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Are Phosphorous and Phosphoric Acids Equal Phosphorous Sources for Plant Growth?
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Title: Are Phosphorous and Phosphoric Acids Equal Phosphorous Sources for Plant Growth?
Physical Description: Fact Sheet
Creator: Brunings, Asha Marcelle
Liu, Guodong
Simonne, Eric H.
Zhang, Shouan
Li, Yuncong
Datnoff, Lawrence E.
Publisher: University of Florida Cooperative Extension Service, Institute of Food and Agriculture Sciences, EDIS
Place of Publication: Gainesville, Fla.
Publication Date: 2012
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Abstract: Phosphorus (P) is one of the 17 elements essential for plant growth and development, and is also a key component in some agrochemicals, such as phosphorous acid. Thus, there are two types of P closely associated with crop production. The similarity of terms such as phosphoric acid and phosphorous acid may create some confusion as to the actual content and efficacy of these products. This 7-page fact sheet explains what phosphorous acid is and examines both its fungicidal activity and nutrient value.
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Publication Status: Published
General Note: "Publication #HS1010"
General Note: "Original publication date April 2005. Revised March 2012."
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Source Institution: University of Florida Institutional Repository
Holding Location: University of Florida
Rights Management: All rights reserved by the submitter.
System ID: IR00000858:00001

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HS1010 Are Phosphorous and Phosphoric Acids Equal Phosphorous Sources for Plant Growth?1Asha M. Brunings, Guodong Liu, Eric H. Simonne, Shouan Zhang, Yuncong Li, and Lawrence E. Datno2 1. This document is HS1010, one of a series of the Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and A gricultural Sciences, University of Florida. Original publication date April 2005. Revised March 2012. Visit the EDIS website at http://edis.ifas.u.edu 2. 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. Datno, former professor, Plant Pathology Department, University of Florida, Gainesville, FL 32611. The use of trade names in this publication is solely for the purpose of providing specic information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication do not signify our approval to the exclusion of other products of suitable composition. All chemicals should be used in accordance with directions on the manufacturers label. Use pesticides safely. Read and follow directions on the manufacturers label.The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or aliations. U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A&M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim DeanPhosphorus (P) is one of the 17 elements essential for plant growth and development (nickel is the 17th) (Association of American Plant Food Control Ocials 2005; Bai, Reilly, and Wood 2006). Phosphorus is also a key component in some agrochemicals, such as phosphorous acid (H3PO3). us, there are two types of P closely associated with crop production. While growers are familiar with P-containing fertilizers, the abundance of terms, apparently similar (such as phosphoric acid and phosphorous acid), may create some confusion as to the actual content and ecacy of these products. Table 1 lists some P-containing compounds used for crop production. 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). e purpose of this article is to explain what phosphorous acid is and to examine both its fungicidal activity and nutrient value. e amount of phosphorus in a fertilizer is represented as the middle number on the bag expressed as phosphorus pentoxide (P2O5) (e.g., 5-10-15). e rst number represents the nitrogen percentage, and the third number represents potassium percentage as K2O. e 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 (HPO4 2-) and/or dihydrogen phosphate (H2PO4 -), depending on the pH of the growing medium. At pH7, both H2PO4 and HPO4 2are 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 HPO4 2-, it is only one out of 100,000 because it always dissociates to H2PO4 and further to HPO4 2-. Both of these H2PO4 and HPO4 2ions are the basic forms taken up by the plant, but H2PO4 is taken up more readily (Street and Kidder 1989) because in most growth conditions, soil solution pH is below 7. Once inside the plant, both ions are mobile.

