as soon as possible.
6. Request a species identification for the lesion nematodes that are found.

Contact your farm advisor for more details about sampling, to help you find a laboratory for extracting and identifying nematodes, and for help in interpreting sample results. Sampling strategies will vary depending on whether they are taken for diagnostic purposes for an established orchard with a suspected nematode problem or in preparation of a new orchard planting. The general soil conditions will also impact the sampling strategy. For example, deeply rooting soils need to be sampled deeper (0–5 footdepth) than those where rooting depth is restricted.

Prevent nematode damage by planting nematode-free certified rootstock. Try to prevent introduction or spread of nematodes through contaminated soil, equipment, or runoff water. As nematodes can actively move only very short distances, the passive transport in these materials is a critical vehicle for them to be introduced to a new field site.

If nematode infestations are suspected, rigorous soil sampling should be done to design management strategies, and inform planting decisions.

If the field was previously planted to trees or vines, sample for nematodes before planting because the soil is likely to have high numbers. Importantly, one P. vulnus per soil sample is the threshold value for a problem with this nematode if the field is a replant site.

When removing old walnut orchards, it is important to reduce nematode numbers as much as possible and destroy the old roots that may harbor nematodes and other pathogens. It is best to plan for pre-plant soil fumigation 3 to 4 months prior to re-establishing the new orchard.

1. Apply triclopyr (Garlon) to the cut trunks in October, which kills the roots.
2. Remove treated stumps and roots no sooner than 60 days after the application to allow for proper herbicide translocation.
3. Leave the land fallow for ripping and cultivation the next season, and follow a fallow period with deep, dried soil where fumigation is planned.
4. Alternatively, plant a cover crop the summer after Garlon application that is not a host for nematodes, such as true sudangrass (Sorghum bicolor) or safflower. This treatment may reduce P. vulnus and Meloidogyne spp. numbers by 30% within the surface 3 feet of the soil profile.
5. Apply a fall fumigation.

Be mindful of the fact that plant-parasitic nematodes are only one part of the problem when establishing a new walnut orchard following another nut crop. For information on this complex problem, including management considerations, see The Replant Problem and its Management - part 1 (PDF); part 2 (PDF); part 3 (PDF).

Rootstocks differ (PDF) in their ability to tolerate different nematode species.

1. Use clonal Paradox VX211 rootstock where lesion or root-knot nematodes are suspected or present. This rootstock has the ability to tolerate the presence of these nematodes, even though the nematodes can still reproduce.
2. English and black walnut are very susceptible to lesion nematode, but Paradox, with its hybrid vigor, is more tolerant to infestation by lesion nematodes. However, it can be damaged when nematode numbers are very high.
3. English and Paradox are vulnerable to root-knot nematode.

For a comparison of rootstock susceptibility to nematodes and disease, see Walnut Trees in the Nursery Trade: Understanding Terminology, How they are Propagated, Availability and Clonal Rootstock Pest Interactions (PDF).

Trees planted on fumigated orchard sites have improved growth and yields compared to those on nonfumigated sites. Broadcast fumigation can reduce nematode populations by 99.9%; thus, nematodes are not a problem for as long as 6 years. The result is excellent root establishment. Strip or spot fumigations may provide only 6 months to a year of nematode-free soil until the walnut roots grow into the untreated areas.

Research suggests that broadcast applications of Telone II, followed by tree row-stripping of chloropicrin, provides the best growth response, with the longest suppression of plant parasitic nematodes. Deeper fumigant placement, greater than the standard 24 inch injection shanks, also helps suppress nematodes.

Fumigation does not remove the need to plant on a nematode-tolerant rootstock.