Baking soda versus potassium bicarbonate for mildew

The claim

Baking soda dissolved in water controls powdery mildew, and potassium bicarbonate is just the fancy version of the same thing.

What the evidence actually shows

Both halves need correcting.

The original Cornell work on roses tested 0.063 M sodium bicarbonate with 1.0% ultrafine spray oil, applied weekly. The combination significantly controlled both powdery mildew and black spot, and — the part usually dropped — the combination was significantly better than either component alone. The oil is not an optional wetting agent. Baking soda by itself is the weaker half of the treatment.

On the two salts, a UK Horticultural Development Council project compared bicarbonate salts across diseases and found the right salt depends on the target: ammonium and sodium bicarbonate were strongest against Botrytis of grapes, while potassium bicarbonate consistently provided the best control of powdery mildew. They are not interchangeable.

The same report notes several practical points rarely mentioned: only potassium bicarbonate holds fungicide approval, it acts as an eradicant rather than a protectant, it needs an adjuvant, it is not persistent so requires repeat application, and it can scorch foliage above about 1%.

The sodium problem

There is also a reason to prefer the potassium salt beyond efficacy. Repeated sodium bicarbonate application adds sodium to soil, which is not something you want to accumulate. Potassium bicarbonate contributes a nutrient the plant uses.

What to do instead

If you are going to use a bicarbonate against powdery mildew, use potassium bicarbonate, with an adjuvant, below 1%, repeatedly, as an eradicant on early infection. Do not expect a protectant effect, and do not drop the oil from the Cornell formula and expect the same result.