Q Why are my camellia leaves covered with black marks?
A The most common cause is sooty mould, a fungus with dark spores which lives on the sugary substance excreted by sap-sucking insects, usually on the leaf undersides, such as cushion scale. Control the insects and wash affected leaves. Look out for colonies of aphids and scale insects on shoots and beneath leaves.
Mealy bugs can also be a problem on plants grown in conservatories. Pick off the worst leaves, and remove insects from the rest by squashing them, or dislodging them with a jet of water.
Leaf-spot diseases are another cause of black marks on camellia leaves. These are far more common under glass, but are unlikely to be serious. Unlike sooty moulds, these marks won’t rub or wash off. Prune out the worst affected parts.