A few days ago I was delighted to see that the first rose had bloomed on my favorite rose bush. However, when I got a little closer, I noticed that the leaves of the bush were dotted with dark spots. Although I was able to admire the beautiful flowers and breathe in their scent, I was disappointed by the unattractive leaves and did not contribute to the overall aesthetic.
While I knew that roses can be affected by various problems, I had not detected this problem in my garden before. And, while I’m not new to gardening, I know it’s a problem that both new and experienced gardeners face.
Eager to understand and resolve the problem, I sought the help of some rose experts to find out what was causing the unsightly marks, how to get rid of them, and how to prevent them from reappearing in the future.
What causes spots on roses?
There are several reasons why roses may develop spots on their leaves. Wes Harvell, Rosarian in Jackson and Perkins, explains that fungal diseases such as black spot, powdery mildew or rust can cause them. “These diseases thrive in moist conditions and manifest as dark spots, powdery spots or rust-colored growths on leaves and stems. If left untreated, black spot disease will progress rapidly throughout the plant and eventually affect all its parts,” he says.
He explains that as the disease spreads, the plant will shed its leaves and each infected leaf can serve as a source of disease transmission. “Despite the plant’s attempts to regrow foliage, new leaves are likely to become infected and the cycle begins again,” she adds.
As the disease spreads, the plant will shed its leaves and each infected leaf can serve as a source of disease transmission.
Roses are one of the common plants that bring insects to your garden, and pests such as aphids, spider mites or rose slugs cause spots by feeding on the plant tissues. Harvell says, “Identifying specific symptoms can help determine the exact disease or pest.”
Robert Silver, Chief Professional Officer, Professional gardening blog, explains more about these insects and what to look out for. “Colonies of green or black pear-shaped aphids clustered on new shoots are a telltale sign, leaving behind sticky cards of honeydew and sooty mold. Their sap sucking can seriously impede development. The barely visible mites also lurk around, leaving tiny yellow or tan specks of their feast.”
Even if you’ve never had problems with your roses before, Harvell warns that this can change as temperatures rise. “It is important to recognize that due to climate changes, diseases are expanding to regions where they were previously absent.”
How to detect black spot, powdery mildew and rust
Black point: If you notice circular black spots surrounded by fringed edges, it is likely black spot fungus.
Powdery mildew: This fungal disease can be detected by a white or grayish coating of powder that resembles dust spread on the foliage.
Oxide: The marks appear as blackish-brown spots with orange, tan, or yellow centers. The leaf may turn yellow and if you turn it over, you will see orange fungal spores that resemble orange powder.
How to correct problems with your roses
If fungal spots appear, Harvell says the first and essential step is to remove and destroy the infected parts of the plants, which will include the leaves and stems. So, grab a pair of the best gardening shears and prune your roses to remove damaged stems.
It is suggested to apply a suitable fungicide once the damaged parts have been removed, which will help control the spread and prevent its appearance. “In cases of pest-induced spots, the use of insecticidal soap or horticultural oils can eradicate pests such as aphids, spider mites or other pests,” he explains.
Silver also suggests that the following products are suitable for fungicide use: “Chlorothalonil can help control black spot and rust, while products containing sulfur or potassium bicarbonate work well against powdery mildew.”
While regular application of fungicide is also key to protecting new growth, Silver also recommends raking fallen leaves to further reduce overwintering spores.
In addition to the techniques mentioned above, Silver says you should make the most of what nature has to offer by “encouraging beneficial insects that prey on pesky rose pests, such as ladybugs and lacewings.”
How to prevent spots on your roses
Preventing future appearances of spots on your roses involves several measures. First, start with the type of roses you grow. Harvell says, “Disease-resistant rose varieties can reduce susceptibility to common fungal diseases.” Therefore, if you are investing in new roses, check their disease resistance before purchasing them.
Silver is happy to admit that he preaches about giving roses a “good deal” and emphasizes that sunlight and airflow are important, and urges you to avoid planting roses in a dimly lit corner of your garden. She “prunes any dense interior growth so that air can circulate freely, as leaves that remain moist invite mold spores and black spots.”
If your roses are susceptible to black spot, powdery mildew or rust, Harvell suggests establishing a regular schedule using a preventative fungicide treatment. Maintaining proper garden hygiene will also help. “Removing fallen leaves and debris can minimize the presence of fungal spores,” she adds.
Fungal growth can also be discouraged by watering the plants at the base in the morning to allow the foliage to dry during the day. Another method is to apply an organic mulch around the base of the plants to help regulate soil moisture and prevent the growth of weeds, which he believes will harbor pests. Silver adds: “Roses don’t need daily watering; a moderate drought is better than a swamp.”
It is also worth inspecting your roses regularly for early signs of disease or pest infestation, as this will allow for quick intervention before the problem becomes serious.
Once the season is over, Silver recommends tidying up. “Clearing every last leaf once winter sets in will deny diseases and pests a cozy place to spend the winter.” Then, just before the first buds emerge in spring, she says, “Spray your roses with horticultural oil, as it softens any straggling fungus or eggs that are trying to ruin the season’s open party.”
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