The Subtropical Fruit Industry and the use of Phosphonates (potassium phosphite, phosphorous acid, fosetyl-al, etc.) compiled by Gerhard Nortjé

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1 The Subtropical Fruit Industry and the use of Phosphonates (potassium phosphite, phosphorous acid, fosetyl-al, etc.) compiled by Gerhard Nortjé This factsheet answers the most common frequently asked questions (FAQs) about the use of phosphorous acid in the South African Subtropical Fruit Industry. PROBLEM STATEMENT: Phosphonate products are commercially used on avocado, macadamia and mango fruit in South Africa, Australia, USA and Brazil. Since Autumn 2013, phosphonic acid has been increasingly detected in conventional and organically grown fruit, ever since a number of provincial and private laboratories established the analytical methods. The suspected origins are primarily the use of potassium phosphonate or fosetyl-al (trade name "Alliette"), given that the actual active substance in both cases is phosphonic acid (see text for details). There is clearly a potential risk of future MRL exceedances outside of Europe when the analysis of Fosetyl-aluminium becomes widespread due to the use of phosphites (phosphonates) as fertilizers and as fungicides. It is therefore becoming increasingly important that SUBTROP communicates with all stakeholders to work together in resolving these issues. What are phosphonates and how do they work (see Table 1)? Numerous phosphonate fungicide and fertilizer products are currently used and sold in the Subtropical Fruit Industry. Although these products have similar active ingredients, they differ in trade name, formulation, label terminology, uses and price. Some of these products are registered as fungicides and have explicit recommendations for disease control. Others, with ingredients that are virtually identical to the fungicides, are sold as fertilizers. Others are registered and used as pesticides. For the purpose of this factsheet, the term phosphonate is used to describe only those products made up of the salts and esters of phosphorous acid (H3PO3). Phosphorous acid is a solid substance that can be purchased through chemical supply companies. When mixed with water, it forms a strong acid called phosphonic acid. This acid is too strong to be used on plants and must be combined with other chemicals to raise the ph of the solution and decrease the potential for plant injury. One means of reducing the acidity of phosphonic acid is to neutralize it with an alkali salt, typically potassium hydroxide (KOH). The resulting solution contains mono-and di-potassium salts of phosphorous acid (often referred to as potassium phosphite) and is the active ingredient in the following products: Avoguard SL, Alliette WG, Fighter 500 SL, Rootmaster 98, LIMA 400 SL, M5, Speedfol K-Phite SP, Ag-Phos 400 SL, Chipco Signa- 1 of 6

2 ture, Brilliant SL, Alude, Magellan, Prodigy Signature, Vital, Vital Sign, Hygrophos, Phosphite 400 SL, Multi-Protek, Metior Series, Phytex, Phosguard, Leafmaster, Fostonic, Mikal M, VeritaFenomin, Roundup, Ethepon, Rootguard and other phosphonate fungicides. Potassium phosphite is also the main ingredient in several phosphite fertilizer products, including K-Phite ( ), Ele-Max Foliar Phosphite ( ) and Nutri Phite P + K ( ). Ammonium-phosphite is the active ingredient in 'Brilliant' SL, a phosphonate product currently in the process of registration for the control of Phytophthora root rot in avocado and macadamia. 'Brilliant' is applied as a drench, through the irrigation system or as a foliar application, but is not currently registered. Phosphonate fungicides and fertilizers should not be confused with phosphate-derived fertilizers such as ammonium phosphate, triple super phosphate, MAP and DAP. Even though phosphonate and phosphate compounds are very similar chemically, they differ significantly in how they act in plants and fungi. Phosphate (HPO4 2- ; H2PO4 - ) is taken up by plants and incorporated into cells where it forms an important energy-yielding molecule (ATP) and structural components of cell membranes and DNA. It is essential for root growth, photosynthesis and respiration in plants. Phosphonate fungicides and fertilizers are absorbed by plants and incorporated into cells as phosphite ions (H2PO3 - ). The fact that this ion has one less oxygen atom than phosphate means that it does not act in the same manner as phosphate in plants. Although the phosphite ion can be transported into plant cells, it does not appear to be involved in any phase of phosphorus metabolism (ATP production, photosynthesis or respiration). Over time, phosphonate fertilizer can be converted by soil bacteria to phosphate, where it can be taken up and metabolized by plants. This conversion can take several weeks and is not thought to be a very efficient means of phosphorus delivery to plants when compared with phosphate fertilizers. Phosphite ions have direct fungicidal effects on certain plant pathogens, a benefit that is not found with phosphate (Table 1, on page 4). Mode of action Phosphorous acid releases the phosphonate ion (H2PO3 - ; also called phosphite) upon disassociation. Phosphonate is easily taken up and translocated inside the plant. The phosphonate 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 Pythium root rots. One of the consequences of phosphonate treatment is that it induces a strong and rapid defence response in the affected host plant and this is sufficient in various crops, for example avocado, mango and macadamia, to fully limit the pathogen. The combined effects of direct inhibition of the pathogen and enhanced host defence responses are responsible for the ability of phosphonates to provide effective, durable control of plant diseases at low application rates. Phosphonates found in the following registered products are used for various diseases in avocado, macadamia and mango: Avocado: Two chemical groups are registered for the control of Phytophthora root rot on avocado: 'Alliette' (foseltyl-al)- is sprayed onto the canopy and converts into phosphorous acid and then to phosphonic acid in the plant, which works partly by direct action 2 of 6

