Along the Garden Path: Black spot fungus disease on roses


Black spot is one of the most common diseases of rose bushes, and if left unchecked, it can cause quite a bit of damage to your rose garden. Caused by the fungus Diplocarpon rosae, black spot begins just as its name suggests, with black spots showing up on the surface of the leaves.

As the spots grow larger, they become ringed with yellow, eventually causing the whole leaf to turn yellow and fall off. The stems may also have black or purplish spots. If not treated, black spot will leave your rose garden not only naked but significantly weakened and unprepared for the next winter.

Like most fungal diseases, black spot thrives in wet, humid weather, usually when temperatures are in the mid-70s Fahrenheit. The spots produce spores, which spread to other leaves and plants. The spores can survive the winter in fallen leaves and infected canes. Rainfall and overhead sprinkling allow the fungus to spread from leaf to leaf and from your gardening tools or clothing to other plants nearby. Black spot can kill all the rose’s leaves.

While black spot may seem relentless in your rose garden, it’s actually fairly easy to manage with the right treatment and prevention strategies.

If you’ve noticed telltale black spots on your roses, you should prune away the infected leaves and infected stems. Also, rake up all the fallen leaves under the plant. Throw the infected debris away, rather than putting it on your compost pile.

Once the infected leaves and stems have been removed, treat your rose with a fungicide. Spray the entire plant, making sure to get the tops and bottoms of the leaves as well as the stems. You can use a chemical fungicide or any number of organic options such as: copper, lime sulfur, neem oil, potassium or ammonium bicarbonate and sulfur. For those with a few roses and don’t want to spray, try Bayer Advanced All-In-One Rose &Flower Care. This contains three systemic products in one – fertilizer, insect control and disease control. This product is applied with a bucket or watering can.

With the right gardening habits in place, you can keep black spot at bay on your roses by watering in the morning as this allows the moisture to evaporate from the leaves. Avoid getting the foliage wet while watering especially if the weather is warm and humid.

Healthy grade No. 1 plants are more likely to provide a strong plant during a growing season compared to a grade No. 1½ or No. 2 rose.

Choose the varieties of roses that are resistant to black spot.

Choose a planting site with plenty of air circulation and be sure to space your roses 3-4 feet apart. Prune dense bushes to let air circulate between the branches.

Roses prefer full sun anyway and choosing a sunny location for your rose garden will help burn off moisture.

Regularly rake and clean up around your roses and throw away any fallen leaves. This is especially important in fall and winter, to remove spores that might be awaiting the spring warm-up to become active. After the cold-weather cleanup, apply fresh mulch to keep any remaining spores away from your roses.

Inspect your roses regularly, and immediately remove and throw away any spotted leaves, both on the plant and on the ground. If the problem isn’t severe, this may be all the treatment you need. Continue treatment every couple of weeks to keep the fungus under control.

By Charlene Thornhill

Along the Garden Path

Charlene Thornhill is a volunteer citizen columnist, who serves The Daily Advocate readers weekly with her community column Along the Garden Path. She can be reached at [email protected]. Viewpoints expressed in the article are the work of the author. The Daily Advocate does not endorse these viewpoints or the independent activities of the author.

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