Do your roses have black spot disease? Here’s how to treat the fungus

If you’ve started noticing some black spots on roses in your garden, chances are they’re infected with black spot disease. This disease, scientifically known as Diplocarpon rosae, is a fungus that causes parts of the leaves to develop black spots, eventually yellowing and falling off. So, why get rid of it? Aside from just causing the leaves to decay, it also weakens the rose plant as a whole. Treating black spots on roses as soon as you notice them is key to keeping the rose plant happy and healthy.

How to identify black spot

Black spot disease is one of the easier fungal plant infections to identify. Black spot on roses usually develops as circular spots on the leaves and can form on both the upper and undersides (though more commonly, the spots will be on top). The spots don’t have clean edges; they look more uneven and feathery.

Identifying black spot disease will take a thorough inspection of the plant since the fungus starts on the lower leaves and makes its way upward. This means that you won’t see the disease just by passing and glancing at the roses until it’s too late; however, you don’t need to worry so much that you’re out inspecting your bushes every day. Once a week should do nicely, and you can make it part of your routine to inspect all your plants weekly for pests, fungus, dead foliage, and the like.

Close-up of black spot on a rose leaf

What causes black spot disease?

Black spots on roses are rather common, actually. Most roses will develop a black spot here or there throughout their lifetime, and sometimes the plant will tolerate it just fine; however, it’s always best to get rid of any fungal infection as soon as you notice it to prevent the problem from worsening.

This particular fungus is caused by spores that are found in unfavorable environments. (We’ll get to good conditions a bit later.) They germinate in the spring, spreading onto the plant through splashing water, and will only start infecting the rose if they’re wet for seven continuous hours. The existing spores have the ability to produce new spores that get splashed onto other leaves, which is how the infection spreads.

Diseased leaves and stems that are left on the ground are also a home to these spores during the winter, so it’s important that you dispose of any diseased tissue properly instead of leaving it to decompose in your garden beds.

How black spot disease affects roses

Rose leaves affected by black spots will eventually turn yellow and fall off. As the disease worsens, it can infect young canes with blisters and flowers with red spotting. Eventually, the entire plant will weaken and possibly lose all its foliage. Roses infected by black spot disease, because of their weakened state, will have fewer flower buds and be more stressed. Remember: one infection or disease makes a plant prone to other problems.

Treating and preventing black spot disease

Unfortunately for leaves that have already been infected by black spot disease, there’s no cure. The best thing you can do is treat the problem and work to prevent future recurrences. This ranges anywhere from pruning off diseased or dead foliage and stems to using topical treatments.

Treatments and preventative aids

The first step to treating and preventing the spread of black spot disease is to trim off and properly dispose of any infected leaves and/or stems. Because the spores can remain on infected tissue throughout the winter, be sure to clean up any leaves that have fallen on the ground beneath the roses, too. Otherwise, come springtime, they’ll start their germination all over again and reinfect your plant.

You should only prune in dry weather. Make sure that your pruners are disinfected. Unclean pruners can potentially introduce more problems and diseases to your plant, and the goal here is to get rid of the fungus and keep the plant healthy.

Along with pruning, you can try these solutions to help treat and prevent future black spot disease on your roses:

  • Insecticidal soaps. Insecticidal soaps with added fungicide are great tools for helping treat and prevent further infections. Simply follow the directions on the bottle you purchase when applying to your plant.
  • Baking soda spray. Baking soda spray is a DIY solution that prevents more than treats, so shouldn’t be used to clear up an infection. To make the solution, dissolve a teaspoon of baking soda into a quart of warm water, then add up to a teaspoon of liquid soap.
  • Sulfur. Sulfur helps prevent fungus diseases, as well as some pests. It should be used as another tool for prevention, if desired. There are forms of sulfur marked as “wettable,” meaning you can mix them with water and spray them onto the plant if that method is better for you.
A close-up of a cluster of red rose blooms

Good conditions are a rose’s best friend

The best thing you can do for your roses is to provide ideal growing conditions. Black spot disease will thrive on roses in the right conditions, and those conditions are usually less than favorable for your plant. A healthy rose (as with any plant) will be less susceptible to disease and pests, so be sure to trim off any dead or dying tissue as soon as you notice it–even if it isn’t infected.

You should also make sure that your roses are planted in well-draining soil to avoid any pooling of water or soggy dirt, especially since they enjoy weekly waterings. They love full sun, and morning sunlight will help dry any excess dew or moisture from their leaves and petals. Above all, don’t crowd them. Good air flow between plants will help keep them healthy and happy.

As long as you catch it early enough, black spot disease doesn’t mark the end for your roses. It just means you’ll have to be a bit more diligent about inspecting your plants, particularly if conditions haven’t been ideal (for example, lots of rain and not a lot of sun to dry out the plants). A healthy plant is a happy plant, and you’re less likely to encounter black spot disease on roses if you care for them as best you can.

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