Ask OSU Extension - How to fight black spot

Q. How do I fight black spot on my roses?

Cheryl from McDonald

A. Black spot is the most dreaded disease of anyone who loves to grow roses. The dark brown to black splotches on the leaves turn a beautiful rose garden into a disease-ridden patch of scraggly looking plants faster than you know it.

It is the most common disease of roses.

It is a fungal pathogen, thus is thrives when humidity is high, when leaves remain wet for extended periods and when temperatures are warm.

The best way to fight this disease is prevention. While some cultivars of roses are more susceptible to the disease, there are many ways to reduce the infection of even those cultivars.

The strategies focus on improved air circulation around the plants and improving sunlight penetration down into the canopy of the plants.

For better air circulation and sunlight penetration, focus on the following:

Give plants some space. Ensure they are not overcrowded. If the tag says they will be 3 inches wide, plant them 3 inches apart or more.

Prune out branches and stems to reduce foliage. Prune in the afternoon when the wounds will heal quickly.

Ensure plants are located in full sun, especially in the morning so the moisture dries off leaves soon after the sun rises.

Spring pruning and cleanup are other important factors in limiting this disease. Rake up all the leaves and plant parts. Remove them from the rose garden.

Before the new shoots appear, prune out diseased canes to avoid re-infection of the new foliage.

Fungicides are an option, but they require a lot of work and planning to be effective. If they are used, be sure to read and follow all label directions. For most home gardeners, preventive measures are more effective.

When choosing new roses, consider disease-resistant varieties. You can download a list of these varieties and learn more information about this disease at: http://go.osu.edu/blackspot

Eric Barrett is OSU Extension educator for agriculture and natural resources in Mahoning County. Call the office plant and pest clinic at 330-533-5538 to submit your questions. Regular clinic hours are 9am to noon Mondays and Thursdays.

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