Fungicidal Activity and Nutritional Value of Phosphorous Acid

Adapted from original article: Brunings, A.M., Liu, G, Simonne, E.H., Zhang, S., Li, Y., and Datnoff. L.E. 2012. Are Phosphorous and Phosphoric Acids Equal Phosphorus Sources for Plant Growth? University of Florida Cooperative Extension Service, HS1010.

Asha M. Brunings, postdoctoral research associate; Guodong Liu, assistant professor; and Eric H. Simonne, professor and Director for Northeast District Cooperative Extension Service, Horticultural Sciences Department. Shouan Zhang, assistant professor, Plant Pathology Department and Yuncong Li, professor, Soil and Water Science Department, Tropical Research and Education Center. Lawrence E. Datnoff, former professor, Plant Pathology Department, University of Florida, Gainesville, FL 32611.

Phosphorus (abbreviated P) is one of the 17 elements essential for plant growth and development. Phosphorus is also a key component in some agrochemicals, such as phosphorous acid (H3PO3). Thus, there are two types of P closely associated with crop production. 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 common phosphorus-containing compounds used for crop production are listed in Table 1. 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 is and to examine both its fungicidal activity and nutritional value.

The amount of phosphorus in a fertilizer is represented as the middle number on the bag, expressed as phosphorus pentoxide (P2O5, such as 5-10-15). The first number represents the nitrogen percentage and the third number potassium percentage as K2O. The P2O5 unit used to represent P content in fertilizer is a conventional unit (in reality, there is little or no P in the form of P2O5 in fertilizer).

As an essential element for normal plant growth and development, P is utilized in the fully oxidized and hydrated form, orthophosphate (H3PO4). Plants absorb and utilize either hydrogen phosphate (HPO42) and/or dihydrogen phosphate (H2PO4-), depending on the pH of the growing medium. At pH7, both H2PO4- and HPO42- are approximately equal in amount. In fertilizers, P is normally not found in the form of phosphoric acid (H3PO4) unless the growth medium is very acidic. At pH levels below 2.1, H3PO4 can become the dominant form, but at pH levels more favorable for plant growth (near neutral pH), the amount of H3PO4 is negligible. Compared with either H2PO4- or HPO42-, it is only one out of 100,000 because it always dissociates to H2PO4- and further to HPO42-. Both of these H2PO4- and HPO42-- ions are the basic forms taken up by the plant, but H2PO4-is taken up more readily because in most growth conditions, soil solution pH is below 7. Once inside the plant, both ions are mobile.

Phosphoric acid should not be confused with phosphorous acid (H3PO3). A little difference in the name or formula of a chemical compound can make a dramatic difference in its properties. The former is a fully oxidized and hydrated form of P, whereas the latter is a partially oxidized and hydrated form. Therefore, phosphorous acid is a powerful reducing agent, but phosphoric acid is not. The former is a diprotic acid (readily ionizes two protons), but the latter is a triprotic acid (readily ionizes three protons). Phosphorous acid dissociates to form the phosphonate ion (HPO32-), also called phosphite. Phosphorous acid and its ionized compounds are often referred to as phosphonate or phosphonite. Like phosphate, phosphonate is easily taken up and translocated inside the plant.

Fosetyl-Al, which was registered by the EPA in 1983, is an aluminum salt of the diethyl ester of phosphorous acid and is sold under the trade name Aliette. It is a systemic fungicide used to control damping-off and rot of plant roots, stems, and fruit, and it may be taken up by the plant. Inside the plant, fosetyl-Al may ionize into phosphonate, and therefore fosetyl-Al belongs to the group of phosphorous acid compounds.

Phosphorous Acid as Fertilizer?

Phosphorous acid is not converted into phosphate, which is the primary source of P for plants. There are bacteria capable of transforming phosphonate into phosphate but this process is so slow that it is of no practical relevance. To date, no plant enzymes have been described that can oxidize phosphonate into phosphate. This is consistent with the fact that phosphonate is stable in plants and is not converted into phosphate. Since phosphorous acid and its derivatives do not get metabolized in plants, claims that phosphonate can contribute to phosphorus nutritional requirements for plants should be taken with caution.

