As the author of this article, I'd like to offer an asterisk to my advice about using Neem oil, since furthur research has resulted in some assertions that it may adversely affect bees and ladybugs.
However, because it is an oil-based spray, you should use it in the early morning or at dusk anyway to avoid burning plants. So, to be safe, just make sure the bees and other active beneficials aren't around when you use it - dusk is my preferred time for application.
Although not the most pleasant stuff to use,I've had real good luck with lime sulfer spray to keep blackspot at a minimum. Using it in the early spring to saturate the beds,and a week later to the canes and plant with addition to horticultural oil,it will kill a lot of the spores causing blackspot. Can also be applied periodcally through the summer.Do not apply during very warm days.Anything much above 70 degrees will risk buring plants.
I lke to apply with a trombone sprayer and a big 5 gallon pail filled with the lime sulfer concentrate and water.The product I use is made by Bonide.
I have suffered with black spot each year, losing a wonderful Mr. Lincoln last year. I got lucky coming upon this website and reading that you should always water roses in the a.m. I had been watering my roses late at night. Now with this add information, I will go out and buy the Neem. This information is GREATLY appreciated. Thank you.