The temporary Fosetyl-Al maximum residue limit (MRL), which covers residues from phosphite-containing products for the European Union, is now extended until March 1, 2019, and is retroactive to January 1, 2016. The decision was officially announced in the January 25, 2016, European Journal. The MRL encompasses almonds, cashews, hazelnuts, macadamias, pistachios and walnuts at the level of 75 mg/kg. Though the temporary MRL has been extended, there have been a number of questions about compound use and timing.
Almond, walnut and pistachio growers can resume the use of phosphite products, which may be important for root health. There is not good data correlating different rates, timings or types of applications with residue levels. This compound moves in both the phloem and xylem of the tree. Preliminary data has shown that residues can carry over from year to year, indicating they do not break down quickly and may be stored in the tree. Because the plant takes up the compound internally, residues are not just on the outside parts of the tree. Research is underway to understand how residues originate. Note that there are no health concerns associated with the residues. EPA has exempted potassium phosphite as a fungicide from tolerance because of its low toxicity, and even the European Food Safety Authority (EFSA) does not have health concerns for phosphite.
Phytophthora Control
Greg Browne from USDA-ARS along with Mario Viveros, then–farm advisor, Kern County, published reports in 2005 showing that judicious applications of phosphonate-containing materials could help control both disease-causing fungi Phytophthora citricola and P. cactorum in almond orchards. They found that a single postharvest foliar treatment with potassium phosphite a month before leaf fall or a single foliar spring treatment with potassium phosphite after leaf-out were effective in controlling Phytophthora. Applications before leaf-out were not effective. This work was co-funded by the Almond Board of California.
More recently, Dr. Themis Michailides has tested efficacy for control of Botryosphaeria in pistachios, which may be applicable to almonds. For those applying materials with phosphite compounded with micronutrients such as calcium or zinc, rather than potassium, consider when the plant is most likely to take up and use the micronutrient from the leaves in order to time the application. Note that while resistance development is slow, it has been found in other crops; thus, repeated doses of phosphite-containing products should be avoided.
The effort to establish a permanent maximum residue limit for the European Union continues, and is expected to be finalized by March 2019. The Almond Board of California will continue sharing updates on this important issue as information becomes available. For questions, please contact Gabriele Ludwig (gludwig@almondboard.com), Beth Van Meter (evanmeter@almondboard.com) or your local farm advisor.
