@john.kempf that’s really interesting, I had no idea (and I’ve used my fair share of potassium phosphite).

@aviani5326 FRAC group 33 (Phosphonate) fungicides are pretty common for downy mildew control in hops which I have experience with. They are effective but select for resistant strains of the disease pretty quickly, so it is best used sparingly with other FRAC group fungicides in between applications. If I remember correctly, hop growers in OR recently had to get a special local need label for an even higher rate as the local downy strains had become resistant.

Unfortunately I see a lot of foliar nutrition products containing potassium phosphite branded as magic plant-strengthening agents that can be used as part of a biweekly foliar feed program. For growers buying in, this is obviously an amazing change to witness, but within a few years the effect will be lost and they’ll be back to the search for fungicides to use in a proper rotation.