Page Banner

United States Department of Agriculture

Agricultural Research Service

Related Topics

Research Project: Development of Disease Resistant Walnut Rootstocks: Integration of Conventional and Genomic Approaches (Root Lesion Nematode)

Location: Crops Pathology and Genetics Research

2013 Annual Report


1a.Objectives (from AD-416):
Evaluate resistance of the walnut rootstock germplasm, interspecific hybrids, and transgenic selections to root-lesion nematode.


1b.Approach (from AD-416):
Fifty open-pollinated (OP) seedlings will be maintained in the greenhouse where they will be challenged with P. vulnus (root-lesion nematode) using screens routinely conducted in the lab. NEM populations will be identified and quantified. MTs with a high percentage of OP seedlings that are highly and moderately NEM resistant will be clonally propagated for further replicated evaluations. Approximately 200 interspecific hybrid seedlings from the selected MPs will be evaluated for resistance to NEM. The resistance ratings will be used to map the genetic loci mediating NEM resistance. Interspecific hybrids showing NEM resistance will be micro-propagated, and validated under greenhouse conditions. Validated NEM-resistant clones will be clonally propagated and tested in fumigated and non-fumigated field trials.


3.Progress Report:

This project was established in support of objective 1 of the in-house project, which is to characterize the etiology, biology, and ecology of key phytopathogenic agents and their interactions with economically important tree and grapevine species. The goal of the project is to evaluate resistance of the walnut rootstock germplasm, interspecific hybrids, and transgenic selections to root-lesion nematodes.

In 2012, we planted 416 Juglans sp trees, each approximately 5 to 30 cm in height, within a Juglans replant field with a 12 year history of being planted to Paradox or Northern California Black (NCB) walnut seedlings. There were usually 8 clonal trees per each of 52 rows. This site was naturally infested with Pratylenchus vulnus population #45 and Meloidogyne incognita. Included for comparison were three randomly placed rows of similarly sized paradox clones including VX211, RX1 and Vlach. In 2013, we planted an additional 416 Juglans trees into an adjacent field site not previously planted to walnut. This later planting was inoculated with P. vulnus population #45 and M. incognita but had not been planted to Juglans for ten years. Meanwhile, in 2013, any missing trees within the 2012 replant site were replaced. All these Juglans hybrids are a product of collaborators in the Plant Sciences Department at University of California, Davis. Hybrids received in the 2013 planting were different from those planted in 2012.

As trees reach 1 meter in height, their root systems are sampled. This sampling effort provides 20 grams of fine feeder roots collected from three locations along each root system without seriously interrupting future tree growth. Collected roots are washed free of soil and placed for 5 days within a Baermann funnel within a mist chamber. Nematodes are collected in test tubes and subsequently counted using a dissecting microscope at 60x and reported as P. vulnus or M. incognita per gram of fresh root.

Individual trees are root sampled a second and third time at six month intervals. Woody root systems with 0 to 0.2 nemas/gram of root across all three samplings within two years are referred to as resistant. Woody root systems with 0.21 to 0.60 nematodes per gram of root are identified as moderately resistant and are of value for future breeding programs because they may possess one or more nematode resistance mechanisms that are different from mechanisms already observed. As of this date, the only source of Juglans to provide resistance against these two nematodes are several J. cathayensis genotypes. We also identified juglans sources that are highly susceptible (>180 nematodes/gram of root).

Tree vigor is quantified annually using measurements of tree height and trunk diameter.


Last Modified: 9/2/2014
Footer Content Back to Top of Page