Endeavour Turf Products Pty Ltd

Here E.T.P & Jerry Spencer take a detailed look at Potassium Phosphite.

POTASSIUM PHOSPHITE

Over the last few years a considerable amount of confusion has arisen in relation to fertilisers and chemicals containing this. Many of us are familiar with MAP and DAP (mono and di- ammonium phosphate) and possibly even MKP (monopotassium phosphate) all of which provide phosphorous derived from phosphoric acid (H3PO4). Plants use phosphate in the form of HPO4 and H2PO4 which once applied to the soil is rapidly converted from fertilisers.

In the USA products containing phosphorous acid (not phosphoric acid), phosphite (not phosphate) and phosphonite/ phosphonate have entered the vocabulary. But what are these and what function do they play in turf management?
Phosphite is completely different in nature from phosphate. Chemically phosphate and phosphate are similar, however phosphate has one more oxygen than phosphite and this markedly changes the nature and reactivity of the resultant molecule.
These three oxygen molecules give phosphate formulations increased mobility in plant tissue and soils so that they can be successfully applied to all areas of the plant.

Phosphite is directly fungistatic i.e. it slows the growth of the disease pathogen and inhibits the formation of spores. This causes the release of stress metabolites (chemicals) by the disease which are recognized by the plant as signals or elicitors causing the plant to enhance its defense response.
One of the results of the defence response by the plant is the accumulation of phytoalexins (immune bodies). In addition hypersensitive cell death occurs (death of disease infected cells) as well as lignifications and cell wall fortification (cell walls are thickened). Lytic enzymes are also produced by the plant which in conjunction with the rest of the plants response can caused death of the disease.

In horticulture growers globally have discovered the following benefits of using these on treated crops:
a) Rapid uptake compared to conventional phosphates
b) Controlled release of P through various stages of crop growth
c) Enhanced plant and root development
d) Improved plant health
e) Increased production of natural fungicides (phytoalexins) effectively providing organic disease control
f) Multiple sites of action effectively countering the development of resistance.
g) Low environmental toxicity effectively being safer than aspirin.
In the 1930’s various phosphorous compounds were studied to look at their efficiency as fertilisers. The conclusion was that phosphite was a poor source of nutritional P as when this was applied to plants they grew weakly.

40 years later it was discovered that phosphites were very efficient against the Oomycota (i.e species of phytopthera and Pythium). The reason for this was that phosphite either activated the defense mechanism in plants or alternatively caused a direct action on this fungal pathogen.
This market was pioneered by Bayer with Aliette® and fosetyl aluminium. When the patent for this expired in the USA several manufacturers created phosphite based fungicides by simple formualtions of phosphite with potassium, ammonium, sodium and aluminium.
Plants can absorb these compounds through roots and leaves and once in the plant the phosphorous acid compounds are very stable. However, plants are incapable of directly using phosphorous acid as a nutrient source. Phosphorous acid compounds can break down in the soil to available forms of P but this is a very slow process and does not provide adequate P nutrition.

The primary difference between Phostrol® and the Bayer CropScience product Aliette® is in the source of their common active ingredient, the phosphite ion. Although both products begin with a different composition, they are converted to the same active form. Phostrol® begins as a unique mixture of phosphorous acid salts and is rapidly converted to phosphite ions. Aliette begins as Aluminum tris (Oethyl phosphanate), is converted to a phosphorous acid, and finally becomes a phosphite ion. Once in
the form of a phosphite ion, fungi are attacked and both products strengthen and encourage the plant’s own defenses. In a variety of research trials, the performance of Phostrol® equals that of Aliette® when the two products are compared on the basis of phosphorous acid concentration.

