This is a fungal root disease which usually affects the trunk of citrus just above soil level.
A section of bark around the base of the tree dies back and falls away from the main trunk. Sometimes oozing gum and cracking around the dead section occurs. Leaves turn yellow. Tree loss can occur if left unchecked.
Cut the bark back to healthy growth and spray foliage with Yates Anti Rot. A spray of Liquid copper can also be applied to the affected bark lesion. Do not allow soil or mulch to pile up above the grafting union. Susceptible trees (especially lemons) should be given a preventative foliar spray with Anti-Rot twice a year. For cultural control, improve air flow by removing any mulch touching the bark of the tree. Improve drainage around the tree if water logging occurs frequently or if the ground is damp often.