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United States Department of Agriculture

Agricultural Research Service

Research Project: Sustainable Vineyard Production Systems

Location: Crops Pathology and Genetics Research

Title: Evaluating paradox walnut rootstocks for resistance to Armillaria root disease

Authors
item Baumgartner, Kendra
item Fujiyoshi, Phillip
item Browne, Greg
item Leslie, Chuck -
item Kluepfel, Daniel

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 10, 2012
Publication Date: January 15, 2013
Repository URL: http://hortsci.ashspublications.org/content/48/1/68.full
Citation: Baumgartner, K., Fujiyoshi, P.T., Browne, G.T., Leslie, C., Kluepfel, D.A. 2013. Evaluating paradox walnut rootstocks for resistance to Armillaria root disease. HortScience. 48:68-72.

Interpretive Summary: Walnut trees cultivated for the nut crop in California is grafted onto a rootstock - Paradox - a hybrid between the wild Northern California black walnut and the cultivated English walnut. Unfortunately, Paradox rootstock is highly susceptible to a fungal root disease, Armillaria root disease. The resistance of new Paradox rootstocks (AX1, Px1, RR4 11A, RX1, Vlach, and VX211) was evaluated, and compared to Armillaria-resistance exhibited by Northern California black walnut rootstock ‘W17’, English walnut scion ‘Chandler’, and Juglans relative Pterocarya stenoptera ‘WNxW’ (Chinese wingnut). Plants were grown in a gel-based medium inside small plastic boxes in the laboratory, and the medium was inoculated with three Armillaria mellea strains in three replicate experiments. At 2 months post-inoculation, the most resistant and most susceptible Paradox rootstocks were AX1 and VX211, respectively, with 9 versus 70% mortality (P < 0.0001, n = 135 plants per rootstock, summed across strains and experiments). This range of resistance may reflect differences in the genetic background, namely that which originates from the maternal parent, between these two Paradox rootstocks (e.g., J. californica for AX1, J. hindsii for VX211). Our finding of similar susceptibility of Chandler and W17 (60 and 69% mortality, respectively) is in contrast with some field observations of greater Armillaria-susceptibility of J. regia relative to J. hindsii. Nonetheless, conflicting reports of the relative resistance of these two species, from trials comparing other genotypes in Armillaria-infected orchards, suggest there is variable resistance within J. regia and J. hindsii.

Technical Abstract: The most common Juglans regia (English walnut) rootstock in California is Paradox, a hybrid between J. hindsii (Northern California black walnut) and J. regia. Unfortuntely, Paradox rootstock is highly susceptible to Armillaria root disease. The relative resistance of new clonal, Paradox rootstocks (AX1, Px1, RR4 11A, RX1, Vlach, and VX211) was evaluated, and compared to Armillaria-resistance exhibited by Northern California black walnut rootstock ‘W17’, English walnut scion ‘Chandler’, and Juglans relative Pterocarya stenoptera ‘WNxW’ (Chinese wingnut). Plants were micropropagated in Magenta boxes, and the tissue-culture medium was inoculated with three Armillaria mellea strains in three replicate experiments. At 2 months post-inoculation, the most resistant and most susceptible Paradox rootstocks were AX1 and VX211, respectively, with 9 versus 70% mortality (P < 0.0001, n = 135 plants per rootstock, summed across strains and experiments). This range of resistance may reflect the broad genetic background, namely from the maternal parent, between these two Paradox rootstocks (e.g., J. californica for AX1, J. hindsii for VX211). Our finding of similar susceptibility of Chandler and W17 (60 and 69% mortality, respectively) is in contrast with some field observations of greater Armillaria-susceptibility of J. regia relative to J. hindsii. Nonetheless, conflicting reports of the relative resistance of these two species, from trials comparing other genotypes in Armillaria-infected orchards, suggest there is variable resistance within J. regia and J. hindsii.

Last Modified: 7/23/2014
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