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Super Paradox walnut rootstock may be near

Apr 3, 2007 10:31 AM, By Joe Grant
UCCE Farm Advisor, San Joaquin County, California

Is there a "Super-Paradox" walnut rootstock? The answer is, "yes" and "no":

Yes, because new Paradox clonal rootstocks with improved resistance to some soil borne pests and diseases are currently under development and their commercial release is likely coming.

No, because they are not available yet and must undergo further tests to rigorously confirm they will continue to perform well under orchard conditions.

UC researchers and farm advisors, in collaboration with California walnut nurseries, started the Paradox Diversity Study (PDS) in the mid 1990s. Initially, seedlings were grown from 36 Paradox seed sources provided by commercial nurseries and the UC Davis walnut breeding program. In 1998, trees grown from the seedlings were planted in four orchard trials in Kings, San Joaquin, Yolo, and Tehama Counties. The idea was to compare growth and nut production of seedlings grown from a wide variety of Paradox sources to see whether some 'Were better than others. The identity of the seed source nurseries was kept confidential. The goal of the project was to compare the various black walnut species used as Paradox seed sources, not to compare nurseries.

The four PDS orchard trials are still ongoing. The plan is to monitor tree growth and yield at each trial through the fifth year of commercial production. Recent advances in genetic testing that allow precise identification of the black walnut parent(s) of Paradox seedlings are helping us understand why some Paradox sources may be better than others. Some clear winners and losers have already emerged; I will have more to report on the results of these trials when they conclude with this season's harvest.

In addition to the seedlings used in PDS orchard trials, many more seedlings were grown from the nursery and UCD sources and subjected to lab and greenhouse tests in the hope of finding individual seedlings that might be resistant to Phytophthora root and crown rot, lesion nematodes or crown gall. Seedlings that survived these initial screenings were re-propagated as "clonal selections" by laboratory tissue culture techniques. The potentially resistant clones were re-tested. (Like the genetic testing discoveries that furthered understanding of Paradox seed source parents, these tissue culture techniques that allow rapid multiplication of clonal trees are recent achievements that have accelerated the effort to develop superior Paradox rootstocks.)

Several clones survived these multiple rounds of testing and are being re-propagated by several nurseries and laboratories to create a larger supply for field testing, the next phase in the evaluation process. Among these are:

VX 211: Considered lesion nematode tolerance because, unlike "regular" seedling Paradox, it survives and grows very vigorously in soils with high lesion nematode populations. VX 211 also has moderate resistance to Phytophthora.

RX1: Moderately resistant to Phytophthora and is currently being tested for response to nematodes. RX1 has survived well in field tests.

WIP3: selected because it avoids blackline caused by the cherry leafroll virus. It is susceptible to Phytophthora and may perform more like an English seeding rootstock. It is being tested for response to nematodes.

Several small-scale orchard trials of these most promising clones are already underway, and more are planned for 2008 planting. Our hope is that these clones will continue to perform well in these trials and, if so, may become available from commercial nurseries as early as 2008 or 2009. Other clones with promising but, as yet, not fully confirmed levels of resistance to these soil borne pests have emerged from this effort, and are at an earlier stage of development than VX211, RX1, and WIP3.

Is there a "Super-Paradox"? Very possibly, yes, in the near future.

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