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2 Phosphor ic acid ( H3PO4) should not be confused with phosphor ous acid (H3PO3). A little dierence in the name or formula of a chemical compound can make a dramatic dierence in its properties.e former is a fully oxidized and hydrated form of P, whereas the latter is a partially oxidized and hydrated form. erefore, phosphorous acid is a powerful reducing agent, but phosphoric acid is not. e 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 (HPO3 2-), also called phosphite. Phosphorous acid and its ionized compounds are oen referred to as phosphonate or phosphonite. Like phosphate, phosphonate is easily taken up (Street and Kidder 1989) and translocated inside the plant. Fosetyl-Al, which was registered by the EPA in 1983 (EPA 1991), 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-o 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 (Cohen and Coey 1986; McGrath 2004).Phosphorous Acid as Fertilizer?Phosphorous acid is not converted into phosphate, which is the primary nutrient source of P for plants (Ouimette and Coey 1989b). ere are bacteria capable of transforming phosphonate into phosphate, but this process is so slow that it is of no practical relevance (Huang, Su, and Xu 2005; McDonald, Grant, and Plaxton 2001). To date, no plant enzymes described oxidize phosphonate into phosphate. is is consistent with the fact that phosphonate is stable in plants and is not converted into phosphate (Smillie, Grant, and Guest 1989). Since phosphorous acid and its derivatives are not metabolized in plants, claims that phosphonate can contribute to P nutrient requirements for plants should be taken with caution. Phophorous acid is used in agriculture but for a dierent purpose than phosphoric acid. Conrming other investigations into the ecacy of phosphorous acid against oomycetes (a group of pathogens that include water molds and downy mildew), Frster et al. (1998) found that phosphite is capable of controlling Phytophthora root and crown rot on tomato and pepper. e authors also tested the ability of phosphorous acid to act as a nutrient source for plant growth and found that P-deciency symptoms developed when plants were grown hydroponically with phosphorous acid as the sole source of P (without phosphate). is means that although phosphorous acid can control oomycetes in a number of host-parasite systems, it is not a substitute for phosphorus fertilization. e 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 OomycetesIt is well documented that phosphorous acid can control diseases caused by pathogens that belong to the Oomycota phylum (or oomycetes) on agronomical and horticultural crops. Oomycetes (Figure 1) are actually not fungi but are frequently grouped with fungi because they form structures (laments) similar to the ones that fungi make. In reality, oomycetes are fungal-like organisms that dier from fungi in that their cell walls do not contain chitin but rather a mixture of cellulosic compounds and glycan (polymeric carbohydrate). Another dierence is that the nuclei in the cells that form the laments have two sets of genetic information (diploid) in oomycetes instead of just one set (haploid) as in fungi (Waggoner and Speer 1995). For most practical purposes, oomycetes are grouped with fungi. Compounds that control plant pathogens belonging to the oomycetes are oen called fungicides. It is important to distinguish between fungi and oomycetes. Chemicals that are used to control one will oen not be eective against the other because of biological dierences. Several important plant pathogens belong to oomycetes (Table 3), such as Phytophthora infestans, the causal agent of late Figure 1. Downy mildew on lettuce Credits: Tyler Harp and Syngenta Crop Protection

