Black spot disease on roses – Preventing and controlling it

Black spot disease on roses – Preventing and controlling it

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Being a lover of roses myself, I have both rose beds which I inherited with the house, but I also grow them in containers. I’ve had my fair share of experience with dealing with blackspot (Diplocarpon rosae). I have also worked in nurseries for over 20 years so have also been the person who deals with treating roses as I have my spraying licence to spray pesticides and fungicides commercially.

Black spot is more about trying to prevent the disease rather than treating it because it’s almost impossible to eradicate it completely. If you get an early start, just as the leaves begin to shoot from the stems, you can have a significant effect on reducing blackspot on your roses. The best way to control blackspot is to spray early on in the season before the disease has a chance to establish itself and spray them again every 2-3 weeks to keep on top of it. This has also proven very useful for controlling mildew fungus, rust as well as aphids at the same time. I usually use a mixed pesticide/fungicide that treats all these diseases as well as the pests. Roseclear is my personal favourite, but there are a few other options usually designed for roses.

What is Black Spot disease?

Black spot disease
Black spot disease

After rust, black spot is probably the most serious disease to infect roses. It’s a fungal disease that affects both the leaves and stems and greatly reduces plant vigour. It’s a very diverse fungus and quickly overcomes the resistance bred into new rose cultivars. As it’s an airborne disease, there’s not much you can do to avoid it so it’s more about combatting it earlier.

How to recognise Black Spot disease

This is where it gets tricky because the signs of black spot vary from variety to variety of roses. In general, you see purplish or black spots on the upper leaf surface. They start off smaller, appearing maybe just on the odd leaf, however, they grow very quickly, and you may notice the radiating strands of the fungus as it spreads.

Fungal rose disease Black Spot, with infected leaves
Fungal rose disease Black Spot, with infected leaves

Around the black spots, the leaves usually turn yellow and then eventually drop to the ground. Even without the discolouration, the leaf may just fall anyway.

If the rose is badly infected with many leaves having black spots, you will probably lose most of the leaves on the plant. This, of course, then limits the amount of energy generated for the plant and severely weakens it further. The problem is, while the rose is weak from black spot, the rose plant is much more susceptible to all kinds of other diseases as well as pests.

What to do about it?

How to help prevent black spot

Pruning rose in spring and removing stems effected with black spot on the stems
Pruning roses in spring and removing stems affected with black spot

The first thing to do is remove any fallen leaves in the fall (autumn) and dispose of them to reduce the chances of the black spot spores spreading from the fallen leaves the following spring. It’s also worth mulching around the base of roses in autumn to help prevent any spores that overwintered in the soil from splashing up in spring when new growth emerges. Mulching also helps retain moisture in the soil too.

Ensure your roses have the best environment for healthy growth; this means planting them in full sun, in well-drained and fertile soil.

In late autumn or spring, prune out all infected stems before the growing season starts and leaves appear.

How to continue to treat roses for blackspot

Healthy rose with very little diseased leaves
Healthy rose with very few diseased leaves

Now as soon as the new growth emerges in spring, spray with a fungicide before signs of black spot. Tebuconazole (Provanto Fungus Fighter Concentrate) and triticonazole (Fungus Clear Ultra) are good choices. Continue to spray your roses every few weeks or as recommended on the fungicide label.

Be sure to remove any leaves that show signs of black spot straight away to help further prevent the spread.

Spraying roses with a fungicide to help prevent black spot, mildew and rust
Spraying roses with a fungicide to help prevent black spot, mildew and rust

The trick is to follow this process all summer and keep removing affected leaves and spraying regularly. Make sure you don’t spray roses when bees are active.

I also like to use a spray that fights blackspot and other diseases but also kills pests such as aphids at the same time because aphids are sure to make an appearance on your roses at some point during the season. Rose sprays such as RoseClear are also systemic which means the pesticide is taken into the plants so will work on aphids as soon as they attack your roses.

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