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Phosphorous Acid: When all P are not Equal

While growers are familiar with phosphorus-containing fertilizer, the abundance of terms apparently 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 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 (Table 2).  The purpose of this article is to explain what phosphorous acid is and to examine both the fungicidal activity and nutritional value of phosphorous acid.

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 these ions may be taken up by the plant, but H₂PO₄^- more readily (Street and Kidder, 1989).  Once inside the plant, both ions are mobile.

How much phosphorus a fertilizer contains is represented as the middle number on the bag (5-10-15, for example) expressed as P₂O₅ (the first number represents the nitrogen grade, and the third number potassium grade as K₂O). The P₂O₅ unit used to represent P content in fertilizer is a conventional unit (in reality, there is little of no P in the form of P₂O₅ in fertilizer).

Phosphoric acid should not be confused with phosphorous acid (H₃PO₃).  A single letter difference in the name of a chemical compound can make a make difference in its properties. Phosphorous acid releases the phosphonate ion (HPO₃^2-; also called phosphite) upon disassociation.  Like phosphate, phosphonate is easily taken up and translocated inside the plant. Phosphorous acid and its related compounds are commonly often referred to as phosphonate, phosphate, and phosphonic acid.  One of the breakdown products of fosetyl-Al is mono-ethyl phosphonite, which may be taken up by the plant.  Inside the plant,  fosetyl-Al may ionized into phosphonate, and therefore fosetyl-Al belongs to the same group of phosphorous acid compounds (Cohen and Coffey, 1986; McGrath, 2004; Wilcox, NY). 

Phosphoric Acid as Fertilizer?

Phosphorous acid does not get converted into phosphate, which is the primary source of P for plants (Ouimette and Coffey, 1989b).  In contrast, some soil bacteria are capable of transforming phosphonate into phosphate.  However, this process is so slow that it is of no practical relevance (McDonald et al., 2001).  To date, no plant enzymes have been described that could oxidize phosphonate into phosphate.  This explains why phosphonate is stable in plants and does not get converted into phosphate (Smillie et al., 1989).  Because phosphorous acid and its derivatives do not get metabolized in plants, claims that phosphonate can contribute to phosphorus nutritional needs of the plants should be taken with caution.

Phophorous acid has properties useful in agriculture.  Like in many other publications investigating efficacy of phosphorous acid against oomycetes, Förster et al. (1998) found that phosphite is capable of controlling Phytophthora root and crown rot on tomato and pepper.  These authors also investigated 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 making the general health of the crop better, thereby improving its natural defense system.  Therefore, no real, valid evidence exists for the claim that phosphorous acid improves plant growth. 

Control of Oomycetes

It is well documented that phosphorous acid is able to control diseases caused by organisms that belong to the Oomycota (or oomycetes) which cause diseases on agronomical crops.  Oomycetes (a group of pathogen that includes water molds and downy mildew) (Fig. 1) 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.  Another difference is the nuclei in the cells that form the filaments each have two sets of genetic information (they are diploid) in oomycetes instead of just one set as in fungi (which are haploid; Waggoner and Speer, 1995).

For most practical purposes, the oomycetes are grouped with fungi, and compounds that control plant pathogens belonging to the oomycetes are often called fungicides.  It is important to dinstinguish between fungi and oomycetes; however, because chemicals that are used to control one will often not be effective against the other based on their different biology.  Several important plant pathogens belong to the oomycetes (Table 3), the most important economically is Phytophthora infestans which causes late blight of potato (Fig. 2).

Phosphorous acid has both a direct and an indirect effect on oomycetes. It inhibits a particular process (oxidative phosphorylation) in the metabolism of oomycetes (McGrath, 2004). For instance, phosphonate compounds are ineffective against phosphonate-resistant oomycetes (Ouimette and Coffey, 1989a).  In addition, some evidence suggests that phosphorous acid has an indirect effect by stimulating the plant’s natural defense response against pathogen attack (Biagro Western Sales, Inc., 2003; Smillie et al., 1989). 

Efficacy

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 (Smillie et al., 1989).  Phosphorous acid does not get converted into phosphate, and is not easily metabolized (Ouimette and Coffey, 1989b).  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.  Plants species may differ in uptake and translocation of phosphonate (Cook and Little, 2001), and there is great variation in sensitivity of individual P. infestans isolates (Bashan et al., 1990, Cohen and Bower, 1984) to phosphonate compounds, which may negatively impact the effectiveness of phosphonate.

