My Central Florida tangerine tree is about 9 years old.

Two years ago it had a magnificent crop, then last year they were much smaller with about half of them bitter and half OK but not super and they were mostly smaller.

Am now getting a lot of branches gradually dying and a small set of fruit.

I can see no evidence of insects.

I have tried fertilizing, extra watering, and oil spray with no luck.

Any other suggestions?

Thanks,