Have you ever noticed that rose plant leaves tend to have black or almost dark purple spots on them?
They may also present with yellowish or very light greenish foliage tone around the dark patches.
This condition is called Rose Black Spot, or in scientific terminology, Diplocarpon rosae.1
What Is Black Spot?
Black spot is an illness of rose leaves where they start to have an unattractive appearance with black or dark spots.
Amidst the vibrant colour of roses, the foliage will turn into a mixture of yellow and light green with dark patches on the upper surface of the leaf.
Over time, the leaves will wither and fall, causing the vigour of roses to drastically drop.
“Black spot is one of the biggest problems with roses,” says Ben Stirton from Country Garden Roses.
“However, a good feeding and watering regime will prevent the rose from getting stressed which will, therefore, make it less prone to disease.”
What Causes Black Spot On Rose Bushes?
“If your roses are well pruned, properly fed and decent recently-bred varieties are chosen, then this shouldn’t be a problem,” says Stewart Pocock from Pocock’s Roses.
However, that does not mean that diseases cannot arise.
The primary cause of rose black spot is a fungal infection; it is the gravest disease for the rose bush.
The fungus, Diplocarpon rosae, weakens the plant’s vitality by infecting the leaves and gradually killing the plant.2
Rose black spot predominantly occurs at the beginning of spring and in very few cases, during the winter.
Like most fungal illnesses, black spot flourishes in moist conditions.
You may notice the black spots produce spores that get carried away by the wind to other foliage or plants, so that other rose bushes also get infected by this as the spores spread.
“When watering, especially at nighttime, keep the foliage dry,” advises Stewart.
Effect Of Black Spot
As the infection takes hold and progresses, the leaf zones around the dark patches turn to a hue of yellowish-green.
When the whole leaf has turned yellow, it becomes weak and eventually drops off.
With most leaves gone, the rose plant is now debilitated and exposed.
“Black spot is unattractive on the plant and a severe attack year after year will weaken a plant,” warns Philip.
“However, a lot of varieties marketed today can recover and grow back with vigour, as long as they have the basic needs of food, water and light provided in generous quantities.”
Treatment Options
The first step in treating black spot roses is prevention, as both our experts agree:
“Being a breeder for so long, I grew the roses without treating them with fungicides to evaluate them for health,” shares Colin, the owner of the award-winning plant nursery Dickson Roses.
“If you make sure your roses are given a balanced diet, it goes a long way to ensuring they thrive.”
“Prevention is better than cure, and sulphur can act as a preventative fungicide and be applied as a spray or drench in the early season,” adds Philip.
However, if the infection had already begun, worry not as you are still able to manage it.
Once you have verified the symptoms of black spot, there are a few treatments you could administer to help save your roses.
1) Remove The Contagion
First things first: it is incredibly important to remove the infection, in this case, the foliage with the black spots.
Taking care of leaf litter and pruning the yellow ones from the canes reduce the risk of spreading the disease.
“Air circulation through the rose is also a good preventative,” says Ben.
“Stripping all the leaves off our roses and clearing the ground underneath if they get either of these diseases prevents the spores from infesting the new growth.”
If you notice unhealthy stems before the leaves appear, eliminate them from your rose plant immediately.
“When deadheading, it can be helpful to trim back harder,” adds Philip.
“Giving more of a summer prune will encourage a larger amount of young vigorous growth which will be more resistant.”
Try to refrain from putting the infected leaves and stems into a compost pile.
Even a gentle breeze could blow the fungal spores back to your rose bushes.
2) Curing With Topical Sprays
One of the best treatments for black spot or any type of fungal disease is routine spraying.
“There are proprietary products on sale to stop the spread of blackspot which can be used according to the manufacturer’s instructions,” says Philip.
With such a wide array of choices, it can be difficult to choose the right fungicidal spraying solution.
Here are a few options to consider:
Baking Soda
Mix 1 teaspoon of baking soda to a quarter gallon of warm water.
