What is causing the black deposits on my variegated holly tree?

The deposit is of sooty moulds and these are living on sugary honeydew being produced by scale insects feeding under the leaves.

If the tree is not too tall, spray with a suitable insecticide to kill scale insects, and that should stop the honeydew production.

The moulds will run out of their food supplies and after a month or two it should be possible to dislodge them with a forceful jet of water.


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