We moved into our home 3 years ago and soon noticed a problem with the plants we brought with us from our other home 30 miles away. It started with the Black-eyed Susans getting spots. It has escalated to the roses not having leaves and the flowering crab trees dying, one each year. The current tree has very few leaves left. We are also having an enormous problem with Japaneese beetles now, too. But that's another issue. . .
A Fungus Among Us
One of the very best fungicides against the black spot fungus that I know of is called Mancozeb. I get mine from an online store called http://www.rosemania.com . It will leave a little bit of yellowish powdery residue on the foliage but that is part of how it works. As for the Japanese Beetles, they can be a very tough customer. Start putting down some stuff called Milky spore now and in a few years it will be built up enough to really make an impact. Until it does, try using some Bayer Tree and Shrub granules. Since you are not talking about fruit trees, the Bayer product is great at getting to the bug problems. I use it once a year for my trees and other shrubs, it does a very good job for me.