If you have noticed a black powdery coating on the leaves of a camellia, a bay tree, a viburnum or any other glossy-leaved evergreen in your garden, what you are looking at is sooty mould. The name describes it well — the leaves look as though they have been dusted with soot from a chimney, dark and grimy, sometimes in patches, sometimes covering the entire upper surface of the leaf. It is one of the more alarming things to find on a plant you value, and one of the most misunderstood. The mould itself is not attacking your plant. It is a symptom of something else entirely, and until you deal with that something else the mould will keep coming back regardless of how often you clean the leaves.

The cause is always the same: sap-sucking insects. Sooty mould does not grow on healthy dry leaf surfaces — it grows on honeydew, a sticky, sugar-rich secretion produced by insects such as scale insects, aphids, whiteflies and mealybugs as they feed on the plant. The honeydew coats the leaves and stems below the feeding site, and airborne fungal spores land on it, germinate, and spread across the surface as the familiar black deposit. In an Irish garden the most common culprit on camellias, bay, viburnum, trachelospermum and holly is scale insect — small, oval, waxy-shelled insects that sit motionless on stems and the undersides of leaves, easy to overlook because they look more like a natural blemish than a living pest. Their eggs overwinter in the plant and hatch in spring, beginning the cycle again. This is why sooty mould tends to be most visible in late winter and early spring — the evidence of the previous season's insect activity is still on the leaves even as the insects themselves are less obvious.

The plants most commonly affected in Irish gardens follow a pattern — they are nearly all evergreens with glossy, leathery leaves, and they tend to be growing in sheltered positions. Camellias grown against a wall or fence, bay trees in containers near the house, viburnums in a sheltered corner, trachelospermum on a south-facing wall — these are exactly the conditions that favour scale insect. The shelter that makes these positions attractive for the plants also creates the warm, still microclimate that insects prefer. It is not a coincidence that the same plants in more open, windier positions tend to have less of a problem.

The first thing to do when you find sooty mould is to look for the insects. Turn the affected leaves over and examine the undersides and the stems carefully. Scale insects are small — two to five millimetres — and oval, brownish or pale in colour, sitting flush against the stem or midrib. They do not move when disturbed. Aphids are easier to spot, clustered on new growth and softer stems, usually green or blackish. Once you have identified what you are dealing with, the treatment becomes straightforward. For scale insects, a thorough drench with a solution of copper sulphate and water will help address the fungal element, and washing the plant down with warm soapy water — a soft cloth or sponge rather than a jet that damages the leaves — will remove the honeydew and the mould from the surface. Repeat the treatment as the season progresses rather than doing it once and hoping for the best.

For the longer term, the health of the plant matters considerably. A plant that is under stress — whether from poor soil, nutrient deficiency, drought, or being kept too long in a pot that no longer suits it — is more vulnerable to insect attack than a plant that is growing vigorously. This is particularly true of camellias and bay trees, both of which are commonly grown in containers and both of which are often left in inadequate compost for longer than they should be. Improving the growing medium, feeding the soil with a product that builds structure and holds nutrients rather than simply delivering a short-term feed, and ensuring the plant is in the right position for its needs — these are the things that reduce susceptibility over time rather than just treating the problem when it appears.

One practical point worth making is that sooty mould can also appear on plants that are sitting underneath or close to an affected plant. The honeydew drips downwards, and a camellia with scale insects overhead can deposit enough sticky residue on the plants below it to trigger mould on those leaves too, even if those plants have no insects on them at all. If you find mould on a plant that does not seem to be infested, look upwards and outwards — the source is likely a neighbouring shrub or tree. This is especially worth checking with sycamore or lime trees overhanging a garden, which can produce significant quantities of aphid honeydew in summer that falls onto everything below.

The mould itself, once the honeydew source is removed or reduced, will gradually diminish. Rain will wash some of it away over time, and the plant's new growth will emerge clean. On badly affected plants with a thick coating you may choose to wipe the leaves individually with warm soapy water — a slow job on a large shrub, but worthwhile on a plant you value. What you should not do is ignore the insect problem and simply clean the leaves repeatedly. That is a cycle with no end. Find the insects, reduce their population, improve the plant's underlying health and growing conditions, and the mould will take care of itself.