Black spot in roses is a detrimental fungal disease that infects the plant and easily spreads to young growth on surrounding plants. Fungus-resistant varieties are available for growers who prefer a low-maintenance rose bush. A management and prevention plan reduces the incidence of black spot in roses, resulting in beautiful, full-bloomed bushes.
Black spot is a common disease in roses caused by the fungus Diplocarpon rosae. The fungus weakens the plant, causing leaf damage and a reduced number of blooms. Black spot generally occurs in the spring and fall seasons. However, excessive rain and moisture will encourage the disease.
Black spot appears as a half-inch circular black spot with edges that are fringed. The disease appears on the upper side of the leaf and occasionally on the underside. Large spots may join together, creating an irregular area on the leaf. As the disease advances, a yellow margin appears and then the leaf falls from the plant. Some rose varieties may show a red-colored lesion. It is not common for black spot to infect the cane or stem of the plant unless the infection has become severe.
Cool temperatures of 65-75 degrees Fahrenheit along with wet conditions are ideal for the fungus and initiate its growth. The fungus will overwinter and resume its infection of your roses. The spores of the fungus spread by attaching to raindrops or splashing water, causing an infection on new plant growth. Once a leaf is infected, it will produce ethylene, causing the leaf to drop. Infected leaves on the ground create a reservoir of spores that may infect other plants if not removed.
To manage the disease, apply a fungicide labeled for use with black spot in the spring at the start of the growing season. It is best to apply the fungicide before bud break. During wet weather, continue to apply the fungicide to prevent an outbreak. Once black spot sets in, it is difficult to manage. If a plant becomes infected, remove and destroy infected canes, branches and leaves. Do not compost infected plants.
Prevention measures are the best management of the disease. Prune rose bushes regularly to promote leaf drying. This includes prompt removal of winter-killed branches and leaves that may be carrying the fungus. Never water plants from the top; instead, apply water to the ground area. Do not plant roses in a dense area with little sunlight, as this promotes disease growth.