Answer: A few possibilities come to mind. Many older citrus trees are the 'sour orange' variety meaning they aren't going to taste good. They were often planted for ornamental reasons rather than for fruit. The previous owner was probably referring to the fact that many sweet-tasting orange varieties were grafted onto a 'sour orange' rootstock, which had positive qualities such as resistance to disease. Sometimes, the sweet variety can die back, and the rootstock takes over. See if you can find the original graft area. It should be a bump or raised area at the base of the tree. If shoots are growing only from below that graft, the rootstock has taken over. If they are growing above the graft, the variety is still viable.
You are correct that watering and fertilizing are important for citrus. First, the longer citrus stays on the tree before harvest, the sweeter it becomes. Citrus takes large amounts of water on a regular basis. It should be watered to a depth of about 3 feet, slowly and deeply, extending out past the tree's canopy. Water about once every 3-4 weeks in winter; about once every 2 weeks in summer, for mature citrus. Citrus requires fertilizing 3 times per year with nitrogen. Feed 1/3 of the tree's annual requirements with each feeding. Feed in February, May, and July. Your title mentioned brown patches, but you didn't include any further info, so I'm not sure what you meant. Feel free to send another email to the Q&A site (not my email address).
Here is a website that contains info on citrus for the low desert, including the best time to harvest specific varieties and irrigation: http://ag.arizona.edu/maricopa/garden/html/pubs/pubs.htm#Cit...
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