The following article provides some good information about phosphorus acid fungicides.

This term “phosphorous acid” 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.  

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. 
We found, for instance, that ProPhyt had efficacy against Downy mildew, Phomopsis,



The phosponate 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 rot

Since these fungicides are actually 
in salt form, care must be taken not to exceed a certain concentration as crop injury may 
result. In addition, if the concentration is too high, the pH may become so low and result in crop injury



Source: Michigan Fruit Crop
Advisory Team Alert, Vol. 20, No. 5, May 10, 2005