JoeSpareBedroom wrote:

Mixed with water and sprayed on. It then dries, leaving a bit of a film.

I usually mix baking soda 1 tsp to a litre/quart, though I suppose you could
use more
(until your sprayer nozzle blocks up).

I read about using skim milk, possibly in rec. gardens. roses, and tried it.
Apparently there's a protein in the milk that stops black spot from replicating
or sending out spores. You use skim milk because it has all the protein and
none of the fat of 2% or homo, which can
stink up your plants. It's fairly cost effective, and easy to do.

You mix 3 parts water to 1 part skim milk.

I've tried it on my roses and think it's as least as effective as baking soda,
and maybe even a bit better. It won't get rid of the black spot that's already
there, but if you spray it twice a week, and after every rain, you'll probably
see new healthy leaves growing. I also spray the ground around the plants as
there are spores in the earth as well.

Flora