There is a more or less common fungus disease caused by the the fungi "Phytophthora". Phytophthora cause the most serious soilborn diseases of citrus. The common name for the disease is Foot Root. You can evaluate the symptoms listed below and see if they match what is going on with your tree. The infection occurs near the ground level. At first the cambium and inner bark are damaged, but the outer bark remains firm with small cracks through which abundant gum exudes. Lesions spread around the circumference of the trunk, slowly girdling the tree, and the bark is eaten away. Severly affected trees have pale, green leaves with yellow veins. Large trees also may be killed, but typically the trunks are only partially girdled and the tree canopy eshibits leaf defoliation, twig dieback, and the tree attempts to make a weak growth flush.(ref. Comp. Of Citrus Diseases). Here is a web site with more information and two pictures for you evaluation.
http://cals.arizona.edu/PLP/plpext/diseases/fruits/citrus/citrusgum.html - Millet