Potassium bicarbonate vs baking soda for powdery mildew
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Potassium bicarbonate is the better choice. A UK Horticultural Development Council project found it consistently gave the best powdery mildew control of the bicarbonate salts tested, and it is the only one with fungicide approval. Baking soda (sodium bicarbonate) worked in Cornell rose trials mainly in combination with 1.0% ultrafine spray oil, and repeated use adds sodium to the soil.
What do people believe about baking soda and potassium bicarbonate?
Baking soda dissolved in water controls powdery mildew, and potassium bicarbonate is just the fancy version of the same thing.
What does the evidence show?
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%.
Why prefer the potassium salt over sodium?
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 works 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.
Key takeaways
- Potassium bicarbonate consistently gave the best powdery mildew control among bicarbonate salts.
- Only potassium bicarbonate holds fungicide approval.
- In Cornell rose trials, sodium bicarbonate plus 1.0% spray oil beat either component alone.
- Potassium bicarbonate acts as an eradicant, needs an adjuvant, needs repeat application and can scorch foliage above about 1%.
- Repeated sodium bicarbonate use adds sodium to soil; potassium is a nutrient the plant uses.
References
Frequently asked questions
Is potassium bicarbonate the same as baking soda?
No. Baking soda is sodium bicarbonate, and the right bicarbonate salt depends on the target. A UK project found ammonium and sodium bicarbonate strongest against Botrytis of grapes, while potassium bicarbonate consistently gave the best control of powdery mildew.
Does baking soda kill powdery mildew?
Not well on its own. The Cornell rose work used 0.063 M sodium bicarbonate with 1.0% ultrafine spray oil, applied weekly, and the combination was significantly better than either component alone. Baking soda by itself is the weaker half of that treatment.
Why is sodium a problem for plants?
Repeated sodium bicarbonate applications add sodium to the soil, where it accumulates. Potassium bicarbonate contributes potassium, a nutrient the plant uses, instead.
How should potassium bicarbonate be used against powdery mildew?
Use it with an adjuvant, below about 1% to avoid scorching foliage, and repeatedly, because it is not persistent. It works as an eradicant on early infection, not as a protectant.
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