The most important use of phosphonic acid is the production of
phosphites (phosphonates) which are used in water treatment. Phosphonic
Acid is also used for preparing phosphite salts, such as potassium
phosphite. These salts, as well as aqueous solutions of pure phosphorous
acid, are fungicides. Phosphites have shown effectiveness in
controlling a variety of plant diseases, in particular, treatment using
either trunk injection or foliar containing phosphorous acid salts is
indicated in response to infections by phytophthora and pythium-type plant pathogens (both within class oomycetes,
known as water molds), such as dieback/root rot and downy mildew.
Anti-microbial products containing salts of phosphorous acid are
marketed in Australia as 'Yates Anti-Rot'; and in the United States of
America, for example, aluminum salts of the monoethyl ester of
phosphorous acid (known generically as 'Fosetyl-Al') are sold under the
trade name 'Aliette'. Phosphonic Acid and its salts, unlike phosphoric
acid, are somewhat toxic and should be handled carefully.