What is Cercospora Leaf Spot?

Cercospora Leaf Spot is a fungal disease that’s not fatal. However it does spread quickly and seems to be more of a problem in shady positions. I have personally noticed the disease quite frequently on hydrangeas including the smooth, panicle, oakleaf and bigleaf types of hydrangea.

How to recognise Cercospora Leaf Spot?

This disease first appears on the underside of older leaves as irregular brown spots that might have halos around them. You may also notice small purple spots on the older leaves that evenly grow and have a grey centre with purple edges. Check particularly the leaves at the bottom of the plant, as well as the flowers and the stems.

The leaves eventually turn yellow, wilt and fall off the plant. Without the energy produced by the leaves, the plant may lose all its foliage in very severe cases but it is never really fatal.

What to do about it?

Prevention

Use a general fungicide on your plants in the spring to help prevent any diseases, including leaf spot which usually appears around July, through to October.

Make sure your plant is in the best growing environment for it. That’s usually a sunny but sheltered location with well-drained soil. The issue is that Hydrangeas are also commonly grown in partial shade which is also a perfect condition for Cercospora Leaf Spot as it likes moist conditions.

And water the base of the plant rather than from above where the water can remain on the leaves.

Treatment

Remove any infected leaves when you notice them. You may need to prune the plant back quite aggressively if it’s heavily infected, but this can also effect flowering the following year. Thoroughly wash your hands and any tools that come into contact with the infected parts of the plant.

Start using an organic fungicide of copper sulphate or potassium carbonate as soon as you notice the problem.

If organic fungicides don’t do the job, move on to chemical ones. A general fungicide labelled for black spot seems to have helped my own plants in the past. But use these carefully as they don’t kill selectively; they destroy everything they come into contact with. It’s best to use these fungicides when your plant’s blooms have faded.