What can I do for blackened holly leaves?

Q

The leaves of my variegated holly are covered in a black oily substance that has made the leaves brittle. What is it and how can I cure it?

Miss H David, South Ruislip, Greater London

AThe black substance on the leaves is sooty mould, which in itself doesn’t do any real harm except that by making the leaves dirty it reduces their ability to photosynthesise and breathe.

It is most easily removed by wiping down the leaves with a damp cloth. Alternatively, you could make up a mild mix of washing-up liquid (a few drops in a bucket of water) and then put this in a garden sprayer. Spray over the plant (the soap helps to loosen the sooty mould) and then wash down with a hose.

The most important thing to tackle is the cause of the sooty mould. It always colonises deposits of honeydew, the sticky secretion of many sap-sucking insects such as aphids and scale insects.

Is your holly planted near or under any larger shrubs or climbers that could have suffered with aphids or scale insects last summer or autumn? Alternatively, if only the lower leaves are affected, it could be that your holly already had the pest problem.

Treat any pests with a pesticide such as Neudorff Bug Free Bug and Larvae Killer, Vitax Plant Guard Pest & Disease Control or Bug Clear for Fruit and Veg. Also encourage predators, as ladybird and lacewing larvae and some birds will eat greenfly, aphids and scale insects.

If you can find the source of the honeydew and deal with that, eventually your holly will grow through the sooty mould and start to look clean again. flowers through most of the summer.

If you opt to grow it as a perennial, it needs to be lifted in October before the really cold weather arrives. Cut stems back to within 6in (15cm) of the crown and overwinter plants in a frost-free greenhouse.

They can be returned to the garden in late May or early June the following year, once the nights have started to warm up.