Add another teaspoon of liquid to the solution and give it a stir.
Transfer the concoction to a bottle and generously spray the leaves of your rose plant.
Rather than a cure, this remedy works best as a precaution against the fungus.
Bordeaux Brew
The deadly combination of copper sulphate and hydrated lime in water kills fungus and deters insects.
But be gentle on the spraying dose because it can scald plants.
We recommend using it as a precautionary measure during spring and late winter.
Neem Oil
Being an organic pesticide, Neem oil is cold-pressed from the seeds of the neem tree.
One of the most active ingredients for repelling pests is Azadirachtin, which is abundantly found in extracted Neem oil.3
The organic oil enters the rose plant’s system and protects from within.
Just watch out not to spray oil on the leaves during hot sun because it burns the foliage.
Sulphur
Spraying sulphur may sound like poisoning your rose bush but in fact, it works like a charm to treat fungus diseases and prevent future occurrences.
We recommend you check the label of the sulphur powder before purchasing.
If the package reads ‘soluble’ for mixing with water, it’s good to go.
3) Prevention Is Better Than The Cure
Perhaps the most common phrase you hear while speaking to a botanist regarding plant health is “prevention works better than cure”.
“In terms of plant diseases, black spot and mildew are the two main diseases roses face,” says David Allison, a Horticultural Advisor for The National Allotment Society.
“You could spray your plants with a fungicide, but prevention is better than cure, so cleanliness is important.
“This includes removing dead leaves on the ground once they have fallen so that any spores are not carried over from one year to the next.”
“Choose varieties with good disease resistance and give them a good environment,” Philip says.
“Are they well fed, watered, sunny and have enough space for decent airflow? A happy vigorous plant will have better natural resistance than stressed plants.”
There’s also this list of preventive measures that ought to keep the roses healthy and your heart happy:
- Use sunlight: Choosing a space with maximum sunlight per day (we are talking around 6-8 hours of it) will improve the resilience of the roses as well as its vigour. As black spots are caused by moisture, constant sunlight and heat, this prevents vapours from forming thus eliminating the risk of infection through fungal diseases.
- Provide space for air: When planting roses, be mindful to give enough space for the plants to ‘breathe’. When planted in close proximity, moisture will form between the rose bushes and eventually cause a fungal infection. Good air circulation through adequate space alleviates this risk and keeps the foliage dry.
- Water sufficiently: When watering, be careful not to wet the top side of the leaves which could retain moisture that never evaporates. Rose plants do not require much irrigation so just a little bit of watering at ground level is more than enough.
- Ensure garden tidiness: A clean garden is a sight to behold. You can start by clearing fallen leaves or stems and practise raking up space around your rose bushes. Not only does this decrease the risk of fungal disease but also improves soil integrity.
“Using wood chip mulch can help to prevent black spot by preventing water and rain splash of fungal spores onto the foliage,” shares Master Horticulturist Colin Skelly.
“It will also help to optimise plant health by conserving soil moisture, providing slow release organic matter and helping to prevent compaction from footfall.”
We hope you gained value from these solutions and that your roses continue to bloom proudly.
References
1. Diplocarpon rosae. (n.d.). NatureSpot. Retrieved March 23, 2023, from https://www.naturespot.org.uk/species/diplocarpon-rosae
2. Black Spot of Rose. (2021, August 31). Cooperative Extension: Insect Pests, Ticks and Plant Diseases. Retrieved March 23, 2023, from https://extension.umaine.edu/ipm/ipddl/publications/5097e/
3. Chaudhary, S., Kanwar, R., Sehgal, A., Cahill, D., Barrow, C., Sehgal, R., & Kanwar, J. (2017, May 8). Progress on Azadirachta indica Based Biopesticides in Replacing Synthetic Toxic Pesticides. Frontier. Retrieved March 23, 2023, from https://www.frontiersin.org/articles/10.3389/fpls.2017.00610/full
