Join the phosphite club
(Sponsored) Potato growers could find themselves getting used to a new weapon in their fight against late blight, as OMEX® Agrifluids agronomist Dean Konieczka explains.
Is there a potato grower anywhere in the world who doesn’t worry about the devastating effects that late blight might wreak in his or her crops? Regular and frequent applications of fungicides, close monitoring of weather and humidity, and a level of attention bordering on obsessive are the only ways to be sure of producing a quality crop, or indeed in some years any crop at all.
While crop protection manufacturers continue to develop new molecules and different modes of action to keep ahead of new variants and resistant strains, the increasing regulation surrounding pesticides has – often for good reason – suppressed the frequency at which novel products reach the market. When they arrive the sheer cost of bringing modern products to market often means they’re more expensive than the products they supersede, while their specificity can result in a very short ‘field life’ before resistance starts to affect their efficacy and viability.
The worry is that as the pipeline of new products starts to dry up, growers will be left with fewer and fewer options to control late blight, while the spiraling cost of extended fungicide programs could persuade more and more growers that the economic returns are not worth the hassle and headache.
But it’s not all doom and gloom. An increasing body of research is indicating that phosphite – as against the more widely known phosphate – could turn out to play a vital role in the struggle to stay ahead of plant pathogens. Several recent papers, the results of studies from teams around the world, have indicated that phosphites can bring all sorts of benefits to a wide range of crops, including wheat, canola, turf and, of course, potatoes – where results show it not only induces defense responses against Phytophthora but also displays a toxic effect against the pathogen.
So what is phosphite? Chemically similar to phosphate, in chemical terms it’s a ‘reduced form’ of phosphate (PO4), meaning that it has one less oxygen atom. But despite phosphorus being one of the major nutrients required for healthy plant development, plants can access it only as phosphate; phosphite doesn’t provide crops with any P nutrition. In fact, if applied directly to crops it would burn the plant tissue.
Instead, it’s applied as a salt, as in OMEX® Agrifluids’ Cell Power® Phorcephite®. In field trials spanning four years, researchers from the Swedish University of Agricultural Sciences found that potassium phosphite, in combination with reduced doses of fungicide, provided potatoes with the same degree of protection against blight as treatments with the recommended full dose of fungicide.
Integrated Crop Management (ICM) doesn’t get much better than this: programs that can truly reduce growers’ reliance on chemical controls. This is the future of crop production – doing more with less. Through increasing our adoption of ICM, we can please consumers, spend less on inputs, minimize the risk of fungicide-resistant populations developing, keep regulators and legislators happy and reduce the crop’s overall environmental impact. All in all, phosphites seem have a lot to offer, as do many other products in the same ‘biostimulant’ class. But how do they work?
Researchers around the world are undertaking detailed studies to better understand the phosphite mode of action. As already mentioned, phosphite can exert toxic effects on certain pathogens. As well as P. infestans, successful trials have also been seen in control of Microdochium nivale in turfgrass. But against true fungal diseases, like Septoria in wheat, there’s no evidence of fungicidal activity.
As for its fertilizing qualities – applications of phosphite have been shown to enhance root growth by as much as 50 per cent in a variety of crops – this is a bit of a mystery, as we know plants can’t convert phosphite into accessible phosphate. It’s led to a flurry of activity amongst plant biologists in a race to identify exactly what mechanisms phosphites employ within the plant.
It also places phosphite products in something of a gray area. Practically, they’re registered either as fungicides or fertilizers, yet technically they’re not true examples of either class. That’s why both in the U.S. and E.U., lawmakers are working towards a proper classification of biostimulants: products that optimize the metabolic efficiency of plants, in order to increase yield, quality and overall plant health.
For growers too, the current lack of official recognition for biostimulants such as phosphite can cause problems, because as the law stands now, a product must be registered as either a pesticide or a fertilizer. Meanwhile firm evidence of performance in the field will really start to help growers feel more confident about adopting biostimulants as a valuable part of their integrated crop management programs.
Several studies have already demonstrated how phosphites are at least as effective as conventional contact fungicides such as mancozeb and chlorothalonil, while work continues to confirm a protecting effect against late blight seen in phosphite-treated potato crops when applied in conjunction with chlorothalonil.
Coming back to those Swedish results, one particular point of interest stands out. Treatment of late blight with a program combining fungicide (in this case, cyazofamid) and phosphite was applied at a 14-day interval and provided the same performance as a 7-day application interval. Imagine the benefits to be enjoyed from halving both your fungicide bill and the number of application passes through your potato fields. Backing up such a strategy with a suitable decision support system, to allow growers to have confidence in their dosing decisions, could make a very valuable contribution towards a more effective integrated crop management program.
What’s more, other studies are showing further beneficial effects from phosphite applications to potato crops. An Argentine study, published in the American Journal of Plant Sciences, reported that application of potassium phosphite to seed tubers not only reduced the time between planting and emergence but also stimulated early growth. This was because the phosphite increased the mycorrhizal colonization of the tubers. There’s also early evidence that phosphite can help in defending crops against another disease caused by a Phytophthora pathogen, in this case the ‘pink rot’ brought about by P. erythroseptica.
This is a fast-moving area, especially as new details on phosphite’s capabilities and mechanisms continue to emerge, as does legislation and the regulatory environment for biostimulants. Our current phosphite product, Cell Power® Phorcephite®, enjoys full U.S. registration as a systemic fungicide for the control of downy mildew, Pythium, Phytophthora and other diseases in a range of crops, with a zero-day harvest interval. It’s a product we’ll continue to develop, aided by our own research as well as that of others.
So if you’re not yet a member of the phosphite club, perhaps the 2021 season will be your time to join?
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The product names and brands referenced here are registered and trademarks of OMEX® Agrifluids, Inc.
© OMEX® Agrifluids, Inc. 2020.
