My rose bush is developing little black spots on some of the leaves. What’s causing this, and what can I do about it?
For questions on roses, I always go to the helpful experts at the Nashville Rose Society, and their Web site. It looks like there are at least two fungal diseases that cause black spots on the leaves of roses: one appropriately called black spot, and the other, anthracnose. You can tell the difference by looking at the edges of the spots. Black spot has the feathery margins, which give rise to some of its other names: leaf blotch or star sooty mold.
Both make a rose look really bad for awhile, which is why people who are serious about growing fancy, beautiful roses stick to a rigid schedule of spraying. Fungicides are to ward off the ugly fungal diseases, pesticides to keep away chewing and sucking bugs.
Both blackspot and anthracnose overwinter on the plant and develop during a cool, moist spring. The best way to minimize the risk of developing diseases is with preventive maintenance: Clean up dead leaves and decaying matter around rose bushes, prune out diseased canes, make sure the rose bush is in a place that has good air circulation, and water in the morning, to give the foliage plenty of time to dry during the day. Fungicides, applied on a regular schedule in early spring, can help prevent infection.