The black spot of a rose is a fungal disease that primarily affects roses. It starts on the lower leaves of rose plants and then gradually moves up the plant. Like many other fungal diseases, it thrives in cool, humid weather and it can overwinter, surviving unnoticed until spring when the roses start growing again.
Unfortunately, black spot fungus is not easy to cure. There are some topical fungicides that you can use to get rid of the black spots the disease causes. There are also a couple of organic home remedies that can work. But, if you are growing roses, it’s best to take preventative steps that will avoid them from attacking your plants.
Why do my roses keep getting a black spot?
Ironically, the rose is the national flower of the U.S. It is a versatile ornamental plant that is commonly grown in home gardens and used by professionals for landscaping purposes.
As Abigayl Novak and Drs. Alicyn Smart and Bruce Watt point out in a University of Maine Cooperative Extension fact sheet, that rose black spots are one of the most common rose diseases in the world. It is widespread globally and frequently causes epidemics.
In the U.S. roses grow in the Department of Agriculture (USDA) plant hardiness zones 2-11. This, in itself, is a factor, because almost everyone can grow roses in their gardens if they want to.
At the end of the day, if your roses keep getting black spots, it’s simply because they are roses!
The sad truth is that it is difficult to cure. But on the positive side, there are steps you can take to avoid it from happening.
What causes black spots on roses?
The fungus Diplocarpon Rosae causes the dreadful black spot roses so often suffer from. If you don’t take action and treat black spot fungus, the disease will weaken your plants and they will produce fewer and fear blooms.
Rose black spot, also known as rose Actinonema, rose leaf Asteroma, star sooty mold, leaf blotch, and leaf spot, needs water to occur. If the spores don’t remain wet for at least seven hours, they won’t germinate.
Once the spores germinate, the fruiting structures of the pathogen form within a couple of weeks. These structures release newly formed spores that are usually carried by splashing water to more roses, causing lots more infections. Even wind, insects, and gardening tools can spread infection.
Temperature is an important factor in the germination of rose black spot disease. A temperature of 65 ℉ is perfect for spores to germinate and the disease will develop rapidly when the temperature is around 75 ℉.
If the temperature rises above 85 ℉ it usually inhibits the spread of roses’ black spots.
Rose powdery mildew is another fungal disease that attacks roses. It is caused by a totally different fungus, Podosphaera pannosa.
Rose rust is also a fungal disease that attacks roses. It is caused by Phragmidium Tuberculatum and is the least serious of the common rose diseases.
How to get rid of black spots on roses?
The only way to get rid of black spots on roses is to prune and discard cane that is infected. You should also remove infected leaves to help stop the disease from spreading.
Rake and discard any fallen leaves. Don’t add them or the infected canes to your compost heap. Rather burn them.
It is possible to manage black spots with fungicides and other treatments. We will discuss these methods in Can roses recover from a black spot below.
How to prevent black spots on roses from growing
The precautionary measures you need to take to prevent black spots on roses from growing are surprisingly simple. First of all, it makes sense to plant resistant varieties in the first place. This lessens both gardener stress and maintenance efforts.
Even if you grow resistant rose varieties, make a point of growing your rose bushes in open sunny locations. This will help the foliage to dry quickly when it gets wet.
Dense plantings may look wonderful – until your rose bushes develop black spots. It is also best to avoid planting windbreaks alongside roses so that the air circulates well.
If you apply a thick layer of mulch around your rose bushes, this will help to prevent water from splashing up against the plant and possibly spreading spores.
When you water your rose bush or bushes, water low down and avoid wetting the foliage. This is particularly important in cloudy weather when the rose bushes will take longer to dry.
Prune infected leaves on an ongoing basis and perform a thorough cleanup in the fall every year to prevent the possibility of the spores overwintering.
Can roses recover from a black spot?
Roses can recover from black spots, but the infected parts of the plant can’t be cured. The best you can do is to control, spray, or treat black spots to prevent the infection of new leaves.
If you want to control black spots, you can use traditional fungicides or natural products to help manage black spot roses.
Traditional fungicides to treat black spot
The Texas A&M AgriLife Extension recommends traditional fungicides that contain chlorothalonil or myclobutanil and natural products that contain neem oil.
The University of Nebraska-Lincoln Extension’s Institute of Agricultural and Natural Resources (IANR) has published an extensive list of fungicides that you can use. But a growing number of home gardeners prefer natural remedies.
Natural products to control black spot
The two most popular natural remedies for controlling black spots on roses are neem oil and ordinary baking soda.
Neem oil is a natural fungicide and it works through the leaves. Just be careful not to use it in the middle of a sunny day or you may end up burning the leaves.
A teaspoon of baking soda dissolved in a quart of warm water is considered by many to be the best homemade recipe for rose black spot. You can add a teaspoon of dishwashing liquid to help the mixture stick to the leaves.
When you spray the baking soda mixture onto the leaves of infected roses, it changes the pH level and makes it more difficult for the disease to infect your precious roses.
Just remember that when it rains, the baking soda will wash off your rose leaves, so respray often to maintain the alkaline conditions.
Roses are beautiful, but when the leaves are attacked by black spots their beauty is quickly compromised.
Our garden tips are designed to explain what a black spot is and why your roses suffer from it. We have also given you lots of tips on what to do to treat black spots and control this horrible disease.