Impact in Walnut Production
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. A. tumefaciens is a ubiquitous, soil-borne bacterium that causes crown gall tumors that girdle trees and decrease plant yield, vigor, and lifespan.
In the nursery, each galled plant and adjacent symptomless plants must be culled.
In orchards, incidence can be as high as 90%, and 15% of all California orchard trees have crown gall symptoms.
The only effective post-plant Crown Gall control strategy is physical removal of galls or trees.
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.
A more sustainable and effective management strategy is identification and deployment of durable disease resistant host genotypes.
We have screened a majority of Juglans species in the USDA National Clonal Germplasm Repository (Davis, California). 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 technique and inoculation and parafilm wrap, shown below in the two photos on the left. Susceptible and putatively resistant genotypes are shown at right, respectively.