Some of the phosphorous acid-related compounds and research on their efficacy against potato late blight are summarized in Table 4.  In most cases, research has been done with foliar applications of phosphorous acid.  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 (Smillie et al., 1989).  The efficacy of different phosphonate compounds against nine Phytophthora spp. which cause stem rots of Persea indica L. and pepper was tested both as a curative or preventive method of control by Ouimette and Coffey (1989a).  Seven though sensitivity of each of the Phytophthora spp. used in their experiments in the laboratory was variable (Table 4), there was little difference in the ability of different phosphonate compounds in controlling the stem rot of pepper either as a curative or preventive agent in pots.  The level of control was better for Persea indica L. than for pepper (Ouimette and Coffey, 1989a).

Fosetyl-Al is a systemic fungicide that is often used against root pathogens because it is mobile in the plant and gets transferred to the roots (Cohen and Coffey, 1986).  Cooke and Little (2001) found that foliar application of fosetyl-Al did not reduce tuber blight on potato caused by P. infestans, while foliar sprays with partially neutralized phosphonate reduced the number of tubers which developed symptoms after inoculation with the pathogen.  Different host plants may take up, transport and metabolize fosetyl-Al differently.  This seems contradictory, since fosetyl-Al releases phosphonate as a breakdown product, but there may be other factors involved, like environmental factors.

Potassium phosphonate negatively affected mycelial growth more than phosphonates that had alkyl groups in general, but some exceptions were noted (Ouimette and Coffey, 1989a). None of the compounds used by Ouimette and Coffey (1989a) 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 phosphate was shown to be effective for control of strawberry leather rot caused by P. cactorum (Rebollar-Alviter et al., 2005).  Phosphonate was shown to be effective when applied to potato foliage against P. infestans and P. erytrhoseptica (causal agent of pink rot), but not against Pythium ultimum (causal agent of Pythium leak; Johnson et al., 2004, Fenn and Coffey, 1984).  Phosphorous acid also appears very effective against downy mildew on grapes (Wilcox, NY), Phytophthora root and crown rot on tomato and green pepper in hydroponic culture (Förster et al., 1998).

For control of oomycetes on turfgrass, Riverdale Magellan (a mixture of phosphorous acid compounds) and Chipco Signature (Aluminum tris (O-ethyl phosphonate)) were found to be equally effective against Pythium blight development on perennial ryegrass (Lolium perenne; Datnoff et al., 2003).  Similarly, different commercial formulations of phosphorous acid suppressed Pythium blight on rough bluegrass (Poa trivialis) during the 2001-2002 season (Datnoff et al., 2005).

The existence of Phytophthora spp. that are resistant against phosphonate has been reported (Brown et al., 2005; Dolan and Coffey, 1988; Fenn and Coffey, 1985, 1989; Griffith et al., 1993; Nelson et al., 2004; Ouimette and Coffey, 1989a).  hence, care should be taken to alternate phosphonates with other effective compounds to prevent a build-up of resistant Phytophtora spp. in the field. 

Conclusion

A clear distinction exists between phosphoric acid and phosphorous acid: the former is a nutritional source of P for plants, and the latter helps control agricultural epidemics of oomycetes.  Claims that suggest that either compound may fulfill the functions of the other are not supported by current literature and are therefore misleading.  Since phosphonates are systemic and very stable in plants, they should not be applied frequently.  Since phosphonate resistant oomycetes have been described, care should be taken to alternate or mix phosphonate with other effective compounds.

References

Bashan, B., Y. Levy, and Y. Cohen (1990) Variation in sensitivity of Phytophthora infestans to fosetyl-Al. Plant Pathol. 39:134-140

Bayer Cropscience (2004) Aliette Product Information. Bayer Cropscience. http://www.bayercropscienceus.com/products/view:aliette/?p=. Last accessed: January 27, 2005

Biagro Western Sales, Inc (2003) Nutri-Phite Fertilizer. Biagro Western Sales, Inc. http://www.biagro.com/nutri_phite/np_html/np_content_intro.html. Last accessed: January 27, 2005

Brown, S., S.T. Koike, O.E. Ochoa, F. Laemmlen, and R.W. Michelmore (2004) Insensitivity to the fungicide fosetyl-aluminum in California isolates of the lettuce downy mildew pathogen, Bremia lactucae. Plant Dis. 88:502-508

Coffey, M.D., and L.A. Bower (1984) In vitro variability among isolates of eight Phytophtora species in response to phosphorous acid. Phytopathology 74:738-742

Cohen, Y., and M.D. Coffey (1986) Systemic fungicides and the control of Oomycetes. Annu. Rev. Phytopathol. 24:311-338

Cooke, L.R., and G. Little (2001) The effect of foliar application of phosphonate formulations on the susceptibility of potato tubers to late blight. Pest Manag. Sci. 58:17-25

Datnoff, L., J. Cisar, B. Rutherford, K. Williams, and D. Park (2003) Effect of Riverdale Magellan and Chipco Signature on Pythium blight development on Lolium perenne, 2001-2002. The American Phytopathological Society. Fungicide and Nematicide Tests (online) Vol. 58:T041. http://www.apsnet.org/online/FNtests/vol58/top.htm

Datnoff, L., J. Cisar, B. Rutherford, K. Williams, and D. Park (2005). Effect of fungicides and other prophylactic treatments on Pythium blight development on Poa trivialis, 2004. In press.

