Andreas Westphal, Ph. D. UC Riverside Professor of Cooperative Extension Nematology

Establishing a new walnut orchard is a long-term investment that requires detailed planning to enable 35 to 40 years of bountiful and sustainable production. Among other soil-borne maladies, soil-dwelling plant-parasitic nematodes can severely damage walnut. On susceptible rootstocks, root lesion nematodes, Pratylenchus vulnus (RLN) can increase to high numbers over time, reaching damaging levels in established orchards, and frequently old orchards leave behind populations of this nematode. RLN is estimated to be present in 85% of California walnut orchard soils. This nematode is highly damaging with a population density of one nematode per 250 cc of soil potentially reducing the growth of a newly planted orchard. Because it can inhabit the upper 5 ft of soil its suppression is challenging. After the ban of methyl bromide fumigation in 2005, alternative fumigants containing 1,3-dichloropropene (1,3-D, Telone) are increasingly regulated. Regulation to start on January 1, 2024, will make 1,3-D use more expensive and potentially less effective.

A California Walnut Board-supported trial was planted in 2021 to ‘Livermore’ walnut on seedling ‘Paradox hybrid’ rootstock following an old walnut planting. In August 2023, a field meeting was held at the trial on the farm of Robert Longstreth in Escalon, CA. The previous orchard had been removed in winter 2020, and the ground deep ripped and leveled. Experimental preplant soil treatments were applied in August 2020. These included the AITC-containing material Dominus, Reklemel (Salibro) (both not currently registered), Velum One, and different forms of anaerobic soil disinfestation (ASD). All treatments received 6 acre inch of water on treatment day except the comparative treatment of Telone II – chloropicrin application. In anaerobic soil disinfestation, easily decomposable substrates are spread on the soil surface, incorporated, drip irrigation lines installed, and totally impermeable film (TIF) used to cover the soil to exclude atmospheric oxygen. Under heavy irrigation, the process starts and microorganisms decompose the substrate, the aerobic microbes eventually deplete the soil oxygen, so anaerobic microbes become dominant and continue decomposing the material while producing secondary metabolites. The created conditions reduce nematode population densities. The potential and limitations of the different treatment options were discussed. In general, alternative preplant soil treatments resulted in less tree growth than Telone fumigation but several of them improved growth over the non-treated control. It remains challenging for treatments to compete with a properly done fumigation but treatment programs are being developed. A more comprehensive field day covering more soil treatment options and additional management tools including improved rootstocks is planned at the Kearney Agricultural Research and Extension Center in Parlier in the fall of 2023.