Hi, I have been spraying both my mandarin and orange trees with Conquer Oil for the past two years and successfully got rid of the plagues of white fly. However my orange in particular has a coating of a black sooty substance on the leaves and on the fruit. The mandarin only has this on the leaves. It is easily washed off the oranges when picked but I would much prefer to treat the trees to get rid of it. Regards Phil

Hi Phil, the black substance is sooty mould which is an air-borne fungus that feeds on the honey dew which is excreted by sucking insects such as aphids, mealy bug or scale insect. It is likely that your trees have scale insect, if you look closely at the stems, branches and leaves there will be small bumps, they could be a creamy colour with a star shape or light to dark brown. There may also be ants crawling amongst the trees, which are also feeding on the honey dew. If you eliminate the insects you will get rid of the sooty mould. Residual sooty mould can be removed by spraying with (conqueror) oil, milk will also do the trick, the mould eventually washes off with water or rain. To eliminate sooty mould you will need to treat the insect pest. Spraying oil should be enough to control scale insect, make sure you get it inside the canopy. Talk to your local garden centre or hardware store for a suitable control. The Sooty mould looks unsightly and won't harm the tree but reduces growth and plant vigour, as do the insect pests that are causing the problem. Lianne.