Black Spot of Rose

Black Spot of Rose, also known as leaf blotch, and leaf spot, is a disease caused by a fungus called Diplocarpon rosae. The optimal conditions for disease development are 75-85° F and high relative humidity. Infection may be greatest on leaves that remain wet for six hours or longer. Leaves and canes can become infected.

Leaf symptoms are roundish, black spots with fringed margins that can be up to ½” in diameter. The spots form on the upper sides of leaves. The tissue surrounding the spots turns yellow. Infected leaves may prematurely drop from the plant. Usually lower leaves are infected first. Excessive leaf drop weakens the plant, predisposing it to other forms of injury such as those caused by temperature extremes.

Cane symptoms are blister-like, purple blotches that later turn black. The fungus probably will not kill the branches, but cane infections can be important in the pathogen’s survival through the winter.

Disease Cycle : The disease cycle typically begins on fallen leaves or canes that were infected the previous season. Rain or sprinkler irrigation splashes fungal spores from infected leaves that were shed the previous year to the plant’s lower leaves. The spores must remain wet for several hours for infection to occur. Symptoms can become visible within 72 hours after infection during warm, wet weather, and a secondary infection cycle can develop within 10 days after the initial infection.

The fungus that causes this disease tolerates a wide range of temperatures, and symptoms can continue to develop all season long if moisture is adequate. The fungus overwinters in infected canes and fallen leaves.

The fungus, Diplocarpon rosae, overwinters on diseased canes and fallen, infested leaves. Spores produced on fallen leaves are spread via rain or water splashed to newly emerged leaves and stem tissue in the spring. Under ideal conditions of leaf wetness, humidity and temperature the spores can germinate and infect in 1 day, cause symptoms in 4 to 5 days, and produce new spores that can infect additional leaves, flowers and canes within 10 to 11 days. Spores are easily moved by air currents

Control: Good ventilation suppresses black spot by promoting rapid drying of rose leaves. Avoid growing susceptible cultivars together because this intensifies and promotes rapid spread of the disease. Development of the disease is retarded in plantings where susceptible and resistant cultivars are mixed. There are many rose species or cultivars available with resistance to black spot. Black spot infection can be suppressed or prevented with the use of fungicides, starting in the spring. Please contact your local county extension office for current information.

Avoid overhead irrigation, and water only during the morning. Increase air circulation to promote drying by pruning and spacing plants. Allow the roses to have enough sun to dry foliage promptly when they are wet.

Sanitation: Remove black spot infested leaves and fallen leaves as often as feasible throughout the season. At the end of the season, rake and dispose of old leaves and prune out black spot infected stems.

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