Lilly Pily high and mighty problem

Hi, I just wanted to consult with you as my High and Mighty 3.5 year old hedge or want to be hedge is now not looking too great. At the beginning it grew fast and healthy, quickly rose up to 3 metres in height. But then we suddenly started to get black dots under the leaves. I thought it would be a seasonal thing and it would go away. Now after this summer the black dots have taken over and are mostly covering the entire lot of leaves on the trees. On closer inspection they actually look like some kind of pest infestation as a multitude of some kind of tiny white fly is seen flying amongst the trees. Not sure if this is related or just because the front of the house is a busy state road but the leaves now look like they are covered with soot. Nothing will get rid of it unless I clean up every leaf one by one. Insects have now attracted spiders to live in the trees. The pretty trees are now looking quite gloomy and are loosing a lot of leaves. Would you please recommend the method to repair the damage? It feels like all the old leaves will all have to go, do I need to give the trees a massive trim? Use chemicals? Am I doing something wrong?

Are they leaving a sticky residue on the leaves? If so then it's probably scale. You'll need to spray them thoroughly with pesticide oil and also treat the surrounding area for ants.

Thank you for your reply. The dots only grow on the underside of the leaf. I just got a magnifying glass and they are either hatched eggs or insect droppings. Caught a few of the white flies lurking over the black stuff. Looks like a fungus is growing on the black stuff. Will the oil fix this? Then should I trim the tree and wait for new growth?

Lilly Pilly are prone to getting psyllid infestations... if that's what it is, you need to trim back the infested growth and regularly spray the trees with something like white oil/confidor. Hopefully not though, as it's a constant problem. You can get psyllid resistant varieties, but my last lot weren't and it was a constant battle

Common Problem of Lilly Pilly is sooty mold. The presence of sap feeding insects like aphids, scale, and mealybugs on the plants causes this. To treat this issue, apply white/pest oil to the vine, making sure to get the undersides of the leaves.