Black spot is a common fungal disease found on roses that causes the leaves to yellow, and then eventually fall off the plant completely. Black spot, which has a Latin name of Diplocarpon rosae, can cause the roses in your garden to look ugly, but unfortunately the effects of the disease run deeper than aesthetics. Over time, they can weaken the plant quite dramatically.
When Is Black Spot Most Active?
Black spot can occur at any time, but it loves hot and humid conditions, particularly wet summers or a warm spring. Very hot days with a damp, much colder night, are also ideal conditions for black spot to thrive.
How Does Black Spot Work?
Black spot fungus disease presents as black spots on the leaves that are uneven on the outside of the circle, and you will often see there is a yellow circle around the black sections.
When the plant has the disease it will be first seen on the lower leaves and then it climbs upwards towards the top of the plant. You may even be unlucky enough to notice black spot on very newly growing leaves and flowers, or even very young plants. Young roses may show blistered canes with black or purple marks.
Once a plant has the infection, you will see a smaller amount of flowers and leaves come through. This can cause the plant to struggle to thrive and if left without treatment, it may totally defoliate and eventually will be sensitive to a myriad of health issues.
How To Prevent Black Spot on Roses
Preventing black spot is much easier than trying to control or treat it. When you have to treat the condition it can be really hard because the spores stay in the soil of the plant. The rain can then splash the spores onto the plant and it becomes re-infected. Try to do the following to help avoid this nasty rose fungus:
- Place a resistant rose plant in a sunny location with lots of drainage and regular watering and fertilising regime
- Give your rose plant lots of room away from other rose plants to allow good airflow between plants
- Prune the rose bush effectively to allow lots of air flow throughout the plant
- Water the base of the plants, avoiding wetting the leaves
- Mulch around the base of the plant to avoid water splashing
- Invest in resistant cultivars
If you can, try to start as you mean to go on and only purchase black spot resistant cultivars. There are many different rose cultivars that are resistant to black spot and other common rose diseases. There is even a rose type in Australia called Brindabella Pink Bouquet which is entirely immune to black spot.
How To Treat Black Spot On Roses
If you notice black spot on you roses it is important to take away all of the infected leaves and cane sections, and do not place an infected leaf or cane in your compost. It is also important to do a thorough leaf cleanup at the end of each season and after pruning, ensuring those leaves do not go into your compost heap.
You should also disinfect your garden tools between pruning and cleanups, and only ever do pruning and cleanups when it is dry.
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You can also try commercial treatments for black spot which will usually involve spraying every week in spring and summer.
There are some homemade solutions you can spray on the plant, such as:
- Neem oil
- Baking soda, warm water and washing up liquid soap mixed
Black spot is a pesky disease that can truly blight a plants aesthetic and strength. Do follow the tips above, and seek advice from your local garden centre if you need specific rose cultivar advice in relation to combatting black spot.
Black Spot FAQs
Pruning diseased leaves and canes is an effective way to treat black spot. You can also purchase commercial sprays, or use homemade solutions.