Root-lesion nematode, Pratylenchus vulnus is pathogenic to most perennial crops and infests 85% of the CA walnut acreage. As the “Hidden Enemy” their direct impact on walnut roots can be debilitating. Although seldom killing tree crops it can rapidly reproduce on walnut and severely stunt the trees. Within a single season, starting from small initial populations, Pratylenchus vulnus can reach damage thresholds and within 2 years at the population threshold P. vulnus can result in a 20% reduction of tree growth. Root damage induced by P. vulnus can result in fewer and smaller nuts and limb dieback in trees of bearing age, and root stunting in replant sites can destroy young trees.

The photo (right) depicts four major nematode species of concern to perennial crop growers in California: root-lesion nematode (P. vulnus), root-knot nematode (Meloidogyne spp.), ring nematode (Mesocriconema xenoplax), and dagger nematode (Xiphinema americanum) observed at 60X. (See the UC Division of Agricultural and Natural Resources overview for more information on these four nematode species.)

After a soil sample indicated hundreds of P. vulnus per sample in his 6 year old orchard the gentleman in the photo (below left) just confirmed for himself missing and damaged roots resulting from nematode infection. The damage was exposed through careful root digging and a bit of whittling on older roots of this tree in an orchard that also exhibited minimal above ground growth.

Symptoms of root lesions caused by nematodes:
1) high nematode counts, 2) lesions present, 3) missing roots, and 4) halted tree growth.



Our screening of Juglans hybrids for nematode resistance occurs in field settings. Trees with nematode counts averaging less than 0.2 P. vulnus per gram of root tip during three sampling periods over two years are considered highly resistant or ’winners’. Trees with 0.21 to 0.6 nematodes per gram of root tip are designated moderately resistant and may have potential use in future breeding efforts. Samples with more than 0.61 nematodes per gram of root tip are considered susceptible.

The four-acre P. vulnus site where our first screening occurs (Pv #45) has been in our possession since 1976 when it was inoculated with a single P. vulnus culture from walnut by Ben Lownsbery at UC Davis. On one edge of this screening field we have grown Paradox and Northern California Black walnut (Juglans hindsii) trees for 12 years to create a replant setting where young hybrids could be tested against P. vulnus  plus the rejection component of the walnut replant problem. This site was planted to Phase 1 hybrids consisting of 8 clones each of 49 hybrids and 3 standard paradox comparisons. We had screened for tolerance to rejection elsewhere but wanted to confirm our findings.


We have screened thousands of Paradox hybrids, J. hindsii, and J. regia seedlings root-lesion nematode resistance (nematodes per gram root) and tolerance (host growth produced regardless of nematode populations). In a 5-year field trial Paradox clone VX211 exhibited significant tolerance to root-lesion nematode and was subsequently released as a commercial clonal rootstock.

In 2005 we broadened our search to additional Juglans species. Of eight Juglans species evaluated an Asian butternut species, J. cathayensis, was the first source to exhibit resistance to root-lesion (P. vulnus) and root-knot (Meloidogyne incognita) nematodes after 2 years. In 2008 we grafted J. regia cv Chandler scion to J. cathayensis with 60% success. Clones of J. cathayensis are under evaluation to confirm these results. We have now identified 3rd-yr root-lesion nematode resistance in 9 new J. cathayensis seedlings.

We now have a J. cathayensis, Jcat #27, which remained nematode-free in vivo for 5 full years but is of smaller size than Jcat #21, the first resistant J. cathayensis seedling. Regardless, the J. cathayensis mechanism affords P. vulnus greater safety within young roots than feeding from outside. [perhaps some of the petri dish pictures here] This is completely opposite to where nematodes prefer to feed on VX211 juvenile tissues. It is noteworthy that we already have foliar-applied Movento nematicide that travels to and performs best within juvenile root tissues. Existence of successful hybrids plus an annual application of Movento might turn walnut roots into a trap crop.