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.
This leaf disease attacks plants throughout the year but is most prevalent during dry months. If left untreated, black spot spreads rapidly and weakens plants severely. It is important to realize that Black Spot is a soil borne fungus, and is present at all times. Proper care, and culture can greatly reduce the instances of this disease.
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.