Location: Crops Pathology and Genetics Research
2013 Annual Report
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.