Roses and other garden plants can suffer from any of a number of fungal diseases. Black spot is one of the most common and most devastating diseases for gardens in humid or rainy climates.
Black spot disease appears on plants' leaves as circular black patches with irregular edges. Members of the rose family are the most common victims. Eventually, the entire plant droops and sheds dead leaves.
This disease is caused most often by the fungus Diplocarpon rosae. The fungus survives through the winter in dead foliage and its spores are splashed onto healthy plants during spring rain storms, thus spreading the disease.
Black spot disease is not merely unattractive. The infection’s steady progression upward along a plant’s stalk eventually kills its host.
Regular spraying with a fungicidal preparation or applications of wettable sulfur are the only known treatments for black spot to date. Removing diseased plants is regarded as a more effective solution.
Commercial growers regard prevention through good garden hygiene as the best method of preventing black spot. Destroying diseased plants and cleaning up dead leaves from the growing area are effective preventative steps.