I've got three established avocado trees, two are over 10 years old. This year they have suffered a big change in foliage with some rusted and what appear to be eaten or damaged leaves. It is dropping a lot of leaves too. They seemed to take a turn after winter, but they usually bounce back in late spring with the new growth. I can't seem to find any obvious small insects on the leaves but I'm basically treating it as either root rot, or maybe mites?

Any advice on treating this, or are they gone? All the new leaf growth is being affected and is curling up, blackening around the edges. There has not been any significant frosts in a couple months. I'm based in Lower Hutt, so not ideal for avocado trees with the wind and winter frosts. But the trees have definitely been fairly healthy until this year.