I want to make a trail spraying or two to test efficacy, before bothering to obtain more, and searched my normal sources for chemicals. It is available of course, but expensive, as a reagent ("real" buyers from scientific supply houses get very deep discounts by bidded contracts; only we 'little people" have to pay the full tab). Instead, the way to buy chemicals reasonably is to find sources of "technical grade" chemical used in manufacturing or larger-scale use (e.g., Epsom salts, magnesium sulfate, from the pharmacy or grocery store shelf). Unfortunately, potassium bicarbonate is not obtainable from The Chemistry Store (supplies soap-making craftsmen) where I get iron (II, i.e., ferrous) sulfate for a good price at a good-enough purity to spray a solution (horticultural copperas has too much debris in it) and get potassium hydroxide for making horticultural (insecticidal) soap.

So I keep my eyes open. Lo and behold, in a homebrewing shop I see 2-oz packets of food-grade potassium bicarbonate at a high price per ounce but still only half the per-ounce cost of the commercial fungicide and one-eighth the cost of the smallest container of the labeled fungicide.

If the trial works, I'll find a homebrewing (beer or wine) supply house that sells in larger quantities, presumably at still lower price.

nuts.com
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Thanks! I likely would not have happened across that.