The Answer:

Blackspot disease, Diplocarpon rosae, is a common and serious fungal disease affecting rose plants. Dark brown or black spots start appearing on infected plants at the start of the growing season. The spots appear on the lower branches and gradually spread upward. Eventually the leaves turn yellow and fall off. The spots can also appear on the stalks, or canes, of rose bushes.

To prevent blackspot, plant rose varieties that are resistant. Otherwise, plant bushes in sunny areas with good air circulation, since that will reduce the time moisture lingers on the leaves. Infection can occur after only seven hours of continued leaf wetness. Avoid dense plantings and watering late at night. Hosing down or misting leaves can also reduce the chances of blackspot. Remove any infected leaves immediately. They should not be composted, since that could enable them to spread.

If blackspot does appear, some fungicides are effective against it.

—The Editors