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"Potassium phosphite, also called «phosphonate», is a salt of phosphonic acid with the formula K2HPO3.
It should not be confounded with phosphates, nor with the organophosphorous insecticides which are also called «phosphonates». Potassium phosphite can be used as a fungicide against oomycetes in various crops. In the early 1990ies, it was extensively tested on organically grown grapevines in Switzerland. It was effective against downy mildew (Plasmopara viticola), and blocked the disease up to 3 days after infection. Phosphite was mobile and very persistent in plants and could be detected in grapes harvested one year after the last application. Analyses of 53 wine samples reveiled that the treatment inevitably leads to phosphite residues in wine, usually ranging between 5000 – 10000 ppb phosphite. Phosphite residues were also found in other crops (e.g. potato, celery) treated with potassium phosphonate.
From a toxicological point of view, these residues are of no concern. However, consumers of organic wine expect to buy a «natural» product, and we assume that they would not approve the presence of such quantities of fungicide residues in organic wine.
Currently, potassium phosphonate is not authorized as a fungicide for organic farming in the EU, and we do not recommend its use in the future”
