Crown Gall

  
Crown gall (CG) is caused by the gram-negative soilborne bacterium Agrobacterium tumefaciens. A. tumefaciens has long term persistence, can be free living or plant associated, and has  the widest host range of any plant pathogen (i.e. “all” dicots). It is also a good rhizosphere colonist and natural genetic engineer.

Though highly vigorous the Paradox rootstock is highly susceptible to A .tumefaciens, which can result in tree girdling and death. Thus goals of our research are to identify crown gall resistant Juglans germplasm in order to generate new “Paradox-like” rootstocks with tolerance/resistance.

 

    

Screening

We have screened a majority of Juglans species in the USDA National Clonal Germplasm Repository (Davis, Calif.). The species that have been screened are Persian walnut (J. regia), close relatives or natural hybrids (J. hopeinsis and J. sinensis), Asian butternuts (J. ailantifolia, J. cathyensis, and J. mandshurica), and North American black walnuts (J. californica, J. hindsii,  J. major, J. microcarpa, and J. nigra). We have also screened more distant relatives within family Juglandaceae, such as Pterocarya spp.

The screening method is a stab and inoculation technique and parafilm wrap, shown below in the two photos on the left. Susceptible and putatively resistant genotypes are shown at right, respectively.

   

Thus far J. microcarpa and Pterocarya spp. exhibited the highest levels of crown gall resistance (>4,200 seedlings examined). To validate plants identified as resistant we screen for false positives by rescoring promising candidates after at least one period of dormancy. Additionally we test rooted cuttings of putative resistant plants for persistent crown gall resistance or tolerance.

Results to Date

Based on our screenings and validations, we are now focused on J. microcarpa and Pterocarya spp. in our CG resistance screening efforts, which includes generating J. microcarpa x J. regia hybrids with CG resistance. J. microcarpa (commonly called Texas black walnut or little walnut) is comparatively smaller than most other Juglans species.

We have identified walnut genotypes and species with minor or no crown gall symptoms with potential utility as rootstocks. These are being in vitro propagated for additional disease testing. Additional “crown gall resistant” interspecific hybrids are being generated, primarily J. microcarpa x J. regia.