How to treat black spot on roses and help prevent it

Last updated on March 31st, 2022

Black spot is one of the most severe diseases to affect roses and there are not many gardeners who could say they have not experienced having roses that have suffered from black spot. The best way to help prevent black spot is to provide the best growing conditions and this means providing plenty of airflow by pruning out stems, positioning them in a sunny location. We also highly recommend spraying all your roses with a fungicide (such as rose clear) at the beginning of spring just as the leaves start to open on the rose bush.

Black spot is caused by a fungus called Diplocarpon rosae that infects the leaves on your rose bush and reduces the strength of your plant. You will know if you have black spot because the leaves will be marked and the markings will persist as long as the leaves are on the rose bush, the leaves usually then start to turn yellow before dropping off.

This fungus is, unfortunately, very diverse and there are new strains rapidly evolving, which means that the new strain starts to adapt to overcome the treatment methods that gardeners typically use. With this in mind, it’s easier to try and prevent black spot in the first place than to try to treat it. Below we go over some of the treatments and how to try and prevent it.

Symptoms of black spot

The most common symptoms include rapidly enlarging patches on the top of the leaf in a black or purple colour. It’s pretty much unmistakable and you will know what it is as soon as it starts. You might also notice that the leaf tissue starts to turn yellow around the perimeter of these black or purple spots and fall off.

In other cases, the yellow does not appear around the perimeter, but the spots still show up, and the leaves still drop. In some situations, the spots don’t grow, but they remain relatively small, and the leaves do not fall off. The reason for such a variety in the symptoms is simply the variety in the strain. It can affect different rose bushes in a slightly different fashion and some roses are more resistant to black spot than others.

If your rose bush is badly impacted it will shed all of its leaves and look bare.

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How to prevent black spot

The best way to care for your rose bush is to know how black spot develops and prevent it. These fungal spores are produced inside of the black spots or lesions, and they spread the infection usually in water. Wet conditions are necessary for the disease to build in the first place, and in the UK, most summers are sufficiently wet, which allows the disease to develop. The fungus will spend the winter inside fallen debris and lay dormant only producing spores come spring.

Clean up all fallen debris

It’s essential that you clean up fallen debris in the winter and not allow it to remain below your rose bush where it could become a host to fungal infections. Equally important is to control the moisture levels around the rose bush to prevent overwatering and mainly to prevent the rose bush from sitting in large amounts of water unnecessarily where fallen leaves can rest and allow a home for fungal spores to develop. 

Provide ideal growing conditions

Providing the best growing conditions is one of the best ways to help prevent it, make sure you provide good airflow by pruning so the stems are not too over congested and so that air can flow between the stems.

Next, try to avoid getting the foliage wet when watering, and try and water around the base of the plant.

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How to treat black spot

If you notice the symptoms, there are control methods you can use. As is the case with most diseases, you can opt for chemical or non-chemical control methods.

Firstly, in terms of non-chemical treatment options, you need to collect and destroy all of the affected leaves that fall off the plant to help stop the disease from spreading. If you notice lesions or purple and black spots on the stems of your bush in the spring, you should prune them before any leaves appear, pruning out infected stems by cutting a few inches below the affected areas. Doing both of these will delay the spread of the disease to other parts of the plant.

Be advised that there are limitations to this treatment option because the spores can still travel in the wind or the rain. With this method hybrid varieties, patio rose bushes, and climbers are usually most acceptable and can be treated if they are. But older species don’t tend to be as affected to the same level. Most rose labels and descriptions usually give an idea of how resistant to diseases roses are, so we recommend looking for roses that are labelled as having good resistance to disease.

Controlling using fungicide sprays such as Rose Clear

We highly recommend spraying all roses with a fungicide before signs appear, just as new shoots appear in spring.

There are chemical control methods you can use, which are slightly faster and more effective than just providing ideal growing conditions. Fungicides that contain tebuconazole, trifloxystrobin, or triticonazole are used to treat black spot on rose bushes.

You can also find products that are a combination of fungicides and insecticides if you want to treat the black spot and diminish any pests at the same time. It’s always advisable to use a product that contains a fungicide only if pests are not a problem.

Last update on 2024-12-03 / Affiliate links / Images from Amazon Product Advertising API