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Identification of tree-crop rootstocks with resistance to Armillaria root disease.
K. BAUMGARTNER (1), P. Fujiyoshi (1), D. Kluepfel (1), G. Browne (1), C. Leslie (2). (1) USDA-ARS, Davis, CA, U.S.A.; (2) Department of Plant Sciences, University of California, Davis, CA, U.S.A.

Armillaria root disease attacks a broad range of tree crops in California. Instead of re-tooling ineffective conventional controls, namely soil fumigation, we focused on identification of <i>Armillaria</i>-resistant <i>Juglans</i> rootstocks. Our work is part of a collaborative project to identify rootstocks with resistance to several major root diseases (e.g., crown gall). Plants were grown in a standard tissue-culture medium, which supports both the plant and the pathogen, <i>A. mellea</i>. This approach overcame the obstacles of greenhouse inoculations by eliminating escapes and facilitating repeatable mortality starting at 3 weeks post-inoculation. Rootstocks were challenged with three <i>A. mellea</i> strains in three replicate experiments. We inoculated paradox hybrids (<i>J. regia</i> x black walnut; AX1, PX1, RX1, RR4-11A, Vlach, and VX211), Northern California black walnut (<i>J. hindsii</i>) ‘W17’, English walnut (<i>J. regia</i>) ‘Chandler’, and relative <i>Pterocarya stenoptera</i> ‘WNxW’. At 2 months post-inoculation, AX1, PX1, and RX1 exhibited the lowest percent mortality for all strains of <i>A. mellea</i> in all three experiments (10, 31, and 34%, respectively; n = 99 observations). The most susceptible to <i>A. mellea</i> infection were VX211, W17, and WNxW (70, 86, and 94% mortality, respectively). Applications of these findings to the field are difficult to evaluate, due to inconsistent observations and poor design of past field trials. As such, we are establishing additional inoculations in both the lab and field to help validate our results.<p><p>Keywords: Fungus, Fruits-Nuts, Walnut

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