Sooty mold

Cladosporium, Capnodium and Alternaria spp.

also called Black mold, Sooty blotch

A black film growing on the honeydew left by aphids, scale, or whitefly above. Deal with the sap-sucking pest and sponge the leaves clean.

Fungus Disfiguring Airborne sporesSpread by contact

What you'll see

A dry, sooty black film on the upper leaf surface that can be wiped or peeled off, leaving green leaf beneath.
The leaves feel sticky and varnished before the black film develops.
The worst deposits are on leaves, paving and furniture directly beneath something else — look up for the real problem.
Aphids, scale insects, whitefly or mealybugs are present on the growth above.
The plant looks tired and dull rather than spotted or lesioned; there is no rot or dieback.

Why it happens

Several harmless fungi that live on honeydew, the sugary excretion of sap-sucking insects. They do not infect the plant at all — they simply grow on the sticky coating, and the damage they do is by shading the leaf and blocking light. The disease you are actually dealing with is the aphid, scale, whitefly or mealybug above.

Any warm, still spell with a heavy sap-sucker population, particularly under glass, on evergreens beneath a lime or sycamore, and on citrus and bay in conservatories.

Treating it

Find and deal with the sap-sucking pest above — squash colonies, wash them off with a jet of water, or encourage ladybirds and lacewings.
Sponge the leaves with lukewarm water and a little mild soap solution, supporting each leaf with your hand, then rinse.
On evergreens outdoors, a good rain or a hose down clears most of it once the pest is gone.
Prune out badly coated congested growth to let light back into the plant.
Do not spray a fungicide; it treats the wrong organism and does nothing about the honeydew.

Keeping it away

Inspect susceptible plants regularly at the shoot tips and leaf undersides where the pests start.
Avoid siting containers under limes, sycamores and other trees that drip honeydew all summer.
Encourage predators by leaving some aphids and planting nectar for hoverflies and lacewings.
Under glass, use biological controls against whitefly, scale and mealybug early in the season.
Keep plants well ventilated and avoid soft, sappy growth from heavy nitrogen feeding.

Who gets it

Citrus, bay, camellia, holly, roses, apples, limes and anything growing beneath an infested tree.