Michael V. McKenry, based at the Kearney Agricultural Center at Parlier, discussed the basic guidelines recently at the Tri-County Walnut Day at Visalia. "First, it’s important to know what pests are in the orchard, before it is pulled, so you know what you are treating for."
Eight-five percent of the walnut orchards in California are infested with root lesion, ring, or root knot nematodes. Oak root fungus is another prevalent pest.
Another component of the "replant problem" is the phenomenon of rejection of new trees where old trees were removed. When soil pests are not present, killing other soil organisms with strip treatments of Telone can bring new tree development in a replanted orchard close to that in virgin soil.
McKenry said he and others are searching for new approaches. "Once we can kill the nematodes inside the roots and we don’t have to penetrate the roots, we can go to softer materials."
The primary tool for management of root lesion nematodes has been pre-plant fumigation with methyl bromide, but that material is being phased out.
In brief, the procedure starts with diagnosis of physical, chemical, and biological problems in the soil, along with sampling to identify nematodes and diseases. It continues with killing old roots and waiting 18 months before replanting.
During that period, soil deficiencies can be corrected and the soil can be dried if Telone is to be applied. The final step is replanting with small amounts of macro and micronutrients.
McKenry said the guidelines spring from several years of small plot experiments. Commercial testing of them has been limited, and revisions will be made as commercial trials produce more information.
Once soil pests are determined, the old trees are removed and the soil is irrigated deeply. The stumps, with the cambium layers exposed, are brushed with a solution of Garlon 3A and MorAct before the end of October.
A deep ripper blade to sever roots from the treated trees or avoiding applications to outer rows of trees will prevent translocation of Garlon from killing adjacent root-grafted trees.
After 60 days, the trees are pushed out and the soil ripped and leveled.
Sudangrass or safflower is spring-planted, but not irrigated after mid-June to deep dry the soil. The sudangrass is harvested and removed once it is four feet high. The sudangrass reduces nematode populations by about 85 percent of what was achieved with methyl bromide. If the land is left fallow the populations can be expected drop by only 50 percent.
A complete year after the Garlon treatment, McKenry said, the old roots are beginning to yellow and darken and 99 percent of the nematodes are gone. "We currently estimate the first-year growth benefit to be about 85 percent of that provided by methyl bromide."
If a Garlon treatment is not made, it can take two and a half years before the roots are free of significant amounts of nematodes.
Referring to the first step of determining soil pests, he said if the soil shows more than one root lesion nematode per sample, or if ring nematodes counts exceed several hundred per 250cc sample, a nematicide will be needed in the top five feet of soil.
Six years relief
"The goal is to achieve six years of nematode relief or 99.9 percent control. Control at the level of 98 percent in soil or roots will provide no more than one year of nematode relief and tree growth will begin to slow in the second and third years after planting," McKenry said.
At rates of 330 pounds to the acre on sand or sandy loam soils, Telone II, Telone C35, In-Line, metam sodium, metam potassium, and chloropicrin all will provide adequate nematode control, particularly when the root refuge for root lesion nematodes has been destroyed.
Choices from the alternatives depend on the grower’s evaluation. A material may be less expensive but may also cost more to apply.
"Unfortunately," he added, "walnuts are usually grown on the best, deep draining soils which are also of finer texture than sandy loam."
Procedures at this point vary according to the nematicide selected, its ability to move through the soil, soil moisture, and the method of delivery, such as by shanking or drip irrigation.
McKenry said the lack of post-plant nematicides and resistant rootstocks for walnuts requires that broadcast applications of biocidal agents be made.
Telone II applied in strips or without additional nematode control at the field surface may not last beyond one year. Strip applications of up to 12 feet wide, he added, will have value only when resistant rootstocks or post-plant nematicides become available or when damaging nematodes are not present.
New bare-root trees grow better with one of several combinations of macro or micronutrient products applied as a starter at planting.
McKenry said several gaps remain in the procedures and need field evaluation. Those include comparison between two and three years of sudangrass and equal amounts of fallow time, use of additional nematicidal agents when root populations of nematodes have been controlled, rootstocks resistant to root lesion nematodes, methods of avoiding Garlon translocation to adjacent root-grafted trees, and rates of return of nematodes in additional commercial settings..