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What is it?

Black spot is a plant disease common in roses. It is caused by diplocarpon rosae, a fungus that is a severe problem in regions of high humidity and rainfall.

What does it look like?

Roses will develop circular, black spots on the leaves. The spots will have irregular edges, and be found on the top and sides of leaves in the spring. Often when the tissue of the plant becomes infected, the entire leaf may turn yellow. The infected leaves may droop prematurely, or drop off the plant entirely. If the infection is allowed to become severe, plants may lose all of their leaves by midsummer, and flower production will be reduced. Quality of the rose overall will be poor.

How does it manifest?

The fungus associated with black spot survives the winter on infected rose canes and leaves. When water is splashed on the plant, spores are spread. Once the fungus enters the tissues of the plant, spots form the size of a pinhead. As the fungus increases, the black spots grow to ¾ inch in diameter, and the spots may eventually join to form irregularly shaped blotches as the infection progresses. Often, when a plant is repeatedly infected it will be killed off.

What can you do about it?

When you notice black spot on your roses, spray them with a fungicide containing triforine or chlorothalonil. Repeat the treatment every 7 to 10 days for as long as the weather remains wet and humid. Cut off all infected canes and avoid overhead watering, which could spread spores. Rake and destroy all fallen leaves in the fall. Once you prune the plants in the dormant season, spray the plant with lime-sulfur spray as an added protection. When new growth starts again in the spring, begin the fungicide spraying program again. An alternative is to seek out plants resistant to black spot.