Last year I used an organic, homemade solution (from a Rodale book, "Great Garden Formulas" by J. Benjamin and D. Martin) on my roses to control blackspot. It was a great success! It is a simple solution of 1 teaspoon baking soda to one quart of water. We sprayed the rose foliage thoroughly about every 7 - 10 days and had almost no blackspot at all. Prior to this regimen, my roses (I have more than 35) had lost nearly all their leaves by Aug-Sept. I highly recommend this simple, inexpensive, very effective spray.

I'm kind of surprised that the baking soda by itself did a good job since what I used for years was 1 tablespoon baking soda and 2-1/2 teaspoons on a light oil in 1 gallon of water. However lately I've been using a 50/50 mixture of fat free milk and water on that same spray schedule to control Black Spot and it works just as well without the mixing problem.

The oil is just to get it to stick, but yes the baking soda will work by itself. The milk is different. The oils in the milk are self sticking, and the enzymes and acids in milk are the things that work on the fungus.

The way I've understood it, the baking soda prevents by making the surface an alkaline environment that is inhospitable to fungi. The oil was to smother any existing spores to prevent spreading. A bit of tobasco for the japanese beetles couldn't hurt also.
What does the milk treat & how. Also, would regular whole milk do well if it is the fat in the milk that is active?

Corn meal is a great generic natural fungicide too.
So is teas made from milk, baking soda, or seaweeds.

Will this work in any zone . Or can you suggest something better for harsh climates like a zone 8 or 9 . Thanks

Either one will work everywhere. The milk for disease control was first used in Brazil to control tobacco leaf mosaic and they used raw milk, something most of us in the USA cannot get.
It's interesting to see something about how they work since none of the research I've seen from Cornell, Michigan State, Purdue, Iowa State indicates they know, yet.

Can you please provide directions for how you apply the corn meal and on what plants?

I use ordinary corn meal on turf at 10-20 pounds per 1,000 square feet.

I use it on my roses at a rate of one heaping handful per month scattered under each rose. That is the rate for all my plants.