Horticulture Magazine

How To Treat Rose Black Spot

Blackspot (Diplocarpon) on a rose bush

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. [source]

What is black spot?

Fungal disease (black spot) on rose petals
Black spot infected leaves on a rose plant.

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.

What causes black spot on rose bushes?

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.

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.

Dark brown and yellow rose leaves infected with black spot
If left untreated, the infection spreads to the entire rose bush.

Effect of black spot on rose plants

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.

Treatment options

The first step in treating black spot roses is prevention.

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

A gardener pruning a rose bush to encourage flowering
Pruning helps to get rid of the infection.

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.

If you notice unhealthy stems before the leaves appear, eliminate them from your rose plant immediately.

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 black spot with topical sprays

One of the best treatments for black spot or any type of fungal disease is routine spraying.

With such a wide array of choices, it can be taxing to choose the right fungicidal spraying solution.

Here are a few options to consider:

Baking soda

gardener spraying a rose bush
Shower some love potion to rose leaves.

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

woman with facemask and knapsack sprayer in a field

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

spraying neem oil on rose plants

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. [source]

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 on a white rose bush

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 cure

Perhaps the most common phrase you hear while speaking to a botanist regarding plant health is “prevention works better than cure”.

white flowering roses in the sunshine

To make things easier for you, we researched and gathered a 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 prevent vapours from forming thus eliminating the risk of the infection or other 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 this decreases the risk of the fungal disease but also improves soil integrity.

We hope you gained value from these solutions and that your roses continue to bloom proudly.

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