How to Manage Pests
UC Pest Management Guidelines
Plant parasitic nematodes are microscopic roundworms that feed on plant roots. They live in soil and plant tissues. The species of nematode most commonly found causing problems in soils of walnut orchards in California is 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. If young trees are replanted 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. Trees show a lack of vigor, poor growth, and reduced cropping when nematode numbers are very high. (Trees cannot regenerate new roots with high populations.)
Symptoms described below are indicative of a nematode problem, but are not diagnostic as they could result from other causes as well. 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. Damage to roots will restrict their ability to take up water and nutrients. Aboveground symptoms of nematode damage are lack of vigor, and 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. 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 an estimate of their population. If a previous orchard or crop had problems caused by nematodes that infest walnut trees, population levels may be high enough to damage to 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.
Divide the field into sampling blocks of not more than five acres each that are representative of cropping history, crop injury, or soil texture. Take soil and small root subsamples from within the root zone at the edge of the tree canopy. Take several subsamples randomly from a block, mix them thoroughly and make a composite sample of about one quart for each block. Place the samples in separate plastic bags, seal them, and place a label on the outside with your name, address, location, and the current/previous crop and the crop you intend to grow. (See IPM for Walnuts, UC/ANR Publication 3270, for more details.) Keep samples cool (do not freeze), and transport as soon as possible to a diagnostic laboratory. Request a species identification if lesion nematodes 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.
Prevention. Plant nematode-free certified rootstock. If the site was previously planted to trees or vines, sample for nematodes before planting because the soil is likely to have high numbers. Numbers of P. vulnus can be reduced by 95% of the original population level by fallowing for 5 years after an application of triclopyr (Garlon) to the fresh cut surface of trunks. Importantly, one P. vulnus per soil sample is the threshold value for a problem with this nematode if the field is a replant site.
A good nonhost for nematodes in walnuts is true sudangrass (Sorghum bicolor) planted in spring and allowed to grow into the fall. This treatment may reduce population levels of P. vulnus and Meloidogyne spp. by 30% within the surface 3 feet of soil profile. Root decomposition can be shortened to 1 year by an application of Garlon to cut trunks of old trees. Follow a fallow period with deep, dried soil where fumigation is planned and plant trees that are on a tolerant rootstock such as VX211. Try to prevent introduction or spread of nematodes through contaminated soil, equipment, or runoff water.
Rootstock selection. Rootstocks differ in their ability to tolerate different nematode species. Paradox, with its hybrid vigor, is the most tolerant to infestation by lesion nematode. However, it will be damaged when nematode numbers are very high. English and black walnut are very susceptible to lesion nematode; English and Paradox are vulnerable to root knot nematode.
Chemical. Trees planted on fumigated orchard sites are generally known to have improved growth and yields compared to those on nonfumigated sites. Broadcast fumigation will 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.
UC IPM Pest Management Guidelines: Walnut