Phosphinic acid (Hypophosphorous acid)

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Click here to get more. Discover the uses and nature of phosphinic acid, also known as hypophosphorous acid, in our comprehensive guide. Learn about its properties, applications, and differences from other phosphorous compounds.

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Introduction

If you've had difficulty sorting through the different products and claims surrounding a group of turfgrass products known as phosphonates (potassium phosphite, phosphorous acid, fosetyl-Al, etc.), you're probably not alone. Numerous phosphonate fungicide and fertilizer products are marketed within the golf turf industry. While these products share similar active ingredients, their trade names, formulations, label terminologies, uses, and prices differ. Some are registered as fungicides with recommendations for disease control, while others, with nearly identical ingredients, are sold as fertilizers. A clear comprehension of these phosphonate products and their field performance can help you navigate the marketing complex and choose what suits your needs best.

What's in a Name?

Phosphonate, in its broadest sense, refers to any compound featuring a carbon to phosphorus bond. Examples of phosphonate compounds include organophosphate insecticides, antiviral medications, flame retardants, and certain herbicides. In nature, phosphonate compounds can be found in some lower life forms such as protozoa, mollusks, coelenterates, and oomycete fungi.

In this article, we use phosphonate to describe products derived from the salts and esters of phosphorous acid [HPO(OH)2]. Phosphorous acid, a solid substance, is obtainable from chemical supply companies. Dissolved in water, it forms a strong acid known as phosphonic acid, which, in its pure form, is too strong for plant use. Thus, it must be combined with other chemicals to raise the solution's pH and reduce the risk of plant damage.

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One approach to reducing phosphonic acid's acidity is neutralizing it with an alkali salt, typically potassium hydroxide (KOH). The resulting solution, containing mono- and di-potassium salts of phosphorous acid (often called potassium phosphite), forms the active ingredient in products like Alude and Resyst. Potassium phosphite is also principal in several phosphite fertilizer products, including KPhite (0-29-26), Ele-Max Foliar Phosphite (0-28-26), and Nutri Phite P + K (0-28-26).

Alternatively, reacting phosphonic acid with ethanol yields ethyl phosphonate. Aluminum ions are then added during manufacture to neutralize the ethyl-phosphonate ions, resulting in fosetyl-Al or Aluminum tris O-ethyl phosphonate. This substance is the active ingredient in Chipco Signature and Signature XTRA Stressgard fungicides.

Phosphonate fungicides and fertilizers should not be confused with phosphate-derived fertilizers like ammonium phosphate or triple super phosphate. Although phosphonate and phosphate compounds are chemically similar, they act differently in plants and fungi.

Phosphate (HPO4; H2PO4-) is absorbed by plants and incorporated into cells where it contributes to key functions like energy production (ATP), and structural components of cell membranes and DNA. It's vital for root growth, photosynthesis, and respiration in plants, thus forming the primary phosphorus source in most turfgrass fertilizers. While phosphate does not have strong direct effects on turfgrass diseases, phosphorus-deficient plants are more vulnerable to certain diseases.

Phosphonate fungicides and fertilizers are absorbed by plants as phosphite ions (H2PO3-). This ion's lack of one oxygen atom compared to phosphate means it does not function the same in plants. Though phosphite ions enter plant cells, they don't seem to partake in phosphorus metabolism (ATP production, photosynthesis, or respiration). Over time, soil bacteria can convert phosphonate fertilizer into phosphate, usable by plants, but this process takes weeks and is inefficient as a phosphorus source compared to phosphate fertilizers. Phosphite ions do, however, exhibit direct fungitoxic effects on certain plant pathogens, which phosphate lacks.

Pythium aphanidermatum growing in cornmeal medium amended with (a) potassium phosphite and (b) potassium phosphate. The potassium phosphite inhibits the growth of Pythium mycelia, while potassium phosphate does not affect growth. Photo: Peter Landschoot, Penn State

Table 1. Understanding Phosphonate Terminology. This table summarizes key terms used to describe phosphonate products.

Phosphonates as Fungicides

The fungicidal properties of phosphonates were discovered by scientists at Rhone-Poulenc Agrochemical Laboratories in France during the 1970s who were screening various chemicals for fungicidal properties. They found that phosphonate salts effectively controlled diseases caused by oomycetes (Phytophthora, Plasmopara, Pythium, and others). This discovery led to the commercial release of fosetyl-Al under the trade name Aliette.

Aliette was initially used to control Pythium diseases on golf courses and was mostly applied to greens and fairways preventatively. Studies in the early 1990s by Dr. L.T. Lucas at North Carolina State University demonstrated that combining Aliette with another fungicide, Fore (mancozeb), improved turf quality and controlled issues like "summer decline of bentgrass" or "summer stress complex." Based on these findings, Chipco Signature, combining fosetyl-Al and a blue pigment, was developed and became widely used on U.S. golf courses. It is labeled for controlling Pythium diseases, yellow tuft in turf, and summer stress complex when used with additional fungicides.

In the mid-1990s, potassium phosphite products entered the turfgrass market as both fungicides and fertilizers. Products registered through the EPA (like Alude and Resyst) include labels for controlling Pythium diseases and summer stress complex with specific fungicides.

Unlike most turfgrass fungicides, which are either contacts or translocated in plant xylem, phosphonate fungicides exhibit significant ambimobility, moving in both xylem and phloem. This unique property enables the fungicide to move from leaf tissues to crowns and roots, making phosphonates ideal for controlling root rot diseases like Pythium root rot and dysfunction caused by various Pythium species.

Phosphonate fungicides are highly effective against Pythium and other oomycete diseases when applied preventatively but have poor efficacy post-infection.

