Phosphorous Acid Fungicides and Nutritional Value

Recently, a number of new fungicides that have phosphorous acid as the active ingredient have come on the market. Other names that you might hear for this are “phosphonates” or “phosphates.” Examples are ProPhyt, Phostrol, and Agri-Fos. Aliette (fosetyl-A1), an older fungicide, is the prototype for this group of fungicides. However, the long standing patent on Aliette had prevented similar fungicides from being developed up to recently.



Phosphorus-containing substances occur naturally (0.1%-0.5%) in foods such as milk, meat, poultry, fish, nuts, and egg yolks.
Phosphorous acid(CAS.NO:13598-36-2) should not be confused with phosphoric acid or phosphorous (P), a fertilizer component. In fertilizers, P is normally found in the form of phosphoric acid (H3PO4), which readily disassociates to release hydrogen phosphate (HPO42) and dihydrogen phosphate (H2PO4-). Both of these ions may be taken up by the plant and are mobile once inside the plant. Phosphorous acid is H3PO3. A single letter difference in the name of a chemical compound can make a major difference in its  properties. Phosphorous acid releases the phosphonate ion (HPO32-; also called phosphite) upon disassociation. Phosphonate is easily taken up and translocated inside the plant. Phosphorous acid does not get converted into phosphate, which is the primary  source of P for plants.


While growers are familiar with phosphorus-containing fertilizer, the abundance of terms, which are deceptively similar (such as phosphoric acid and phosphorous acid), may create some confusion on the actual content and efficacy of these products. . Some claims found in commercial literature and product descriptions refer to phosphorous acid as a “supplemental fertilizer,” while others present it as a fungicide. The purpose of this article is to explain what phosphorous acid actually is and to examine its fungicidal activity and nutritional value.
Phosphorus (abbreviated P) is one of the essential elements for normal growth and development of plants. In fertilizers, it is normally found in the form of phosphoric acid (H3PO4) (Table 1), which readily disassociates to release hydrogen phosphate (HPO42-) and dihydrogen phosphate (H2PO4-). Both of these ions may be taken up by the plant but H2PO4- more readily. Once inside the plant, both ions are mobile.


Because phosphorous acid and its derivatives do not get metabolized in plants, they are fairly stable and probably contribute little or nothing to P nutritional needs of the plants. Some researchers have investigated the ability of phosphorous acid to act as a nutrient source for plant growth and found that P-deficiency symptoms developed with phosphorous acid as a sole source of P. This means that although phosphorous acid can control diseases it is not a substitute for P fertilization. The inverse is also true: phosphate is an excellent source of P for plant growth, but is unable to control diseases other than improving the general health of the crop. So applying high amounts of P fertilizer will not work as a disease control measurer.


Researchers have found that phosphorous acid fungicides are especially effective against Oomycete pathogens, such as Phytophthora, Phythium, and Downy mildews in a number of crops. Phosphorous acid has both a direct and indirect effect on these pathogens. It inhibits a particular process (oxidative phosphorylation). In addition, some evidence suggests that phosphorous acid has an indirect effect by stimulating the plants natural defense response against pathogen attack. This probably explains the much broader spectrum of activity observed in fungicide efficacy trials in small fruit crops in Michigan.



According to the Center for Science in the Public Interest (CSPI), a consumer watchdog group not affiliated with the food industry, only a small fraction of the phosphate in the American diet comes from additives in soft drinks. Most comes from meat and dairy products. So your reason for not drinking Coke should be its sugar content and artificial food colorings, not the phosphoric acid.