I have one citrus in my tiny yard: a standard with 7 varieties (Valencia, Lane's late navel, Moro blood orange, Chandler pommello, honey mandarin, Bearr's lime and Meyer lemon) on it. It has been in the ground for 8 years, and the trunk is about 3 inches in diameter. It has a rather schizophrenic growth pattern, in that the lime, pommello, Moro blood orange branches grow vigorously, the lemon is more or less holding its own, and the Valencia, navel, and mandarin branches look really poor. They have very sparse foliage, little new growth, and some twigs are dead. These sick-looking branches are bearing moderate amounts of fruit that is developing normally. The sick branches have only begun to deteriorate in the last 2 years, but the lime, pommello, and blood orange branches look great and are heavy with fruit. What's the matter with these underperforming branches/varieties? Can I make them healthy again? How?