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3 blight of potato (Figure 2) and the culprit of the Irish Potato Famine between 1845 and 1849; P. ramorum the causal agent of sudden oak death (Parke and Lucas 2008); and Pythium and Peronospora species, among others (Fry and Grnwald 2010). Phosphorous acid has both direct and indirect eects on oomycetes. It directly inhibits a particular process (oxidative phosphorylation) in the metabolism of oomycetes (McGrath 2004). An indirect eect is the stimulation of the plants natural defense response against pathogen attack (Biagro Western Sales, Inc. 2003; Smillie, Grant, and Guest 1989). It should be noted, however, that phosphonateresistant oomycetes have been reported (Ouimette and Coey 1989a). In addition, some evidence suggests that phosphorous acid has an indirect eect by stimulating the plants natural defense response against pathogen attack (Biagro Western Sales, Inc. 2003; Smillie, Grant, and Guest 1989).EcacyA major factor in the ability of phosphorous acid to control oomycetes appears to be its chemical stability in the plant (Smillie, Grant, and Guest 1989). Phosphorous acid does not convert into phosphate and is not easily metabolized (Ouimette and Coey 1989b). e stability of dierent phosphonate-related compounds may depend on environmental factors, such as climate or crop type. Because phosphonate is systemic and stable in plants, it should be applied infrequently in order to avoid accumulation problems. Plant species may dier in phosphonate uptake and translocation (Cooke and Little 2001), and individual P. infestans isolates show great variation in sensitivity (Bashan, Levy, and Cohen 1990; Coey and Bower 1984) to phosphonate compounds, which may impact the eectiveness of phosphonate. Table 4 summarizes some of the phosphorous acid-related compounds and research on their ecacy against potato late blight. In most cases, phosphorous acid is applied to the foliage. e compound is translocated from the shoots to the roots and can, therefore, also control oomycetes that aect roots. Phosphorous acid was shown to be eective when applied as a root drench against P. cinnamomi P. nicotianae and P. palmivora in lupin, tobacco, and papaya, respectively (Smillie, Grant, and Guest 1989). e ecacy of dierent 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 by Ouimette and Coey (1989a). Although there were notable dierences in the sensitivity of the Phytophthora spp. in their experiments (Table 4), 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 (Ouimette and Coey 1989a). Similar to other phosphonate-based systemic fungicides, fosetyl-Al is oen used to treat plants infected with root pathogens because it is mobile in the plant and is transferred to the roots (Cohen and Coey 1986). Cooke and Little (2001) found, however, 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. is result implies that dierent host plants may take up, transport, and metabolize fosetyl-Al dierently. In general, potassium phosphonate negatively aected mycelial growth more than phosphonates that had alkyl groups, with some exceptions (Ouimette and Coey 1989a). None of the compounds used by Ouimette and Coey (1989a) were able to control infections by Phytophthora spp. completely when they were used as a curative or protective agent. All the compounds were equally eective when used as a protective agent (by root dip). Potassium phosphite controlled strawberry leather rot caused by P. cactorum (Rebollar-Alviter, Madden, and Ellis 2005). It also controlled downy mildew of basil in its early stages (Roberts et al. 2009). Phosphonate was shown to be eective 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, Inglis, and Miller Figure 2. Potato late blight caused by Phytophthora infestans. Credits: Tyler Harp and Syngenta Crop Protection

