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.

New label requirements

On the federal level, the Environmental Protection Agency recently completed a re-registration process for common fumigants in almonds, including methyl bromide, chloropicrin and metam sodium. As of January 2011, there are new label requirements that include the development of a fumigant management plan, new safeguards for worker and applicator protection, and new staffing requirements. Additional restrictions will be placed on those labels in early 2012 to include more extensive buffer zones or tarps, restrictions on use in sensitive sites such as schools and prisons, and warning requirements for neighbors within a certain range of application site.

With these new labels, it is important growers read the label and associated booklets carefully before making an application.

Another compound, methyl iodide, which is already approved at the federal level, has been proposed for approval in California, although various lawsuits and environmental opposition are jeopardizing the future state registration of this promising compound.

In the San Joaquin Valley, VOC rules enacted in 2009 have imposed severe limitations on the rate, timing and application method of common fumigants during the ozone period from May 1 through Oct. 30.

These restrictions can make fumigation applications less efficacious and often more expensive due to required mitigation measures. For instance, the VOC window pushes applications into the fall and winter months when soils are less likely to be dry and warm— preferred conditions for optimum efficacy. Methyl bromide (which performs better than other available compounds under cooler, moist conditions) can be used either alone or in combination with Pic from May to October. However, the new VOC rules require that methyl bromide be applied during that window using an expensive high density polyethylene (HDPE) tarp.

Further complicating this regulatory maze, these VOC requirements do not necessarily correspond with the EPA requirements enacted under the new labels. As mentioned above, your local ag commissioner is a good place to start to help sort out what restrictions exist and which compounds are available to you in your area at the time you plan to apply. For the latest reports on Almond Board–funded research projects on fumigation, go to AlmondBoard.com/farmpress17.