Baking Soda for Black Spot: Correct Ingredient, Ratio, and Limitations

A British gardener asked if a black spot recipe using "baking soda" meant baking powder or bicarbonate of soda, and about the 3 tsp per bucket with dish soap formula.

Bicarbonate of Soda

"Baking soda" = bicarbonate of soda (not baking powder). Correct ratio: 1 tablespoon per quart of water. Soap is just a sticker, not essential.

Preventive Only

This spray makes leaves less attractive to spores—it must be applied BEFORE disease appears. Once black spot is visible, it's too late. Better: sulfur + dormant oil spray before bud break to kill overwintering spores. Summer oil spray after leaf-out.