Hi, Can anyone please help.... the cause and treatment of the black on the leaves? Thanks for any advice!! Andrew
Hi Andy
The black deposit on you leaves is sooty mold. This is an indication of the presence of sap sucking insects that are secreting honeydew. The honeydew forms a film over leaf surface and the sooty mold grows on that.
You need to remove sooty layer as that blocking the sun on the leaves and compromising the plants ability to photosynthesise. Use a hose to blast the mold off the leaves and then you need to look for the insects that are produce the honeydew and treat them.
Most likely they are aphids, scale or psyllids all sap sucking insects. Aphids can be controlled with Pest Oil, a contact insecticide like Pyrethrum or a systemic insecticide like Bug Killa.
For scale and/or aphids the easiest way is to use an systemic insecticide, the insects will take up the chemicals with the sap. You can treat scale with pest oil but it is all about timing as the pest oil will not work whilst the insects are protected by the scale. You need to spray them at the time the nymphs leave their scale home and search for a safe place further along the branch to build their own home. This is the time to spray the branches with pest oil and you need to do this every 2-3 weeks from December to mid- January.