Black spot is a fungus that attacks mostly roses and it survives in rose leaf litter and also overwinters in it. This condition can be deadly. It starts off with small black to purple spots that spread rapidly. Black spot can cause defoliation in no time if left uncontrolled. Black spot occurs in areas of high humidity and also places with high rainfall.
Black spot on roses (left)-Crocus Gardeners by Nature, Black spot (right)-R,C, Lambe
1) Neem oils have been highly effective.
2) If infestation is not severe, cut off the affected leaves and burn them. In the fall be sure to clean up all rose debris and burn this as well. a severe pruning of roses in the fall will help them off to a good start in the spring.
3) Buy disease resistant roses or plants.
4) 1 tsp of baking soda to one spray bottle can also help clear it up.
5) Avoid watering the leaves as this can increase the problem.
6) Organic fungicides are also effective.