Potassium bicarbonate spray

How it works

Raises the pH on the leaf surface and pulls water out of fungal threads, collapsing them. It is one of the few organic sprays that is genuinely curative on mildew that has already appeared, not just preventative.

Recipe

5 g potassium bicarbonate + 2-3 ml soft soap (helps it stick) per 1 litre water. Use potassium bicarbonate, NOT sodium bicarbonate - baking soda leaves sodium in your soil year after year.

How to apply

At the first white patches, spray all leaf surfaces every 7-10 days. Also useful preventatively on courgette, vine, rose and cucumber from midsummer.

Targets — pests

Not recorded — the field is blank rather than guessed.

Targets — diseases

Powdery mildew, Black spot, Botrytis (light)

Supplies: Potassium bicarbonate, Potassium soft soap, 5 L pressure sprayer

Days to harvest after spraying: 1

Best months: not recorded

Cautions

Do not exceed 10 g per litre - it will burn leaf margins. Avoid full sun. Rinse the sprayer well; the salt corrodes metal parts.

The thing people get wrong

Powdery mildew and downy mildew are completely different organisms and bicarbonate only touches the first. If the fuzz is on the UNDERSIDE with yellow patches above, that is downy mildew - bicarbonate will do nothing, and you need airflow and copper instead.