Thanks all for the replies.
I was going to get rid of my Black Genoa, but after the reading the above comments, I think I might give it a second chance and see how it goes with a repot + some trace elements / fertiliser.
If it doesn’t pick up by next summer, I’ll try the whitewash as Jdsfrances suggested - but not sure if its insect damage as I haven’t see anything on the leaves / bark of the tree??
Hydrated lime (also known as builders or slaked lime) is quite safe to use on fruit trees. I have use it a couple of times on my apples to treat apple scab and it works like a treat! But I never used it for insect control. Jdsfrance, how much lime to water do you normally use?