Gardening: Remedies for Valencia oranges going sour, mango trees under attack

Question: I have a Valencia seedless orange tree that I set out about 40 years ago. The fruit has always been sweet and juicy. But the past couple of years, the oranges are larger with thicker skin, not much meat and less juice. Can you tell me what I am doing wrong? — Charles Johnson, North Hills
Answer: In all likelihood, you are seeing sour fruit from a branch or branches that have grown off the bottom of the trunk.
Nearly all fruit trees, at least those available in