My roses get black fungus every year, I know how to treat but how do you prevent ?
You will prevent black fungus on your roses by spraying them with a dormant oil or Bordeaux mixture when plants are dormant , which is often in winter. Be sure to space the roses to allow for good air flow and avoid watering them from overhead.
Plenty of air current around them and not having wet foliage thru the night (water in morning, not evening). Also, keep ground underneath clean and free of clippings and leaves.