Cause Pratylenchus vulnus is a nematode that lives in soil but is rarely found in Oregon. Root-lesion nematodes are migratory endoparasites where part of the population is in soil and part in the roots at all times. All walnut rootstocks are susceptible but trees on Paradox hybrid are sometimes more vigorous than nearby trees on black walnut rootstock. Can cause serious replant problems so be sure to sample before replanting into an area that has had walnut.
Symptoms Trees show few distinctive symptoms aboveground. Infected trees may grow slowly with dieback and/or chlorosis, which may occur in the upper branches. Damaged trees may be grouped in patches in the field. Roots have brown to dark, elongate lesions, about 0.5 inch in diameter, in the inner root tissue. These lesions can be seen after the outer bark has been scraped away.
Sampling The number of nematodes in the soil can be a rough guide to possible plant damage. Always include soil and roots in samples sent for nematode analysis if trees are present. Take samples any time if the numbers are interpreted in relation to nematode population dynamics. In most crops, populations are relatively low through winter and spring but increase rapidly through summer.
- If possible rotate plantings to new land.
- Use certified trees.
Chemical control Preplant soil fumigation.
- Basamid G. Avoid application when soil is over 90°F. Do not apply within 3 to 4 feet of growing plants or closer than the drip line of larger plants. Do not harvest within one year of application. 5-day reentry. Restricted-use pesticide.
- Dominus at 10 to 40 gal/A. Apply as a shank injection to an entire field or raised beds when soil temperatures are above 60°F but not greater than 90°F. 5-day reentry.
- Nema-Q was ineffective alone but reduced nematodes when combined with DiTera in California. Efficacy in the Pacific Northwest is unknown. 24-hr reentry. O
- MeloCon WG (Paecilomyces lilacinus strain 251) at 2 to 4 lb/A plus a soil wetting agent can be applied to established plants, although it might be better used when applied to plants just before planting. Stable for only days at room temperature, weeks in the refrigerator or for a year if frozen. Unknown efficacy in the PNW. 4-hr reentry. O
- DiTera DF (Myrothecium verrucaria strain AARC-0255 fermentation solids and solubles) is registered for many nematodes on walnuts. Was ineffective alone but reduced nematodes when combined with Nema-Q in California. 4-hr reentry. O
Note: Some registered products offer only suppression of this disease and thus are not recommended for use. These products include Movento.
References Lownsbery, B.F. 1956. Pratylenchus vulnus, primary cause of the root-lesion disease of walnuts. Phytopathology 46:376-379.
Nyczepir, A.P. and Halbrendt, J.M. 1993. Nematode pests of deciduous fruit and nut trees. pp. 381-425 in K. Evans, D.L. Trudgill, and J.M. Webster (eds.). Plant Parasitic Nematodes in Temperate Agriculture. CAB International, Wallingford, England.