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4 2004; Fenn and Coey 1984). Phosphorous acid is also eective against downy mildew on grapes and against Phytophthora root and crown rot on tomato and green pepper in hydroponic culture (Frster et al. 1998). Studies have shown that phosphonate can control the sudden oak death pathogen in vitro and in planta (Garbelotto, Harnik, and Schmidt 2009; Garbelotto and Schmidt 2009). 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 eective against Pythium blight development on perennial ryegrass (Lolium perenne ) (Datno et al. 2003). Similarly, dierent commercial formulations of phosphorous acid suppressed Pythium blight on rough bluegrass (Poa trivialis ) during the 2004 season (Datno et al 2005). e existence of Phytophthora spp. that are resistant to phosphonate has been reported (Brown et al. 2004; Dolan and Coey 1988; Fenn and Coey 1985, 1989; Grith, Coey, and Grant 1993; Nelson et al. 2004; Ouimette and Coey 1989a). Hence, care should be taken to alternate phosphonates with other eective compounds to prevent a buildup of resistant Phytophtora spp. in the eld.ConclusionBoth 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 agrochemical compounds: e former is a nutrient 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 eective compounds.ReferencesAssociation of American Plant Food Control Ocials. 2005. Model for Fertilizer Regulation in North America. Accessed October 19, 2011. http://www.aapfco.org/aapfcorules.html Bai, C., C. C. Reilly, and B. W. Wood. 2006. Nickel Deciency Disrupts Metabolism of Ureides, Amino Acids, and Organic Acids of Young Pecan Foliage. Plant Physiology 140 (2): 433. Bashan, B., Y. Levy, and Y. Cohen. 1990. Variation in Sensitivity of Phytophthora infestans to Fosetyl-Al. Plant Pathology 39 (2): 134. Bayer Cropscience. 2004. Aliette Product Information. Accessed March 12, 2011. http://www.bayercropscience.us/ products/fungicides/aliette/. Biagro Western Sales, Inc. 2003. Nutri-Phite Fertil izer. Accessed March 12, 2011. http://www.biagro.com/ nutri_phite/np_html/np_content_intro.html Brown, S., S. T. Koike, O. E. Ochoa, F. Laemmlen, and R. W. Michelmore. 2004. Insensitivity to the Fungicide FosetylAluminum in California Isolates of the Lettuce Downy Mildew Pathogen, Bremia lactucae . Plant Disease 88 (5): 502. Coey, M. D., and L. A. Bower. 1984. In vitro Variability among Isolates of Eight Phytophtora Species in Response to Phosphorous Acid. Phytopathology 74 (6): 738. Cohen, Y., and M. D. Coey. 1986. Systemic Fungicides and the Control of Oomycetes. Annual Review of Phytopathology 24: 311. http://www.annualreviews.org/doi/ abs/10.1146/annurev.py.24.090186.001523. Cooke, L. R., and G. Little. 2002. e Eect of Foliar Application of Phosphonate Formulations on the Susceptibility of Potato Tubers to Late Blight. Pest Management Science 58 (1): 17. doi: 10.1002/ps.408 http://onlinelibrary.wiley. com/doi/10.1002/ps.408/full. Datno, L., J. Cisar, B. Rutherford, K. Williams, and D. Park. 2003. Eect of Riverdale Magellan and Chipco Signature on Pythium Blight Development on Lolium perenne, 2001. F&N Tests 58 (T041): 1. http:// www.plantmanagementnetwork.org/pub/trial/fntests/ reports/2003/T041.pdf Datno, L., J. Cisar, B. Rutherford, K. Williams, and D. Park. 2005. Eect of Fungicides and Other Prophylactic Treatments on Pythium Blight Development on Poa trivialis 2004. F&N Tests 60 (T033): 1. http://www.plantmanagementnetwork.org/pub/trial/fntests/reports/2005/ T033.pdf Dolan, T. E., and M. D. Coey. 1988. Correlative In vitro and In vivo Behavior of Mutant Strains of Phytophthora

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5 palmivora Expressing Dierent Resistances to Phosphorous Acid and Fosetyl-Na. Phytopathology 78 (7): 974. Environmental Protection Agency (EPA). 1991. R.E.D. Facts: Fosetyl-Al (Aliette). Accessed October 19, 2011. http://upload.wikimedia.org/wikisource/en/f/fa/Fosetylal_red-facts_1994.pdf Fenn, M. E., and M. D. Coey. 1984. Studies on the In vitro and In vivo Antifungal Activity of Fosetyl-Al and Phosphorous Acid. Phytopathology 74 (5): 606. Fenn, M. E., and M. D. Coey. 1985. Further Evidence for the Direct Mode of Action of Fosetyl-Al and Phosphorous Acid. Phytopathology 75 (9): 1064. Fenn, M. E., and M. D. Coey. 1989. Quantication of Phosphonate and Ethyl Phosphonate in Tobacco and Tomato Tissues and Signicance for the Mode of Action of Two Phosphonate Fungicides. Phytopathology 79 (1): 76. Frster, H., J. E. Adaskaveg, D. H. Kim, and M. E. Stanghellini. 