Sooty mould

Symptoms

On leaves, shoots and fruit a black, soot-like coating forms that can be partly wiped off with a finger. These fungi do not infect the plant tissue – they grow on the sweet, sticky honeydew excreted by aphids, whiteflies, mealybugs, psyllids and scale insects. The coating therefore appears exactly where these pests feed, and its main harm is that it masks the leaf blade, reduces photosynthesis and disfigures the plants. The symptom is often seen on citrus, lime, maple and on houseplants and greenhouse plants. The coating may be thin and cobweb-like or thick, crusty and dull black — depending on the fungus species and the amount of honeydew; on evergreen plants it can persist through the whole winter. A key identifying feature: after the blade is wiped, healthy green tissue emerges from beneath the black layer — sooty mould leaves no spots, necrosis or pitting, because it does not penetrate the leaf. The extent of the symptom is also characteristic: the soot settles where the sticky honeydew drips, so it appears not only on leaves but also on objects beneath the plant — the pavement, garden furniture, a windowsill or car bodywork — which in itself reveals pests feeding higher up. The sooty mould fungi themselves are dark-pigmented (melanised) saprotrophs, which is why the layer is deeply black. Direct damage to the plant is usually moderate — weakening through leaf shading, loss of vigour, poorer appearance and reduced market value of fruit or ornamental plants — but a dense, long-lasting coating on many leaves genuinely limits assimilation and growth. Sooty mould is best treated as a visible signal that a colony of sap-sucking insects is developing higher up on the plant (or on a tree above it).

Treatment

It is not the coating itself that is controlled but its cause – the sap-sucking insects that produce the honeydew. Once the pest population is reduced (sprays, washing off, beneficial predators), no new coating forms and the old one gradually comes off with rain or when wiped. Leaves of houseplants can be gently cleaned with mild soapy water to restore light access. The first step is to establish which pest is producing the honeydew — the undersides of leaves, shoot tips and the base of leaf stalks should be checked for aphids, whiteflies, scale insects, mealybugs and psyllids, as well as ants, which 'farm' and spread them. The method is matched to the specific culprit (see the methods below): from introducing predators and washing the plants, through oil- and potassium-soap-based products, to a systemic insecticide in a heavy infestation. An important practical note: spraying with a fungicide does not solve the problem — sooty mould is only a surface layer and regrows on each new portion of honeydew as long as the pests are feeding, so a fungicidal treatment is pointless here. Once the insects have been effectively reduced, the old coating weathers over time and is washed away by rain and cleaning — patience is needed, because fully clearing the leaves can take several weeks. Houseplants and young trees can be sped up by wiping the leaves with a soft cloth dampened with water and a mild (potassium) soap; fruit intended for sale simply needs washing.

Prevention

The basis is controlling the honeydew-producing pests – regular inspection of plants, curbing ants (which protect aphids), supporting beneficial predators and avoiding nitrogen over-fertilisation that favours aphids. Good air circulation and not planting under trees that drip heavily with honeydew reduce the risk. Regular monitoring is cheaper and more effective than washing off later — the earlier and smaller the colony of aphids or scale insects is detected, the less honeydew and the weaker the coating. Keep nitrogen fertilisation in check: an excess pushes out soft, sappy growth that is a magnet for aphids. Curb ants — sticky bands on trunks, barriers and removal of nearby nests break the mutualism in which ants protect aphids and scale insects from predators; without that 'protection' beneficial insects reduce the pests more quickly. Support natural enemies (ladybirds, lacewings, hoverflies, predatory bugs, parasitic wasps) by planting nectar-rich plants and avoiding routine sprays with broad-spectrum products that wipe out beneficial insects along with the pests. Maintain an airy position and adequate spacing, and do not place susceptible plants, garden furniture or parking spots under trees known for abundant honeydew (lime, maple, some fruit trees colonised by aphids). New plants — especially potted ones — should be inspected for scale insects, mealybugs and whiteflies before joining the collection, as these easily 'arrive' with a purchase and start a new focus of honeydew.

How to tell it apart from look-alikes

Feature	Sooty mould	Powdery mildew
Coating colour	Black or dark brown, soot-like	White to pale grey, powdery
Link to pests	Develops on the sweet honeydew of insects — almost always accompanied by aphids, whiteflies, scale insects or ants	Independent of pests; favoured by warm, humid weather and dense plantings
Does it attack the tissue	No — the fungus lives only on the surface; after wiping, the tissue is healthy and green	Yes — the hyphae penetrate the epidermis (haustoria); the tissue gradually yellows, deforms and dies
Location	Along the path where honeydew drips, also on shoots, fruit and objects beneath the plant	On the plant itself — young growth, upper leaf surfaces, buds
Response to fungicide	A fungicide does not solve the problem as long as pests are feeding	Clears after fungicides (sulphur products, triazoles)

Control — organic and chemical methods

Organic and biological methods

Method	How it works	When to use
Predators and parasites of honeydew-producing pests	They reduce aphids, whiteflies and scale insects, so the source of honeydew disappears and the coating stops growing.	Growing season, at the first signs of a pest colony
Washing plants with potassium-soap water	Physically washes off the soot and some soft-bodied pests and restores light to the leaves; without reducing the pests the coating returns.	Houseplants, young trees, at a low population
Curbing ants (sticky bands, barriers)	Cutting off the ants that protect and spread aphids and scale insects lets predators effectively reduce the pest population.	From spring, when ants start being active on trunks

Chemical methods

Method	How it works	When to use
Oil-based contact product	Coats and smothers the pests and their eggs by blocking gas exchange, and additionally loosens the existing coating.	Outside heat, full sun and flowering — risk of leaf scorch
Systemic insecticide against sap-sucking insects	Taken up by the plant, it strikes the insects feeding in the tissue, removing the source of honeydew at the root.	In a heavy, persistent infestation, mainly on ornamental plants; do not use on flowering plants visited by pollinators

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.