Black spot fungus causes rosebushes to weaken, die

I think roses are my favorite flower, but they are not a low-maintenance plant. They are attacked by a host of insects, including the dreaded Japanese beetle (more on this in a future article), as well as several diseases. The most common disease of roses is black spot caused by the fungus Diplocarpon rosea. It can cause extensive damage if it isn’t controlled. As one might guess from the name, the infections appear as black spots on the top side of the leaves. These spots can be up to a half-inch in diameter and often have a fringed border. The leaf will start to turn yellow around the spots and the whole leaf eventually may turn yellow and drop off. As the infection spreads, and the leaves continue to fall off, the plant becomes weak. As the plant grows weaker, fewer and fewer smaller flowers will form. The plant may become so weakened that severe winter damage will occur, causing its death.

So, what to do? The disease spreads by spores that blow or splash from leaf to leaf and from plant to plant. The spores first come from leaves left on the ground over winter and also from old infections on the stems. After the first infections occur, the black spots will produce spores all summer. If there aren’t any spores, there can be no infection. Clean up everything that might produce the spores. Prune out canes that look suspicious and pick up all fallen leaves. Pick leaves that have black spots off the plant before they can start producing more spores. Burn or bury these leaves because they will keep producing spores. Always do this cleaning during dry weather because the spores need water to germinate. Keep the plants as dry as possible. Water only at the base of the plant and grow the plants where there is plenty of wind so the leaves will dry quickly if they do get wet. When you buy new roses, consider buying varieties that are disease-resistant. As a last resort, there are several fungicides that are effective in controlling the disease. Remember that these chemicals only protect the leaves and must be present when a spore lands to be effective.

For more information, contact plant diagnostician Bruce Watt in the Pest Management Office at the University of Maine Cooperative Extension Office in Orono at (800) 287-0279 or http://pmo.umext.maine.edu.