Pulling out a walnut orchard with the intention of replanting a new one requires careful planning to overcome tree health challenges.
University of California orchard systems specialists recommend identifying the potential problems by soil sampling, developing a plan for tree removal, soil remediation and fumigation, then choosing rootstocks to match conditions.
An 18-24 month interval from cutting trees to replanting a new orchard leaves sufficient time to address the potential for the most serious challenges: replant disease, root lesion nematode and crown gall.
Orchard removal should start with a soil test for nematodes. If nematodes are present, tree removal starts in the fall. Katherine Jarvis-Shean, University of California Cooperative Extension (UCCE) orchard systems advisor in Sacramento, Solano and Yolo counties said when trees are being cut, growers can kill the roots chemically or they use a backhoe to remove stumps. The key to controlling root lesion nematode is to remove or kill as much root material as possible.
The chemical kill involves painting the stumps within a few minutes after cutting with Garlon undiluted or with the surfactant MorAct. Luke Milliron, UCCE sustainable orchard systems advisor in Butte, Glenn and Tehama counties said the stumps should be left for 60 days to allow the herbicide to kill as many roots as possible.
Removing roots and planting an interim crop that pulls moisture from the ground prepares the orchard site for fumigation if soil samples determine a need. Soil moisture of 12 percent is the target.
Soil moisture at 12 percent will help the fumigant move through air spaces in the soil and be more effective. Ripping and reworking the soil in the summer can also dry down the soil, but is not always sufficient. Soil also needs to be warm—above 55 degrees F at one foot depth.
Milliron said studies have shown that two products—Telone II—a nematicide and Chloropicrin applied together will target both root lesion nematodes, crown gall and replant disease. Picking out as many roots and buried gall tissue from the replant site is also critical, since these woody tissues may protect disease inoculum from the fumigants.
With replant disease, growers are fighting a complex of pathogens that feed on the roots of mature trees. These pathogens will affect growth of newly planted trees if they are replanted in the same soil.
The future health of the young trees planted at the old orchard site depends on the rootstock. UC recommendations for orchards where nematodes were present in the previous orchard is the clonal Paradox rootstock VX211. Where crown gall pressure is high, RX1, which showed moderate resistance, is an option. For more information on selecting the right rootstock for an orchard’s conditions, see http://www.sacvalleyorchards.com/blog/walnuts-blog/selecting-the-right-clonal-rootstock-for-managing-soil-and- pest-problems/.