Phosphorous acid is used in agriculture but for a different purpose than phosphoric acid. Confirming other investigations into the efficacy of phosphorous acid against oomycetes, (a group of pathogens that include water molds and downy mildew) Förster et al. (1998) found that phosphite is capable of controlling Phytophthora root and crown rot on tomato and pepper. The authors also tested the ability of phosphorous acid to act as a nutrient source for plant growth and found that P-deficiency symptoms developed when plants were grown hydroponically with phosphorous acid as the sole source of P (without phosphate). This means that although phosphorous acid can control oomycetes in a number of host-parasite systems, it is not a substitute for phosphorus fertilization. The inverse is also true: phosphate is an excellent source of P for plant growth but is unable to control pathogen attack by oomycetes, other than by improving the general health of the crop and, therefore its natural defense system. At this point in time, no evidence exists to substantiate the claim that phosphorous acid provides P for plant growth.

Control of Oomycetes

It is well documented that phosphorous acid is able to control diseases caused by pathogens that belong to the Oomycota (or oomycetes) on agronomical and horticultural crops. Oomycetes are actually not fungi but are frequently grouped with fungi, because they form structures (filaments) similar to the ones that fungi make. In reality, oomycetes are fungal-like organisms that differ from fungi in that their cell walls do not contain chitin but a mixture of cellulosic compounds and glycan (polymeric carbohydrate). Another difference is the nuclei in the cells that form the filaments have two sets of genetic information (diploid) in oomycetes instead of just one set (haploid) as in fungi.

For most practical purposes, the oomycetes are grouped with fungi. Compounds that control plant pathogens belonging to the oomycetes are often called fungicides. It is important to distinguish between fungi and oomycetes. Chemicals that are used to control one will often not be effective against the other because of biology differences. Several important plant pathogens belong to the oomycetes such as Phytophthora infestans, the causal agent of late blight of potato and the culprit of the Irish Potato Famine between 1845 and 1849; P. ramorum, the causal agent of sudden oak death; and Pythium and Peronospora species, among others.

Phosphorous acid has both a direct and an indirect effect on oomycetes. It directly inhibits a particular process (oxidative phosphorylation) in the metabolism of oomycetes. An indirect effect is the stimulation of the plant’s natural defense response against pathogen attack. It should be noted, however, that phosphonate-resistant oomycetes have been reported. In addition, some evidence suggests that phosphorous acid has an indirect effect by stimulating the plant’s natural defense response against pathogen attack.


A major factor in the ability of phosphorous acid to control oomycetes for long periods of time appears to be its chemical stability in the plant. Phosphorous acid does not convert into phosphate and is not easily metabolized. The stability of the different phosphonate-related compounds may depend on environmental factors such as climate or crop type. Because phosphonate is systemic and stable in the plant, it should be applied infrequently in order to avoid accumulation problems. Plant species may differ in phosphonate uptake and translocation, and individual P. infestans isolates show great variation in sensitivity to phosphonate compounds, which may negatively impact the effectiveness of phosphonate.

Table 1. Agriculturally relevant P-containing compounds.



What is it?



The chemical element indicated with the symbol P is a structural component of many things, including biological membranes, DNA, RNA, and ATP, and it is essential for numerous biochemical processes in all organisms. It does not occur as a free element in nature.

Phosphoric acid


Also known as orthophosphoric acid or phosphoric (V) acid, it is a mineral (inorganic) acid. This chemical compound normally does not exist in P fertilizers unless the fertilizer is put in a strong acidic solution. The P form in the fertilizer includes either phosphate salts or esters. Potassium or diammonium phosphate exemplifies the former, whereas phytate is an example of the latter. For acidic soils, phosphate rock can be directly used as a P source.

Dihydrogen phosphate


It is a partially disassociated form of H3PO4, in which P is most readily taken up by the plant. It is the major form of phosphate when pH is greater than 2.

Hydrogen phosphate


It is a partially disassociated form of H3PO4, in which P can also be taken up by the plant. This form dominates when pH is greater than 7. At pH7, both dihydrogen phosphate and hydrogen phosphate are approximately equal in amount.



