Gabriele Ludwig, Associate Director, Environmental Affairs; Bob Curtis, Associate Director, Agricultural Affairs; Almond Board of California

This article is the third in a series on considerations for replanting an almond orchard. (For the first in the series, please see Almond replant strategy pays dividends down the road. For the second in the series, please see Rootstock decisions vital to replanted almond orchards.)

Soil fumigation is often a major consideration when replanting an almond orchard. This final article in our three-part series on almond orchard replant considerations will focus on some of the many decision points and regulatory hurdles surrounding soil fumigation.

Site-specific criteria and objectives should be well defined before embarking on a soil fumigation plan. It is important to first identify through soil sampling and orchard history what the issues are at the specific site that might require fumigation. From there, you can determine the criteria for whether or not to fumigate for those issues, and which materials or combination of compounds work best under that specific situation.

In many cases soil fumigation is the only truly effective method for addressing aggressive pathogens and nematodes in the orchard. Decisions, however, don’t stop at that determination. They are further complicated by the myriad of regulatory hurdles related to soil fumigation. For that reason, we recommend that once you have met the criteria for soil fumigation during orchard replant, your first step should be to meet with your county agricultural commissioner’s office to find out what compounds are available for your county, and what regulatory restrictions are placed on site and timing of application.

Prior to replanting an orchard, take the time to sample soils for nematode populations. Low levels of nematodes can often be managed through cultural practices such as weed management prior to orchard removal, cover crops and resistant rootstocks. Merced County Farm Advisor David Doll says that if sampling reveals high levels of nematodes, consider a broadcast or row-strip Telone fumigation treatment.

For replant disease, if fallowing for a year is not an option, row-strip or tree-site fumigation treatments with pure chloropicrin or Telone C35 with chloropicrin have shown significant yield and growth response in young trees. Fumigation with a broadcast treatment of methyl bromide combined with chloropicrin (Pic), will not completely manage the problem, but can significantly reduce pressures from localized aggressive pathogens such as Armillaria and Phytophthora.

Despite what we know about the best compounds, criteria and conditions for fumigation, in the end, regulatory issues may play an even larger part in the grower’s fumigant decisions. There are three major regulatory issues that have an impact on the grower’s choice of soil fumigant materials at the international, federal and state level. In some cases, those issues can impose contradictory and even conflicting requirements.

Internationally, under the Clean Air Act’s Montreal Protocol, methyl bromide has been phased out for use as a soil fumigant, except under highly specific critical use exemptions (CUE). Methyl bromide is still an important fumigant for almonds in certain situations; it is still considered the best material for oak root fungus, for instance. While some CUEs for almonds do still exist, the phase-out has made methyl bromide available in only limited supply and in many cases, cost-prohibitive.