understanding phosphorous acid products Phosphites are labeled and sold as both fungicides and fertilizers. The reason for the fertilizer label is that the phosphite will be exposed to oxygen and microbial action over time and, thus, transformed into phosphate fertilizer. The problem is that, according to some scientists, this process is too slow to be of any practical value (it takes at least six weeks). For example, bone meal is a form of phosphorus certified for use in organic food production, but you can imagine how immediately available to plants the phosphorus in bones might be—yet bone meal has no label restriction in this regard. Nevertheless, it is for this reason that thirteen states—California, Georgia, Indiana, Kentucky, Michigan, Minnesota, Missouri, North Carolina, Oklahoma, Oregon, Pennsylvania, Texas and Washington—do not recognize phosphites as a source of phosphate fertilizer. In these states, a phosphite with an analysis of 0-2825 would have to be labeled 0-0-25. In California, it would be labeled as 0-28*-25 (the asterisk is required since a state law was passed in 2006). So, there are often arguments about whether phosphites are really fungicides or fertilizers—sort of like the old Saturday Night Live skit: “It’s a floor wax, no it’s a dessert topping; actually, it’s both!” The mode of action in which phosphite materials control fungal disease is not clear, but it is thought that there is direct activity against the pathogen and an effect on the crop plant. Phosphites are known to jump-start the plants’ immune system by ramping up the production of phytoalexins. There is evidence for the existence of other lesser-known modes of action, thus making the formation of resistance to phosphites very unlikely (given that the pathogen has to overcome these multiple systems). It is for these reasons that phosphites are so effective for disease control and why there are literally dozens of manufacturers worldwide selling them as fertilizer and fungicide. Phosphites are routinely used in fungicide rotations to manage resistance. The phosphites belong to resistance management class FRAC 33. Phosphites are usually created by combining a strong inorganic acid (low pH solvent) with a strong inorganic base (high pH solvent) to form phosphite salts. Inorganic, in the chemical sense, means there is no carbon in the molecule.
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Phosphites have another interesting characteristic: once absorbed into the plant they travel in both the xylem and phloem conductive tissues, meaning that they are transported to all plant tissues in a systemic fashion. Moreover, phosphites are readily absorbed by root, stem and leaf tissues, making for versatile application procedures (such as drenches, root dips, tree bark treatments, soil application, soil injection, chemigation and foliar and aerial application methods). And besides being useful during the crop season, many phosphites are labeled for post-harvest applications by either spraying the crop as it enters a storage bin or by using a vaporizing system. As a rule, phosphites are very safe for humans and the EPA often approves them as reduced-risk pesticides.
Phosphites
have another interesting characteristic: once absorbed into the plant they travel in both the xylem and phloem conductive tissues, meaning that they are transported to all plant tissues in a systemic fashion.”