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34 POTATO GROWER | JUNE 2021 Diggin' In Diggin' In Phosphite can act as both a fungicide and a biostimulant. Two Faces FUNGICIDES | Industry Report The "dual personality" of phosphite merits a closer look, says David Booty, technical development manager for Omex Agrifluids. Potato growers may find they have more than one opportunity to use it. Yes, phosphite — not phosphate. While it's chemically similar to the widely used and essential phosphate — the "P" in the NPK fertilizer mnemonic — phosphite (PO 3 ) is actually a "reduced form" of phosphate (PO 4 ). One less oxygen atom makes all the difference; plants cannot access phosphorus in the phosphite form. Some growers will already be aware of phosphite's role in a disease suppression program, where it has shown remark- able activity against diseases caused by oomycetes such as downy mildew and late blight. In numerous trials, research- ers and agronomists have observed how phosphite, when ap- plied in combination with reduced fungicide doses, was able to offer potato crops a similar degree of protection against late blight as full-dose fungicide treatments. This has proven valuable in recent years, in the wake of ever-tighter crop protection product legislation that has cur- tailed the use of go-to fungicide actives such as mancozeb and chlorothalonil. It's important to note that this is disease suppression, rather than disease control. Phosphites won't ever deliver 100 percent control, nor will they act against true fungal dis- eases like Septoria in wheat. And scientists are still debating the exact mode of action; it is thought to "switch on" certain key genes in the host plant (i.e., potato) that are instrumen- tal in defense, but other theories abound as well. What's also crucial — and where some growers have come unstuck — is that not all phosphites are created (or at least registered) equal. While they may be chemically identical, a grower who wants to adopt a form of integrated crop management through introducing phosphite to the fungicide program must use a product that has a pesticide registra- tion. Omex Cell Power Phorcephite, for example, has full U.S. registration as a systemic fungicide for the control of downy mildew, Pythium, Phytopthora and other diseases, across a range of crops including potatoes, where an added attraction is its zero-day harvest interval. But Omex also has another phosphite registration, a prod- uct called 4pHoric. This is registered as a fertilizer, because this is where phosphite starts to get really interesting. Phosphites truly have two faces. Applied at high doses, they provide the fungicidal inhibition we've just discussed. But when applied at low levels, below that capable of pro- viding fungicidal activity, they can also act as a valuable biostimulant.