Dolan, T.E., and M.D. Coffey (1988) Correlative in vitro and in vivo behavior of mutant strains of Phytophthora palmivora expressing different resistances to phosphorous acid and fosetyl-Na. Phytopathology 78:974-978

Fenn, M.E., and M.D. Coffey (1984) Studies on the in vitro and in vivo antifungal activity of fosetyl-Al and phosphorous acid. Phytopathology 74:606-611

Fenn, M.E., and M.D. Coffey (1985) Further evidence for the direct mode of action of fosetyl-Al and phosphorous acid. Phytopathology 75:1064-1068

Fenn, M.E., and M.D. Coffey (1989) Quantification of phosphonate and ethyl phosphonate in tobacco and tomato tissues and significance for the mode of action of two phosphonate fungicides. Phytopathology 79:76-82

Förster, H., J.E. Adaskaveg, D.H. Kim, and M.E. Stanghellini (1998) Effect of phosphite on tomato and pepper plants and on susceptibility of pepper to Phytophthora root and crown rot in hydroponic culture. Plant Dis. 82:1165-1170

Griffith, J.M., M.D. Coffey, and B.R. Grant (1993) Phosphonate inhibition as a function of phosphate concentration in isolates of Phytophthora palmivora. J. Gen. Microbiol. 139:2109-2116

Heffer, V., M.L. Powelson, and K.B. Johnson (2002) Oomycetes. The Plant Health Instructor.  doi:10.1094/PHI-I-2002-0225-01. ©2002 The American Phytopathological Society. http://www.apsnet.org/education/LabExercises/Oomycetes/Top.html. Last accessed: January 28, 2005

Helena Chemical Company (2001) Introduction to Helena’s Latest Products. Helena Chemical Company. http://www.helenachemical.com/proprietary/latest_products.htm. Last accessed: January 27, 2005

Helena Chemical Company (2002) Helena ProPhyt. A systemic fungicide containing potassium and phosphate (promotional brochure). Helena Chemical Company.

Johnson, D.A., D.A. Inglis, J.S. Miller (2004) Control of potato tuber rots caused by oomycetes with foliar applications of phosphorous acid. Plant Dis. 88:1153-1159

McDonald, A.E., B.R. Grant, and W.C. Plaxton (2001) Phosphite (phosphorous acid): its relevance in the environment and agriculture and influence on plant phosphate starvation response. J. Plant Nutr. 24:1505-1519

McGrath, M.T. (2004) What are Fungicides? Online. The Plant Health Instructor. doi: 10.1094/PHI-I-2004-0825-01. ©2004 The American Phytopathological Society. http://www.apsnet.org/education/IntroPlantPath/Topics/fungicides/pdfs/CommonAndTradeFungicides.pdf . Last accessed: January 28, 2005

Nelson, M.E., K.C. Eastwell, G.G. Grove, J.D. Barbour, C.M. Ocamb, and J.R. Aldredge (2004) Sensitivity of Pseudoperonospora humuli (the causal agent of hop downy mildew) from Washington, Idaho, and Orgeon to fosetyl-Al (Aliette). Online. Plant Health Progress doi:10.1094/PHP-2004-0811-01-RS. ©2004 Plant Management Network. http://www.plantmanagementnetwork.org/sub/php/research/2004/aliette/. Last accessed: January 28, 2005

Nufarm USA (n.y.) Phostrol®. Nufarm Ltd. http://www.ag.us.nufarm.com/. Last accessed: January 27, 2005

Ouimette, D.G., and M.D. Coffey (1989a) Comparative antifungal activity of four phosphonate compounds against isolates of nine Phytophthora species. Phytopathology 79:761-767

Ouimette D.G., and M.D. Coffey (1989b) Phosphonate levels in avocade (Persea americana seedlings and soil following treatment with fosethyl-Al or potassion phosphonate. Plant Dis. 73:212-215

Pesticide Action Network (2004) PAN pesticide database. Pesticide Action Network North America. http://data.pesticideinfo.org/Index.html. Last accessed: January 27, 2005

