In honey-do wrote:
I recently read an article in an organic magazine that suggested using
Baking Soda and horticulture oil as a remedy for black spots on roses.

Both of those work for me, but black spot hasn't been a big problem
lately here in Sydney.

I found an old wives' cure as good as any. Mix one part milk to six
parts water and spray the leaves thoroughly. In sunlight one of the
milk proteins turns toxic to fungus (including mildew and black spot)
and it also provides the oil-like coating to form a protective film.

When I use fungicides I find that both triforine and myclobutanil
products work well (these are ingredients, not brand names).