It is a completely disassociated form of H3PO4.Under growth conditions, it is present in negligible amounts, less than 1:100,000 that of either dihydrogen phosphate or hydrogen phosphate.

Phosphorous pentoxide


It is a formula used to express P-content of fertilizers. It is a white and anhydride form of phosphoric acid. It is a powerful desiccant.

Phosphorous acid


It is a powerful reducing agent used for preparing phosphite salts, such as potassium phosphite. These salts, as well as aqueous solutions of pure phosphorous acid, control a variety of microbial plant diseases caused by Oomycota.

Dihydrogen phosphonate


It is a partially dissociated form of H3PO3, the major form of phosphonate at pH > 1.

Hydrogen phosphonate


A completely dissociated form of H3PO3, it dominates at pH > 7. The hydrogen has a covalent bond with phosphorus that cannot be readily dissociated.

There has been a lot of research on phosphorous acid-related compounds and their efficacy against potato late blight. In most cases, research has been done with foliar applications of phosphorous acid. In most cases, phosphorous acid is applied to the foliage. The compound gets translocated in the plant to the roots and is therefore effective against oomycetes that affect roots. Phosphorous acid was shown to be effective when applied as a root drench against P. cinnamomi, P. nicotianae, and P. palmivora in lupin, tobacco, and papaya, respectively. The efficacy of different phosphonate compounds against nine Phytophthora spp. that cause stem rot of Persea indica L. and pepper, was tested both as a curative and preventive method of control. Although there were notable differences in the sensitivity of the Phytophthora spp. in their experiments, there was little variation in the ability of phosphonates to control stem rot of pepper, regardless of its use as a curative or a preventive agent in pots. A greater level of control was obtained for Persea indica L. than for pepper.

Similar to other phosphonate-based systemic fungicides, Fosetyl-Al is often used to treat plants infected with root pathogens because it is mobile in the plant and gets transferred to the roots. It was found that foliar application of fosetyl-Al did not reduce tuber blight on potato caused by P. infestans, while foliar sprays with phosphonate reduced the number of symptomatic tubers. Different host plants may take up, transport, and metabolize fosetyl-Al differently. This result implies that different host plants may take up, transport, and metabolize fosetyl-Al differently.

In general, Potassium phosphonate negatively affected mycelial growth more than phosphonates that had alkyl groups, with some exceptions. None of the compounds used were able to control infections by Phytophthora spp. completely when they were used as a curative or protective agent. All of the compounds were equally effective when used as a protective agent (by root dip). Potassium phosphite controlled strawberry leather rot caused by P. cactorum. It also controlled downy mildew of basil in its early stages. Phosphonate was shown to be effective when applied to potato foliage against P. infestans and P. erythroseptica (causal agent of pink rot) but not against Pythium ultimum (causal agent of Pythium leak). Phosphorous acid is also effective against downy mildew on grapes, and against Phytophthora root and crown rot on tomato and green pepper in hydroponic culture. Studies have shown that phosphonate can control the sudden oak death pathogen in vitro and in planta.

For control of oomycetes on turfgrass, a mixture of phosphorous acid compounds and Aluminum tris [O-ethyl phosphonate] were found to be equally effective against Pythium blight development on perennial ryegrass (Lolium perenne). Similarly, different commercial formulations of phosphorous acid suppressed Pythium blight on rough bluegrass (Poa trivialis) during the 2004 season.

The existence of Phytophthora spp. that are resistant to phosphonate has been reported. Hence, care should be taken to alternate phosphonates with other effective compounds to prevent a buildup of resistant Phytophthora spp. in the field.


Both phosphoric acid and phosphorous acid are essential agrochemicals in crop production. Under normal plant growth conditions, both dissociate and exist as corresponding anions, phosphate and phosphite. A clear distinction exists between the two agrichemical compounds: the former is a nutritional source of P essential for plants, and the latter helps control agricultural epidemics of oomycetes. Phosphate and phosphite are not equivalent inside the plant. Phosphoric acid or phosphate cannot function as phosphorous acid or phosphite and vice versa. Since phosphites are systemic and very stable in plants, they should not be applied frequently. To help delay the development of phosphite-resistant oomycetes, care should be taken to alternate or mix phosphite with other effective compounds.


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