I have used tea tree oil to treat powdery mildew successfully. It did nothing to treat existing blackspot. I would be leery of using it as a preventative substitute in Cornell Formula because it is heavier than horticultual oil. There may be some unnecessary risk of damaging the leaves.

I tried teatree oil in water and in spoapy water, it did nothing for my roses with mildew. I find that milk and water spray works better.

I have tried grapefruit seed extract against mildew and it did have some effect but not as good as the poisons on spray cans. Works very well against spidermites ! Can harm leafs if dilution is too strong.

At some point I started to use seaweed meal in the soil around my roses. I am convinced that this streangthens the roses considerably against fungus problems like blackspot and mildew. I use it every spring now and I find it a must when I am growing my garden organic.

I'll never understand why folks keep seeking more expensive, untested replacements for scientifically proven materials, such as the horticultural oil and baking soda in the Cornell formula. Oh, well. Whatever floats your boat, I guess.

human nature.

I am trying tea tree oil on blackspot and I think it is helping. It is supposed to have antifungal properties and I had some around so... why not?

I can't be sure of the result because I've hit my roses with several treatments at once. I added a few drops of tea tree oil, a teaspoon of baking soda and a drop of dish soap to a quart of water in which I had boiled fresh rosemary and mint. (Rose Pharm product's active ingredients are oils of rosemary and mint.)

I am spraying my roses (in pots) with this mixture every couple of days. I scratched cornmeal into the surface of the soil (this is rumored to support beneficial fungi that fight the blackspot fungi).

I'm new at this so it's all an experiment. I ordered 8 roses by mail and the Cl. Iceberg arrived with blackspot, so now I'm fighting it on my Katy Girl and a few of the others.