3 on the fungus and partly by increasing the natural defence mechanisms of the plant; 'Fighter', 'Rootmaster' and 'Avoguard' (various formulations of phosphorous acid) - are injected directly into the sap flow of the tree. These products are currently the most widely used by avocado farmers (Figures 1 and 2, below). It seems from work currently done (Mcleod, 2016) that early season injections (until Mid-December) can result in higher residue levels than late season injections. The main problem in Macadamia is slow decline or tree die-back where branches die off and also bacterial canker. These trunk disease symptoms are due to Phytophthora root rot. Currently (June 2016) registration trials are in the final phase for some of the above-mentioned products to control Phytophthora trunk disease. Tree nuts, including macadamias, almonds, hazelnuts, pistachios and walnuts, have been granted a temporary MRL of 75 ppm to allow these industries time to provide the necessary information for the establishment of a permanent MRL for phosphonate. The almond industry has submitted appropriate data to the EU for the establishment of an MRL for phosphonate. The information submitted by the almond and other tree nut industries, is sufficient for the establishment of a permanent MRL for other tree nut crops, including macadamias. Mango Blossom Malformation (MBM) in Mango is a huge problem which has the possibility to destroy the mango industry in South Africa. A phosphorous acid product with the name LIMA was registered conditionally as a spray to control Blossom Malformation for local market fruit only. Further residue trials are underway for full registration. MBM is caused by Fusarium spp. This disease causes significant losses in yield. Previously, removal of affected panicles has been the only means to control this disease. However, phosphorous acid cannot be used on export mangoes as the for phosphonates is 2 ppm, while the South African MRL is 50 ppm. It has been established that phosphonates can be very resilient in wood tissue of perennial crops and hence the possible residual effect in fruit of these crops. Figures 1 and 2: Phosphonate trunk injections of avocado trees 3 of 6

