What's with the drip line above the tree? Nothing should be spraying the trunk of a citrus tree? There is moss growing on the tree trunk too, that's not good, sure evidence that it is too moist for that tree. It looks like a large tree so if you move that sprinkler to the drip line area around where the trees canopy ends it should help. Also post some pictures of the whole tree and it's leaves so we can help diagnose any other possible issues going on.
Evan