Strategies for breeding walnut (Juglans spp.) rootstocks with resistance and tolerance to plant-parasitic nematodes

Walnut production in California utilizes seedling Paradox rootstocks (Juglans hindsii × Juglans regia hybrid) to provide vigorous root systems that promote scion vigor and productivity. Until recently, efforts to capture disease resistance among these hybrid seedlings were limited. For decades, the widely distributed (85% of California walnut acreage) root lesion nematode, Pratylenchus vulnus, was managed using pre-plant soil fumigation, primarily with methyl bromide. Using novel breeding and propagation strategies developed as part of our walnut improvement program, it is possible to generate and clonally multiply plants from interspecific crosses. This strategy uses a combination of in vitro embryo- rescue and micropropagation of the resulting germinated epicotyls to propagate interspecific crosses of heterozygous parents. These have been screened for their response to P. vulnus. Determining resistance and tolerance to P. vulnus has been especially challenging because these traits need to be measured under field conditions for multiple years. Here, we report the development of an enhanced screening approach that facilitates selection of resistant and tolerant genotypes after the second growing season. Using this procedure, we have identified candidate Juglans genotypes as putatively resistant and tolerant to P. vulnus. These selections continue to be monitored for their durability of nematode resistance and tolerance, along with important horticultural traits.