We have some olive trees in pots 3 of which have these deposits of a white material in clumps. We have a launderette nearby and at first I thought it was soap powder that had been borne on the wind - that's the kind of appearance it has - but one of the trees which is at a fair distance from it is still showing these deposits. One of our trees is free of it.
Any ideas on what it is and how to treat it would be appreciated. Many thanks.
It sounds very much like Woolly aphid, Catherine. These are aphids which exude a white waxy substance to protect themselves. You can gently scrub them off using some soapy water and a small brush or rag. Some folk also use a cotton bud dipped in methylated spirit to kill and remove them. Woolly aphid itself doesn't cause much harm, but left untreated other diseases can develop around the damaged areas.
Thank you so much for the advice. My apologies for not replying sooner.