|MACCREE, MALENDIA - US Department Of Agriculture (USDA)|
|HACKETT, WES - University Of California|
Submitted to: Walnut Research Conference
Publication Type: Research Technical Update
Publication Acceptance Date: 3/1/2010
Publication Date: 1/1/2011
Citation: Kluepfel, D.A., Aradhya, M.K., Maccree, M., Moersfelder, J.W., Mcclean, A.E., Hackett, W. 2011. Evaluation of wild juglans species for crown gall resistance. Walnut Research Conference. pp. 239-242.
Technical Abstract: During the 2009 USDA, ARS, National Clonal Germplasm Repository (NCGR) germplasm screening season, we examined a total of ~300 seedlings from "new" genotypes for their resistance to A. tumefaciens. This consisted of seedlings from 89 mother trees representing J. regia and its conspecific taxon, J. sinensis, five species of black walnut (J. hindsii, J. nigra, J. microcarpa, J. californica and J. major), and three species of butternuts (J. ailantifolia, J. mandshurica, and J. cathayensis). As in our previous screening, the phenotype after A. tumefaciens inoculation ranged from total resistance to delayed gall development after dormancy to rapid gall formation 3 week post inoculation. Similar to 2008, we identified Crown gall (CG) resistant J. microcarpa, J. cathayensis, J. californica, J. major, and Pteracarya sp, individuals. This year we retained 43 individuals exhibiting a range of CG resistance (i.e., no tumors through limited tumor develoment (<25% girdling) which represented 15% of the total number screened. Interestingly, the 43 individuals retained were harvested from 25 different mother trees in the NCGR collection. Conclusion: CG resistant Juglans and Pteracarya genotypes have been identified in our screening assay conducted under greenhouse conditions. We have uncovered a high degree of variability in tumor formation rates among different host genotypes. Interestingly we also discovered the importance of monitoring putatively resistant selections through a dormancy cycle to confirm resistance. In addition, a limited number of rooted cuttings from CG resistant selections (i.e., open pollinated half sibs) continue to show CG resistance. Demonstration of the genetic stability of CG resistance is an essential prerequisite to incorporation of a given genotype into the walnut rootstock breeding program. We are examining why only a small percent of the original individuals selected as CG resistant remained resistant as rooting cuttings. The few rooted cuttings that appear to be remaining CG resistant are being cloned and multiplied for use in limited field studies.