4 Table 1: Phosphonate terminology (this table summarizes some of the important terms used to describe the phosphonate products) Phosphate Phosphorous acid Phosphonic acid Phosphite Ethyl phosphonate Phosphoric acid Broadly, any compound containing a carbon to phosphorus bond. More commonly used to describe products made of the salts of esters of phosphorous acid or the principle component of phosphate fertilizer usually in the form of ammonium-phosphate, potassium-phosphate or calcium-phosphate. Plants take up and use phosphate ions (H 2PO 4 - or HPO 4 2- ) for ATP, DNA, photosynthesis, respiration and other metabolic functions. Phosphate does not have fungicidal properties. Anhydrous solid substance often cited by its chemical formula H 3PO 3 or HPO (OH) 2. The basic ingredient of phosphonate products. Strong acid produced by dissolving phosphorous acid in water. The term phosphonic acid is often used synonymously with phosphorous acid. Alkali metal salts of phosphorous acid. The most common phosphite is potassium phosphite and is made by mixing a solution of potassium hydroxide with phosphonic acid. Potassium phosphite is also referred to as mono- and dipotassium salts of phosphorous acid on some phosphonate acid product labels. Plants take up phosphite ions (H 2PO 3 - ) but they are not used in phosphorus metabolism. Phosphite products have fungicidal properties. Organic (carbon-based) compound bonded to an aluminium ion forming aluminium tris (O-ethyl phosphonate) or fosetyl-al; the active ingredient in Alliette and Chipco Signature fungicides. Strong acid used in the manufacture of phosphate fertilizer. What is SUBTROP doing? SUBTROP, as part of the South African Fruit Industry Focus Group and under the HORT- GRO umbrella, is actively working to address the problem. A google survey with growers yielded very important baseline information with regards to the various products used and for what purposes. Regular communication to growers and industry with regards to new developments, will form part of the ongoing actions of SUB- TROP. Current recommendations Use only registered chemicals as recommended by SUBTROP (SAAGA, SAMGA, SAMAC and SALGA) (see the links below, footnote (*) on page 5, and Table 2, on page 6): hnical-info/food-safety-registeredchemicals-mrls, Use each specific chemical strictly according to the product label. Adhere to MRLs and keep detailed records of chemical use, Supply SUBTROP with all relevant information regarding chemical residues and reasons for crop rejections on local and export markets, Take part in SUBTROP surveys with regards to this issue via , google surveys, etc. References Briedenhann, Phosphonates, Adapted. Pathway from rock-phoshate to 4 of 6

5 Phosphonic acid, Phosphite and Phosetyl- Al. Fosetyl-Al, Phosphorous acid and Phosphonates MRLs. CRI Cutting Edge Research News from Citrus Research International. July Compiled by Paul Hardman, Industry Affairs Manager, Citrus Growers Association. Fosetyl-Aluminium, aluminium tris (ethylphosphonate). FAO Specifications and Evaluations for Agricultural Pesticides. Food and Agriculture Organization of the United Nations. Mcleod, A. Timing of phosphonate tree injections. Pers. comm. 5 June MRLs for Fosetyl and Phosphites, Briefing Note, 29 th June Audax Information Management Systems Ltd. PO Box 5021 READING RG8 0WZ. Phosphate vs. Phosphite. Dr. Ilango van- Ramasamy, AgriInfoTech, Inc. Accessed 27 th June Phosphonate residues in products exported to the EU - Cautionary Note. Fresh Notes (123) June Compiled on behalf of the Phosphonates Focus Group under HORTGRO. Phosphonic acid, potassium phosphonate (potassium salt of phosphonic acid), fosetyl-aluminium. Factsheet. Summary of current knowledge, April Bundesverband Naturkost Naturwaren. Koordinationsstelle der BNN-Monitorings, Michaelkirchstraße 17-18, D Berlin. Smillie, R; Grant, B.R and Guest, B. The mode of action of Phosphite: evidence for both direct and indirect modes of action on three Phytophthora spp. in plants. Disease control and pest management. Vol. 79, No. 9, 1989.The American Phytopathological Society. For more information, call SUBTROP at Disclaimer: Any recommendation contained within this has been compiled with information currently available and in good faith, but with the express condition that SUBTROP accepts no responsibility for any loss or damage resulting directly or indirectly from the use thereof. *'Ridomil' (metalaxyl) - is a soil applied granular formulation, which kills Phytophthora in the soil; this fungicide does not contain phosphonates and can be used as an alternative to phosphonates; 5 of 6

6 Table 2: Registered products for various applications containing Phosphonates Crop Chemical product Active ingredient Application SA MRL German supermarket MRL - Rootmaster 98 - Avoguard 500 SL - Fighter 500 SL Phosphorous acid / potassium phosphonate Root and trunk rot Avocado - Aliette WG Fosetyl-Al Weed control 0.1 Litchi Weed control 0.1 Ethapon SL Ethepon Plant growth regulator 0.05 Mango - Nirvana - Glufos 200 SL Weed control 0.1 LIMA Potassium Phosphite Blossom Malformation 2 50 Macadamia - Nirvana - Glufos 200 SL Weed control Ethepon SL - Ethepon 480 SL Ethepon Abscission of mature nut in husk 6 of 6