Adapted from the article “Best Management of Replant Alternative Fumigants” by Carolyn DeBuse, Former UCCE Farm Advisor, Solano, Yolo Counties and Michael McKenry, Extension Nematologist Emeritus and “Orchard removal and site preparation for walnut planting” by Joseph Connell, UCCE Farm Advisor Emeritus, Butte County in the October 2012 Sacramento Valley Walnut News.

Replanting walnuts into an orchard with root lesion nematode, Pratylenchus vulnus is difficult after the phase-out of methyl bromide, but still feasible if the following steps are taken.

Nematode Sampling. Send soil samples to a lab to determine if nematodes are present. Plan on a broadcast soil fumigation if P. vulnus is present anywhere within the field at populations greater than 1 P. vulnus per 250 cc of soil and the previous crop was a perennial.

 October- Mid-November: Kill and Remove Previous Orchard Roots. Living roots left in the soil from the previous orchard will harbor nematodes and pathogens that will quickly invade new root systems. Thus, roots must be killed or physically removed. Garlon herbicide can completely rid old walnut roots of P. vulnus within 6 months. However, fumigation is still necessary after Garlon. Five-years after Garlon kills roots up to 5% of the P. vulnus population can remain alive in soil without feeding, making fumigation is still necessary. Glyphosate is best for killing Prunus species roots (prunes, almonds, and peaches). Garlon has had better results killing walnut roots.

Herbicide Approach

1) Irrigate the orchard, then cut down trees, leaving the stumps.

2) Paint the cut tree stumps with herbicide by the end of October. For walnut stumps, use 50ml Garlon 3A plus 50ml MorAct. For almonds, prune, and peaches use a straight 50:50 mix of Roundup® (41% glyphosate) and MorAct®. A sponge fastened to the end of a broom handle into the liquid, then onto the freshly cut surface has worked well. These mixtures are thick and paint-like and may damage sprayers or their components, and, they are also likely to clog the sprayer. Check the label for use procedures.

3) Wait at least 60 days before removing stumps and ripping the ground. With the herbicide method cross ripping and hand-removing roots is not necessary.

4) Within five months nematode populations within roots can be reduced by 99%. Now, any fumigant used does not need to penetrate and kill old roots. However, the fumigant does need to penetrate as much soil volume as possible to kill free nematodes in the soil.

Traditional Root Removal

If you choose not to use Garlon or glyphosate to kill the roots, they should be removed in the traditional style of deep ripping to bring roots to the surface and then hand removing them from the orchard site. It will take over 2 ½ years for the nematode population to be reduced to the same level herbicide achieves in one.

March-May: Plant a Cover Crop or Leave Fallow. Plant safflower or true sudan grass. These do not support nematode development and will deep-dry the soil profile. Or leave the site fallow for at least one year between orchard removal and replanting to starve nematode and soil pathogen populations and to dry out the soil. Sudan grass can reduce the soil dwelling nematode population by 85% while fallowing the soil only reduces them by 50%.

September-November: Fumigation. Dry soil is critical for fumigation success. True fumigants move 10,000 to 30,000 times faster in soil air spaces than in soil water. Rip and rework fallow soil through the summer to dry soils to the 5 feet. Fumigate after September but before 2 inches of rainfall occurs and before temperatures drop below 55°F (at one foot depth). Soil moisture content on a dry-weight basis at the time of application should be at or below 12% throughout the top five feet of soil to be successful with 332 lb/acre Telone II.

Walnut soils in the Sacramento Valley are almost always high in clay content. These finer- textured soils are harder to dry because their soil air spaces are more abundant and also smaller than those of the coarser-textured San Joaquin Valley soils. In finer-textured soils expect reduced overall fumigant movement particularly because molecules of Telone II and Chloropicrin move slower through soil while degrading much faster than molecules of methyl bromide decreasing fumigant effectiveness.

Fumigation Choice Depend on Soil Texture, Moisture and Temperature.

• Sandy or Sandy Loam Soils: All true fumigants (Telone II, Telonce C-35, and chloropicrin) move best through coarser-textured soils because these soils dry deeper, faster and gases move through them more easily. Fumigants that must be moved with water, including In-line, Metam Sodium, and Metam Potassium also perform best in these soils. They provide adequate nematode control in coarser textured soils if uniformly mixed with at least 6 acre inches of water; enough to move them to five-foot depth.

• Silty Loams or Clay Loams: For best results (control to five foot depth) on finer-textured soils pre-rip soil in one direction to 4 to 5 foot depth on 2 foot centers. Disc and ring roller to close soil. Fumigate using a shank mounted with 2 wings along its length just above the shank delivery points which are located 15-20 inches and 24-30 inches beneath the field surface (“Buessing shanks”). Follow shank application with disc and roller.

Product mix and application rate at different depths depends on soil moisture. At less than 12% moisture content, apply 332 lb/ac Telone II equally mixed between the two delivery points. At 12 to 15% moisture content, apply 332lb/ac Telone II at upper depth plus 150 lb/ac chloropicrin at deeper depth. At 15 to 19% moisture content, apply 332 lb/ac Telone II at upper soil depth plus 250 lb/ac chloropicrin at deeper depth.

(Telone II and chloropicrin need a restricted materials permit that you can apply for at your County Ag Commissioner’s office)

Rootstock. It’s recommended, when removing an orchard, to replant to a different crop or to a rootstock resistant and tolerant to pest presence. Nematodes feed on walnut root tips. English, Black and Paradox rootstocks are all highly susceptible to root lesion nematode, P. vulnus, supporting thousands of P. vulnus per gram of root at the terminal 12 inches of roots. Only one clonal Paradox, VX211, is currently known to tolerate nematode feeding and this tolerance can be overcome if nematode populations are too high at planting time.

Nutritional considerations:  Soil is depleted of many micro and macro nutrients by previous crop production and fumigation can lower available phosphorus (P) and zinc (Zn). It is important to consider these nutrients when replanting an orchard.  Fertilization of the young trees may be needed. Leaf analysis should be done in July of the first year to confirm nutritional status.

For more information go to Dr. Michael McKenry’s website.