Rebollar-Alviter, A., L.V. Madden, and M.A. Ellis (2005) Efficacy of Azoxystrobin, Pyraclostrobin, potassium phosohite, and Mefenoxam for control of strawberry leather rot caused by Phytophthora cactorum. Online. Plant Health Progress doi:10.1094/PHP-2005-0107-01-RS. ©2005 Plant Management Network. http://www.plantmanagementnetwork.org/pub/php/research/2005/leather/. Last accessed: January 28, 2005

Smillie, R., B.R. Grant, and D. Guest (1989) The mode of action of phosphite: evidence for both direct and indirect action modes of action on three Phytophthora spp. in plants. Phytopathology 79:921-926

Street, J.J., and G. Kidder (1989) Soils and plant nutrition. Fact Sheet SL-8. Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First published 1989, revised 1997

Waggoner, B.M., and B.R. Speer (1995) Introduction to the Oomycota. University of California at Berkeley. http://www.ucmp.berkeley.edu/chromista/oomycota.html . ©1994-2004 Regents of University of California. Last accessed: January 26, 2005

Wilcox, W. (n.y.). ProPhyt, Aliette, and phosphorous acid. The Lake Erie Regional Grape Program. http://lenewa.netsync.net/public/ProPhyt_Aliette_phosphorous_acid.htm. Last accessed: January 27, 2005

 

Table 2. Marketing of products with active ingredient phosphorous acid or related compounds.1
Product	Company	Active Ingredient	Marketed as	Reference
Terronate wdg	Agriliance, llc	Fosetyl-Al	Fungicide	Pesticide Action Network, 2004
Aliette®	Bayer Cropscience, lp	Fosetyl-Al	Fungicide	Bayer, 2004
Nutri-Phite®	Biagro Western Sales	Phosphite and organic acids	Fertilizer	Biagro Western Sales, Inc., 2003
CP home and garden fungicide	Contract packaging, Inc.	Fosetyl-Al	Fungicide	Pesticide Action Network, 2004
Tree tech brand aliette injectable	Florida Silvics, Inc.	Fosetyl-Al	Fungicide	Pesticide Action Network, 2004
Ele-Max® foliar phosphite	Helena Chemical	Phosphorus acid2	Foliar fertilizer	Helena, 2001
Ele-Max® soil phosphate		Phosphorus acid	Soil fertilizer	Helena, 2001
ProPhyt®		Potassium phosphite	Systemic fungicide	Helena, 2001, 2002; Wilcox, NY
Phostrol®	Nufarm America	Phosphorous acid	Fungicide, biochemical pesticide	Nufarm, NY; Pesticide Action Network, 2004
Riverdal Magellan		Phosphorous acid	Fungicide	Pesticide Action Network, 2004
Plant synergists phosphorous acid technical	Plant Synergists, Inc.	Phosphorous acid	Fungicide	Pesticide Action Network, 2004
1 Disclaimer: Products and companies are mentioned for educational purposes and are not recommended over similar products in this document. 2 It is unclear whether “phosphorus acid” means phosphoric or phosphorous acid. The word “phosphite” in the name implies that phosphorous acid is the active ingredient. However, the fact that the product is marketed as a fertilizer implies that the active ingredient is phosphoric acid.

 

Table 3. Genus of oomycetes that cause disease on horticultural crops and that are likely to be controlled by phosphorous acid (Heffer et al., 2002).
Genus	Disease
Aphanomyces	Root rot
Bremia, Peronospora, Plasmopara, Pseudoperonospora, Sclerospora	Downy mildew (Fig. 2)
Pythium	Root rot and damping-off
Phytophthora	Late blight of potato and tomato, foliar blights on peppers and cucurbits, root and stem rots
Albugo	White rust on cruciferous plants

 

Table 4. Control of potato late blight by phosphorous acid and related products.
Compound	Efficacy	Application	Reference
Fosetyl-Al	Not good in field	Foliar spray	Cook and Little, 2001
Phosphonate	Good in field, variable against oomycetes in the lab	Foliar spray	Cook and Little, 2001
Phosphonate compounds	Good in pots	Root dip	Ouimette and Coffey, 1989a
Phosphonate	Variable against P. infestans isolates in the lab	To detached leaves	Bashan, 1990
Phosphorous acid	Good against P. infestans in the field	Foliar spray	Johnson et al., 2004

 

Figure 1. Downy mildew on lettuce.
Photograph courtesy of Tyler Harp and Syngenta Crop Protection.

 

Figure 2. Potato late blight, caused by Phytophthora infestans.
Photograph courtesy of Tyler Harp and Syngenta Crop Protection.
 

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Asha M. Brunings¹, Lawrence E. Datnoff¹, and Eric H. Simmone^3
1Plant Pathology Department
²Horticultural Sciences Department
Vegetarian 05-03