Integrated Methyl Bromide Alternatives in Walnut Orchards
Project Leaders: Bob Beede, UCCE, Hanford; Joe Connell, UCCE, Oroville; Greg Browne, USDA-ARS, Davis
Cooperators: Mike McKenry, UCR, Parlier; Bruce Lampinen (UCD, Plant Sciences), Joe Grant, UCCE, Stockton; Karen Klonsky (UCD, Agricultural and Resource Economics), Dan Kluepfel (USDA-ARS, Davis), Doug Verboon (Kings Co walnut grower), and Jeff Cripe (Butte County walnut grower)
Pre-plant soil fumigation with methyl bromide (MB) has been used widely to control nematodes and other soilborne pathogens before replanting walnut orchards. Over time, walnut orchards commonly build and support relatively large populations of the root lesion nematode (Pratylenchus vulnus). Conventional walnut rootstocks also are susceptible to and encounter the ring nematode (Criconemella xenoplax), root knot nematodes (Meloidogyne spp.), and the dagger nematode (Xiphinema spp.) (Westerdahl and McKenry, 2002). Pre-plant treatments with MB have afforded temporary suppression of Armillaria melea (cause of “oak root fungus” disease), Phytophthora species (cause of Phytophthora crown and root rots), and Agrobacterium tumefaciens (cause of crown gall disease) (Mircetich et al., 1998; Yakabe et al., 2010). It also is possible that preplant soil fumigation results in additional favorable shifts in soilborne microbial communities that stimulate tree growth in replanted walnut orchards, but this has not been specifically explored.
When properly applied, pre-plant soil fumigation with MB has effectively managed the known nematode pests of walnut and given partial control of the other soilborne pathogens across all soil types commonly used for walnut production in California. Although many walnut growers have shifted towards use of 1,3-dichloropropene (1,3-D) as an alternative to MB, this shift is not considered to be stable for several reasons, including, but not limited to:
- Compared to MB, 1,3-D diffuses poorly through clay, clay loam, and silt loam soils often used for walnut production in the upper San Joaquin and Sacramento Valleys.
- Soil fumigation is becoming very challenging in California due to regulatory restrictions. Rate limits, buffer zone requirements, 1,3-D township use caps, and regulations designed to reduce emissions of volatile organic compounds seriously complicate effective use of 1,3-D and other MB alternatives over much of California’s walnut acreage.
With the goal of reducing dependence on pre-plant soil fumigation, the California walnut industry has intensified efforts to develop rootstocks with genetic resistance to the complex of soilborne pathogens that affect walnuts. Walnut rootstocks ‘VX211’, considered tolerant (but not resistant) to the root lesion nematode, and ‘RX1’, considered moderately resistant to Phytophthora citricola and P. cinnamomi have recently been released by UC/USDA-ARS for commercial use (McGranahan et al., 2010a; McGranahan et al., 2010b).
Nevertheless, these rootstocks only address some of the replant problems faced by walnutproducers, and further rootstock development and testing are needed. In the meanwhile, effective pre-plant soil treatments are needed by walnut orchardists for management of walnut replant problems.