During the past decade, Chipco Signature® fungicide (active ingredient fosetyl-Al) became a "keystone" product for disease control programs on many golf courses across the country. This is in part because of the general enhancement in quality on stressed turfgrass that has been reported in some studies, including selected tests at the University of Kentucky. The product is also known to provide control of Pythium cottony blight in many circumstances, particularly when sequential applications are made.
Fosetyl-Al is absorbed into the plant and converted to a phosphite (=phosphonate) ion (PO3-). An interesting and important fact is that the phosphite ion is the principal chemical responsible for disease control. Although a variety of chemically simple, inexpensive phosphite salts were known to provide disease control in the late 1970's, the language of Rhone-Poulenc's patent apparently permitted exclusive development of phosphite materials as fungicides. After fosetyl-Al recently came off patent, several fungicide manufacturers and formulators brought to market phosphite materials as fungicides. Turf fungicides for which I have current labels include Magellan®, Resyst®, Alude®, Prodigy Signature®, and Vital®.
The active ingredients in all these turf fungicides are phosphite salts, such as potassium phosphite, ammonium phosphite, sodium phosphite, and aluminum phosphite. These salts release phosphite ions onto the leaf surface, which are taken up by the plant. Thus, in theory, these phosphite fungicides should provide equal performance to fosetyl-Al. In reality, determining how generics compare to a well-established fungicide like fosetyl-Al will take several years of study. However, a status report of the research to date will undoubtedly be of interest to the many users of generic phosphite fungicides.
In 1990 (Lovatt), discovered that P deficiency in citrus caused changes in nitrogen metabolism and that when potassium phosphite was applied the biochemical response and normal plant growth were restored. Further work showed that fruit set and yield of avocadoes were improved when potassium phosphite was applied as a foliar spray.
Cook, Landschoot and Schlossberg (2005) carried out a study to determine if active ingredient and formulation of phosphonate fungicides potassium phosphite or fosetyl Al (Aliette and Chipco Signature)] provide similar control of Pythium blight when applied at equivalent rates of phosphorous acid, the active compound for controlling this disease.

Results of this study showed that when phosphonate fungicides with different active
ingredients and formulations [potassium phosphite (Alude® and the phosphorous
acid/potassium hydroxide treatment) and fosetyl Al (Aliette® and Chipco Signature®)] were applied at the same rate of phosphorous acid, no differences in Pythium blight control occurred among these products.

The Pythium fungus overwinters in soil and plant debris. Due to its spore structure, water is very important to the spread of this pathogen across turfgrass (Pythium is considered a "water mold") Oospores, resistant reproductive fungal structures (Image
2), release motile zoospores which can swim in films of water and spread the disease from one area to another.

Its spread is associated with water movement. When run-off drains through symptomatic turf, the surface water can transport spores. Also, the fungus is readily spread by equipment after affected areas are mowed while wet.

Cultural Control Options

• Modifying the environment may help reduce the severity of Pythium blight.

• Water management and proper drainage to avoid waterlogged root zones during summer are especially important.

• Selective pruning of trees and shrubs will help circulate air and dry turfgrass surfaces, effectively limiting the duration of the dew period.

• Avoiding conditions that produce excessive nitrogen fertility during midsummer will reduce vulnerability to Pythium outbreaks.

• Because of the pathogen’s survival and spread characteristics, Pythium outbreaks normally occur in the same “problem” areas each year as extreme weather conditions prevail.

• Delay mowing in those areas until surfaces are dry.

Chemical control

A number of options are available for Pythium control.

• A useful hint is to apply fungicides past the green onto the collar to provide an extra buffer zone of greens protection from Pythium that may be outside of the green

• When using fosetyl-Al research suggests that two or more consecutive applications of this fungicide are necessary for good control under severe disease pressure. Additionally, Phosphite/phosphonate materials like fosetyl-Al (e.g. Signature) should only be applied to the plant surface. If watered-in, it turns out the soil may change their protective chemistry.

References available upon request but special thanks to Agrichem Pty Ltd for ‘Phosphites The future of disease control –Phytopthera, Pythium and downy mildew (February 2004) and in particular Figures 1, 3, 4a (including labels)