Q.Can you tell me what this is and how I can fix it please
Unsure what this is.
It looks like it might be some kind of infection, bacterial or even fungal - pretty hard to tell. You might be farther ahead to get an in-person diagnosis from a tree professional, someone who knows not only the problems of trees in your area, but the treatments that are available to you. You could call a certified arborist, or take some leaf samples to a nursery near you. You might call the nearest university that has a horticulture or botany dept. Or you could call the botanic garden in the nearest city for advice, either from them directly, or on connecting with the local Master Gardener Association.