David Ross wrote in
:

Bicarbonate of soda is alkaline, which does little to inhibit
fungus (including mildew). You need an acid. Try vinegar.

I don't know where you got that idea. Regular household bleach
(alkaline) has labeled instructions for killing (non-powdery) mildew. On
the other hand, milk and consumer grade hydrogen peroxide probably have
below neutral pH despite being slippery.

At any rate, I've been lead to believe (or maybe it's just my pet theory)
that merely changing the pH of the leaf surface is enough to inhibit
powdery mildew.

For home remedies, I've heard more people using potassium bicarbonate
(than sodium) and the milk. For non-home remedies I was surprised that
Neem is labeled as useful against powdery mildew. There is also
something called "Remedy" or R-something which is just potassium bi/mono?
carbonate. There are also some cultural things you can do to curb the
spread (control moisture, cut down on water splash, others I forget,
water the soil, not the foliage like paggers said).

IIRC, some say potassium in general in important for disease resistance
in plants.