Ask-a-Dave's-Gardener: Help with Navel orange tree, 1 by Philibuster

Philibuster wrote:
Hi all. I have an old navel orange tree in my backyard that has probably been there for 40-50 years. When we bought the house the tree had been neglected and was only producing about a dozen oranges. Over the years we were able to get it to thrive again with regular watering and removal of dead branches. This year our harvest was much smaller than usual and in the springtime the new leaf growth we are accustomed to was almost non existent. It has been very hot the last few weeks here in the San Fernando valley of Los Angeles but while regular watering has worked with my lime tree and other potted fruit/vegetables, my orange tree isn’t looking great. Today I noticed a hard flaking/scaling almost fungus looking substance on one of the main trunks and wonder if that could be some kind of disease or if there’s anything else to do. I fertilized with organic fertilizer around Memorial Day. The dirt in my yard is generally not quick draining.