1998. Eect of Phosphite on Tomato and Pepper Plants and on Susceptibility of Pepper to Phytophthora Root and Crown Rot in Hydroponic Culture. Plant Disease 82 (10): 1165. Fry, W. E., and N. J. Grnwald. 2010. Introduction to Oomycetes. e Plant Health Instructor. doi:10.1094/PHII-2010-1207-01. http://www.apsnet.org/edcenter/intropp/ PathogenGroups/Pages/IntroOomycetes.aspx. Garbelotto, M., T. Y. Harnik, and D. J. Schmidt. 2009. Ecacy of Phosphonic Acid, Metalaxyl-M and Copper Hydroxide Against Phytophthora ramorum In vitro and In planta . Plant Pathology 58 (1): 111. Garbelotto, M., and D. Schmidt. 2009. Phosphonate Controls Sudden Oak Death Pathogen for up to 2 Years. California Agriculture 63 (1): 10. Grith, J. M., M. D. Coey, and B. R. Grant. 1993. Phosphonate Inhibition as a Function of Phosphate Concentration in Isolates of Phytophthora palmivora . Journal of General Microbiology 139 (9): 2109. Heer, V., M. L. Powelson, and K. B. Johnson. 2002. Oomycetes. e Plant Health Instructor. doi:10.1094/PHII-2002-0225-01. http://www.apsnet.org/edcenter/intropp/ LabExercises/Pages/Oomycetes.aspx. Helena Chemical Company. 2002. Helena ProPhyt: A Systemic Fungicide Containing Potassium and Phosphate (promotional brochure). Memphis, TN: Author. Huang, J., Z. Su, and Y. Xu. 2005. e Evolution of Microbial Phosphonate Degradative Pathways. Journal of Molecular Evolution 61 (5): 682. Johnson, D. A., D. A. Inglis, and J. S. Miller. 2004. Control of Potato Tuber Rots Caused by Oomycetes with Foliar Applications of Phosphorous Acid. Plant Disease 88 (10): 1153. McDonald, A. E., B. R. Grant, and W. C. Plaxton. 2001. Phosphite (Phosphorous Acid): Its Relevance in the Environment and Agriculture and Inuence on Plant Phosphate Starvation Response. Journal of Plant Nutrition 24 (10): 1505. McGrath, M. T. 2004. What are Fungicides? e Plant Health Instructor. doi: 10.1094/PHI-I-2004-0825-01. http:// www.apsnet.org/edcenter/intropp/topics/Pages/Fungicides. aspx 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 Oregon to Fosetyl-Al (Aliette). Plant Health Progress. doi: 10.1094/ PHP-2004-0811-01-RS. http://www.plantmanagementnetwork.org/sub/php/research/2004/aliette/. Nufarm USA. Phostrol Nufarm Ltd. Accessed March 12, 2011. http://www.nufarm.com/USAg/Phostrolr Ouimette, D .G., and M. D. Coey. 1989a. Comparative Antifungal Activity of Four Phosphonate Compounds against Isolates of Nine Phytophthora Species. Phytopathol ogy 79 (7): 761. Ouimette, D. G., and M. D. Coey. 1989b. Phosphonate Levels in Avocado (Persea americana) Seedlings and Soil Following Treatment with Fosethyl-Al or Potassium Phosphonate. Plant Disease 73 (3): 212. Parke, J. L., and S. Lucas. 2008. Sudden Oak Death and Ramorum Blight. e Plant Health Instructor. doi: 10.1094/ PHI-I-2008-0227-01. http://www.apsnet.org/edcenter/ intropp/lessons/fungi/Oomycetes/Pages/SuddenOakDeath. aspx

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6 Pesticide Action Network. 2004. PAN Pesticide Database. Accessed March 12, 2011. http://pesticideinfo.org/. Raid, R. N., E. McAvoy, and D. D. Sui. 2010. Evaluation of Fungicides for Management of Downy Mildew on Sweet Basil. Phytopathology 100 (6, Supplement): S107. Raid, R. N. 2008. Evaluation of Prophyt, Alone and in Combination, for Post-Infection Control of Downy Mildew on Basil, Fall 2007. Plant Disease Management Reports 2: V070. doi: 10. 1094/PDMR02. http://www.plantmanagementnetwork.org/pub/trial/pdmr/reports/2008/V070.pdf Rebollar-Alviter, A., L. V. Madden, and M. A. Ellis. 2005. Ecacy of Azoxystrobin, Pyraclostrobin, Potassium Phosphite, and Mefenoxam for Control of Strawberry Leather Rot Caused by Phytophthora cactorum . Plant Health Progress. doi: 10.1094/PHP-2005-0107-01-RS. http://www. plantmanagementnetwork.org/pub/php/research/2005/ leather/. Roberts, P. D., R. N. Raid, P. F. Harmon, S. A. Jordan, and A. J. Palmateer. 2009. First Report of Downy Mildew Caused by a Peronospora sp. on Basil in Florida and the United States. Plant Disease 93 (2): 199. Smillie, R., B. R. Grant, and D. Guest. 