A Unique Mode of Action

The mode of action of phosphonate fungicides has long been debated. Some researchers believe that these products act directly on fungal pathogens, while others theorize that a combination of direct fungal inhibition and stimulation of host plant defenses prevents disease.

Early research with phosphonate fungicides incorporated into artificial media showed no direct effect on fungi like Pythium aphanidermatum, leading to the assumption that they worked by stimulating plant defenses. Later studies, however, revealed that phosphonate fungicides could inhibit fungi directly when media phosphate concentrations were low, allowing competition between phosphite and phosphate ions for transporters across cell membranes. This competition suggested that phosphonates disrupt phosphorus metabolism in fungi by causing accumulation of polyphosphate and pyrophosphate, diverting ATP from other pathways and reducing fungal growth.

More recently, studies on Phytophthora palmivora indicated that phosphonate fungicides inhibit key enzymes needed for growth and development, suggesting that their action is primarily direct inhibition of the pathogen, reducing rapid resistance development potential compared to other systemic fungicides.

Research has also shown that phosphite ions, which do not affect phosphorus metabolism in plants, may still aid in disease prevention by stimulating plant chemical defenses. Eucalyptus studies demonstrated that Phytophthora-infecting plants treated with phosphonates produce phytoalexins, defensive chemicals against the pathogen.

Due to the complexity of studying host defense mechanisms, much less is known about this mode of action compared to direct fungitoxic effects. However, many pathologists believe it is a likely factor in disease suppression.

Resistance Risk

The extensive use of phosphonate products in disease control and turf quality enhancement has raised concerns about pathogen resistance development. While no confirmed reports of resistance exist in turfgrass, resistance to phosphonate fungicides has emerged in other crops, such as lettuce downy mildew caused by Bremia lactucae in California, emphasizing the need for careful management to prevent resistance.

Phosphonates as Fertilizers

Initial research in the 1930s and 40s explored phosphonates as fertilizers due to concerns over disruptions in phosphate shipments during wartime. However, studies found that phosphonates were not effective substitutes for phosphate fertilizers, with lower crop yields compared to phosphate-treated plants, attributed to the slow conversion of phosphite to phosphate in soil primarily by bacteria.

Despite these findings, some companies market phosphonates as phosphorus and potassium fertilizers. Research at Penn State University showed that potassium phosphite does not supply usable phosphorus to turfgrasses in sand culture, reinforcing the inefficiency of phosphites compared to phosphate fertilizers.

Annual bluegrass treated with potassium phosphate (left) vs. potassium phosphite (right). The latter shows phosphorus deficiency symptoms, indicating it does not supply usable phosphorus. Photo: Peter Landschoot, Penn State

Claims of consistent enhanced rooting with phosphonates require more evidence. A two-year study at North Carolina State University found no effect on bentgrass root mass, highlighting the need for further research on the potential benefits under different conditions and product formulations.

While phosphonates' effects on phosphorus metabolism or yield in grasses appear limited, their occasional improvement in turf quality may stem from fungitoxic action against minor root pathogens, leading to healthier roots. More research is needed to determine the true causes of these benefits.

Literature Cited

1. Adams, F. and J.P. Conrad. 1953. Transition of phosphite to phosphate in soils. Soil Science 75:361-371.
2. Anonymous. 2005. Greenbook turf and ornamental reference for plant protection products. Vance Communication Corp., New York, NY.
3. Brown, S., S.T. Koike, O.E. Ochoa, F. Laemmlen, R.W. Michelmore. 2004. Insensitivity to the fungicide fosetyl-aluminum in California isolates of the lettuce downy mildew pathogen, Bremia lactucae. Plant Disease 88:502-508.
4. Dorer, S.P. 1996. Nutritional effects of a fungicide combination on summer bentgrass decline. Master of Science Thesis, North Carolina State University, Raleigh, NC.
5. Griffith, J.M., A.J. Davis, and B.R. Grant. 1992. Target sites of fungicides to control oomycetes. pp. 69-100. In: Target sites of fungicide action. W. Koller (ed.), CRC Press, Inc., Boca Raton, FL.
6. Guest, D. and B. Grant. 1991. The complex action of phosphonates as antifungal agents. Biological Reviews 66:159-187.
7. Jackson, T.J., T. Burgess, I. Colquhoun, G.E.S. Hardy. 2000. Action of the fungicide phosphite on Eucalyptus marginata inoculated with Phytophthora cinnamomi. Plant Pathology 49:147-154.
8. Lucas, L.T. 1995. Development and management of summer decline of bentgrass. PACE Insights 1(1):2-3.
9. MacIntire, W.H., S.H. Winterberg, L.J. Hardin, A.J. Sterges, and L.B. Clements. 1950. Fertilizer evaluation of certain phosphorus, phosphorous, and phosphoric materials by means of pot cultures. Journal American Society Agronomy 42:543-549.
10. McDonald, A.E., B. Grant, and W.C. Plaxton. 2001. Phosphite (phosphorous acid): Its relevance in the environment and agriculture and influence on plant phosphate starvation response. Journal Plant Nutrition 24:1505-1519.
11. Mudge, L.C. 1997. Fungicidal compositions for the enhancement of turf quality. United States Patent #5,599,804.
12. Niere, J.O., G. DeAngelis, B.R. Grant. 1994. The effect of phosphonate on the acid-soluble phosphorus components in the genus Phytophthora. Microbiology 140:1661-1670.
13. Sanders, P., W.J. Houser, and H.Cole, Jr. 1983. Control of Pythium spp. and Pythium blight of turfgrass with fosetyl aluminum. Plant Disease 67