Phosphorous Acid Fungicides and Phosphorus Nutrients

Phosphites and Phosphonites are alkali metal salts of phosphorous acid and sold as fungicide that control a number of crop diseases particularly those caused by Phytophthora spp. Fosetyl-Al the active ingredient in Aliette is an example of a fungicide in this class that is registered on several crops in Canada for the control of diseases caused by Phytophthora. There are several other different metals salts of phosphorous acid sold as fungicides in the US that are not available in Canada. Fosetyl-Al, was one of the first phosphorous acid type fungicides developed that can move both up and down in plants. Once inside the plant, it is broken down rapidly into phosphorous acid, which is stable but extremely soluble in water and toxic to many Phytophthora species.

Phosphorous acid type fungicides works in two ways. They act directly on the invading fungus to stop its growth and sporulation. They also act indirectly by stimulating the plant to activate their own defense system, thus helping to prevent future infections.
 Plants that have their defense system activated prior to invasion by a pathogen can defend themselves much more effectively.

There is no evidence that phosphorous acid type fungicides can be used directly by plants as a source of nutritional phosphorus. It is possible that when these products are applied to plant root systems or in the soil, soil microbes could convert these compounds to phosphate which would be available to the plant as a phosphorus nutrient. This is a slow process and the amount of phosphate produced would not be sufficient to satisfy the phosphorus requirements of plants. There is evidence however that phosphorous acid type fungicides may actually intensify the deleterious effects of phosphorus deficiency by 'tricking' deprived plant cells into sensing that they are phosphorus sufficient, when, in fact, their cellular phosphorus content is extremely low. Therefore, it is a good idea to make sure that the plants are not deficient in phosphorus prior to using a phosphorous acid type fungicide

In contrast to phosphites and phosphonites, the form of phosphorus plants take up as a nutrient is known as orthophosphate. Fertilizers containing phosphorus are water soluble and once they dissolve, orthophosphate is available for uptake by plants. Following application to the soil, orthophosphate is prone to fixation reactions after which it is no longer available to plants. At lower pH, it forms insoluble salts with aluminum and iron. At higher pH, it reacts with calcium. Either way, these forms are not available to plants, which is why limiting the exposure of phosphorus fertilizer to soil by banded application is a common practice.