Understanding and Managing Sudden Oak Death in California Use of phosphites (phosphonates) to manage SOD

Phosphites (also known as phosphonates) are a group of fungicides which have potential for use in managing SOD. Phosphites are selective, systemic fungicides that have been used to manage Phytophthora diseases for a number of years. They have a high level of environmental safety and very low nontarget toxicity. The most widely used material in this fungicide group is potassium phosphite (also known as potassium phosphonate, or mono- and di-potassium salts of phosphorous acid). This material is sold under a number of trade names. Potassium phosphite moves systemically, in both xylem and phloem, of trees.

At least two general modes of action have been identified for phosphites (Guest and Grant 1991). At high concentrations in the plant, phosphite is directly toxic to Phytophthora species. At lower concentrations, phosphites act indirectly by increasing a plant's natural resistance response to Phytophthora infection. At these lower concentrations, phosphite appears to interfere with Phytophthora metabolism, causing the release of compounds that trigger host defense responses. Phosphites are primarily effective against Phytophthora species and have little or no activity against most diseases caused by true fungi.

Phosphite is most likely to be effective when applied as a preventative, before trees are infected. In field tests, phosphite has not been effective against cankers in previously-infected tanoaks, even if external cankers symptoms were not visible at time of application.

Most studies indicate that phosphite does not prevent infection by P. ramorum, but may reduce the rate at which P. ramorum cankers expand. If disease pressure is high, phosphite may not suppress disease progress enough to provide practical levels of disease control. In a recent controlled study involving over 470 large tanoaks, 3 years of preventative annual phosphite bark spray applications did not reduce disease incidence or mortality when SOD subsequently spread into the stand. Phosphite applications were started well before P. ramorum was detected in or near this stand.

Phosphite solutions can be phytotoxic to some species if applied at sufficiently high concentrations. Leaves of many species, including tanoak, may develop extensive browning (necrosis) if they are sprayed with the highly concentrated phosphite solution used in trunk spray applications. Mosses, which are common on oak and tanoak trunks in some locations, are also killed by the spray. Injected phosphite solutions can also cause necrosis in the bark and sapwood. Phosphites should be tested for potential to cause phytotoxicity whenever it is used on a new plant or if a modified application method is used. Environmental and user safety—

Phosphites are considered by the EPA to be biopesticides because these salts are closely related to common, widely occurring substances. However, phosphonate salts are not naturally occurring mineral substances. Information on toxicity and environmental fate of this material was summarized in an EPA fact sheet EPA fact sheet 076416 issued 10/98.


A potential for exposure exists to nontarget insects, fish, and other wildlife with foliar spray applications. However, test results indicate that the compound is practically nontoxic to birds and freshwater fish, and, at most, slightly toxic to aquatic invertebrates. Low toxicity, the proposed rate of application, and mitigating label language present minimal to nonexistent risk to wildlife."

Phosphorous acid and its ammonium, sodium, and potassium salts are also exempt from food tolerance, for both crop and postharvest uses (USEPA 2006). In exempting phosphites from tolerance, the EPA cited both the low toxicity of phosphites and the fact that phosphites had already been used widely as fertilizers.. In a preliminary study, acorns from tanoak trees treated with phosphite showed no substantial differences in nutritional quality when compared with nontreated trees (Meyers and others 2007).

Although phosphites are relatively safe materials, phosphite solutions, particularly in their concentrated form, may be harmful if inhaled, ingested, or absorbed through the skin and must be handled safely to avoid exposing applicators or others.. Formulations of potassium phosphite typically have the "Caution" signal word on the label. Applicators and others handling the material for mixing and loading must wear protective eyewear, long pants and a long-sleeved shirt, waterproof gloves, shoes, and socks. Depending on the application method, additional personal protective equipment may be necessary.

Potassium phosphite is corrosive to many metals, especially brass, which is commonly used in sprayers and injection equipment. Sprayer components that are exposed to potassium phosphite solution should be made of plastic or stainless steel. To minimize the potential for corrosion, thoroughly rinse application equipment with water immediately after use.