1989. e Mode of Action of Phosphite: Evidence for Both Direct and Indirect Action Modes of Action on ree Phytophthora spp. in Plants. Phytopathology 79 (9): 921. Street, J. J., and G. Kidder. 1989. Soils and Plant Nutrition. Fact Sheet SL-8. Gainesville: University of Florida Institute of Food and Agricultural Sciences. Waggoner, B. M., and B. R. Speer. 1995. Introduction to the Oomycota. Regents of University of California. University of California at Berkeley. Accessed March 12, 2011. http://www.ucmp.berkeley.edu/chromista/oomycota.html Table 1. Agriculturally relevant P-containing compounds Name SymbolWhat is it? Phosphorus P 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 H3PO4Also 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 exemplies the former, whereas phytate is an example of the latter. For acidic soils, phosphate rock can be directly used as a P source. Dihydrogen phosphateH2PO4 -It is a partially dissociated 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 phosphateHPO4 2-It is a partially dissociated for 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. Phosphate PO4 3-It is a completely dissociated 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. Phosphorus pentoxideP2O5It is a formula used to express the P content of fertilizers. It is a white and anhydride form of phosphoric acid. It is a powerful desiccant. Phosphorous acidH3PO3It 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 phosphonateH2PO3It is a partially dissociated form of H3PO3, the major form of phosphonate at pH > 1. Hydrogen phosphonateHPO3 2-A completely dissociated form of H3PO3, it dominates at pH > 7. The hydrogen has a covalent bond with phosphorus that cannot be readily dissociated.

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7 Table 2. Marketing of products with active ingredient phosphorous acid or related compoundsz Product Company Active ingredientMarketed as Reference K-Phite 7LP Plant Food Systems Inc.Monoand di-potassium salts of phosphorous acid Fungicide Raid, McAvoy, and Sui 2010 Terronate WDG Agriliance LLC Fosetyl-Al Fungicide Pesticide Action Network 2004 AlietteBayer Cropscience LPFosetyl-Al Fungicide Bayer Cropscience 2004 Nutri-PhiteBiagro 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 Soil Phosphate Foliar Phosphate Helena ChemicalPhosphorus acidyFoliar fertilizer Helena 2002 ProPhytHelena ChemicalPotassium phosphiteSystemic fungicideHelena 2002; Nufarm USA, n.d. PhostroNufarm AmericaPhosphorus acidBiochemical pesticide Pesticide Action Network 2004 Riverdale MagellanNufarm AmericaPhosphorous acidFungicide Pesticide Action Network 2004 Plant Synergists Phosphorous Acid Technical Plant Synergists, Inc.Phosphorous acidFungicide Pesticide Action Network 2004 z Products and companies are mentioned for educational purposes and are not recommended over similar products in this document.y 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 phosphate.Table 3. Genera of oomycetes that cause disease on horticultural crops and that are likely to be controlled by phosphorous acid (Heer, Powelson, and Johnson 2002) Genus Disease Aphanomyces Root rot Bremia, Peronospora, Plasmopara, Pseudoperonospora, Sclerospora Downy mildew (Figure 2) Pythium Root rot and damping-o Phytophthora Late blight of potato and tomato, foliar blights on peppers and cucurbits, root and stem rots Albugo White rust on cruciferous plantsTable 4. Control of potato late blight by phosphorous acid and related products Compound Ecacy Application Reference Fosetyl-Al Not good in eld Foliar spray Cooke and Little 2001 Phosphonate Good in eld, variable against oomycetes in the lab Foliar spray Cooke and Little 2001 Phosphonate compoundsGood in pots Root dip Ouimette and Coey 1989a Phosphonate Variable against P. infestans isolates in the lab Foliar spray to detached leavesBashan, Levy, and Cohen 1990 Phosphorous acid Good against P. infestans in the eldFoliar spray Johnson, Inglis, and Miller 2004 Potassium phosphite Eective against Peronospora belbahrii (basil downy mildew) early in the trial Foliar spray Raid 2008