Black Fungus on Flower Leaves
Roses are very susceptible to the fungus that causes tiny black spots to appear on flower leaves. In the case of this popular flower, the culprit is Diplocarpon rosae.
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Description
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This fungal infection is identified by the black spots on the leaves; the common name for the disease is "Black Spot." Though these spots start out tiny if untreated, they can reach 12 millimeters in diameter. The spots look like circles with frayed edges. Diplocarpon rosae thrives in a wet, warm environment.
Plant Damage
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Black Spot can cause rose bushes to lose their leaves and can also weaken the stems. As the black spots get larger, the leaves turn yellow, die and drop off. In advanced cases, reddish spots may appear on the stems and branches of rose bushes.
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Control
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Once the spots have appeared, the infected leaves should be removed and thrown away. Leaves that have fallen from the rose bush should also be discarded. If the weather is warm, don't water in the late afternoon to keep humidity levels down.
Chemicals
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Copper oxychloride, also called Mancozeb, is an antifungal that can be used as a preventative measure or as a treatment if rose bushes are already infected and control is not possible by picking off the leaves. Multiple treatments are usually required.
Black Soot
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Another cause of black spots on roses is the mold that grows on honeydew sap left by aphids. They may look similar, but the moldy sap will just brush off. Excessive numbers of aphids can harm rose bushes, so treatment with an insecticide might be worth considering. This is also commonly found on gardenias.
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References
- Photo Credit Image by Flickr.com, courtesy of John Morgan
