Phosphorous Acid Fungicides

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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. In Australia, where the patent did not apply growers have been using these types of fungicides for over a decade

The term “phosphorous acid” should not be confused with phosphoric acid or phosphorous (P) which is 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.  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 defence response against pathogen attack. This probably explains the much broader spectrum of activity observed in fungicide efficacy trials in small fruit crops in Michigan. We found, for instance, that ProPhyt had efficacy against Downy mildew, Phomopsis, and Black rot (but not much against Powdery mildew) in grapes. We also have evidence of activity of these compounds against Anthracnose in blueberries. 

The phosphonate ion is highly systemic and fairly stable in plants. The systemic activity allows them to be applied as foliar fungicides for prevention of Phytophthora and Phythium root rots. They also display curative activity but follow label directions. These fungicides are sold as solutions of potassium and/or sodium salts of phosphorous acid. To compare them, one should look at the “phosphorous acid equivalent,” which should be listed on the label. Since these fungicides are actually in salt form, care must be taken not to exceed a certain concentration as crop injury may result, follow label directions.

(There has been some minor changes to the above article, original at /Phosphorous AcidFungicide _Ellis.pdf )            

The following information collected by George Allen:

It is widely known that phytophthora in avocados can be controlled by the injection or spraying of phosphorous acid products. It is also useful on a wide range of plants that suffer from phytophthora invasion & other plant fungi type diseases to varying degrees. Plants that are affected include Mangoes, Blueberry, Apple, Loquat, Plum, Avocado, Coconut, Citrus, Cucurbits, Asparagus, Grape, Lettuce, Onions, Pineapple, Potatoes, Stonefruit, Strawberry, Raspberries, Blackberries, Tomato, Chilli, Egg plants, Potatoes, Oak trees, Beech trees, Durian, Cocoa, Pepper, Taro, Rubber, Rambutan, Longan, Papaya (Pawpaw), Santol, Guava, Rosella, Vanilla; the list is enormous. It looks like most fruit trees are affected to some extent by fungus-like diseases that can be controlled by these products. In our garden partial dieback indicates that Rollinia, Custard Apples, Soursop, Mango, Pawpaw, Jackfruit and a lot of the Myrtaceae are affected.  In Australia major parts of native flora are under attack and one of the main defences being used is phosphorous acid products/salts. This is a strange substance; it appears to be very safe, extremely stable in the plants (3 to 5 years) and to have no negative effects except for leaf burn when applied too heavily. Only recently has it been found existing naturally in plants. Yates do have a product on the home gardening market, but it is very low key. Use it and watch your trees respond. Do some research on the internet: you will be surprised and wonder why it has remained so obscured.      Highly Recommended for use on your trees