This is a well known disease in fruit trees. I am sharing with you my limited experience with gummosis on one of our Satsuma trees and how I am treating it.
In simple terms, foot rot gummosis in citrus occurs when zoospores splash into wound or bark crack around the trunk base. Zoospores travel thru rain and/or irrigation, contact the roots, germinate and cause root rot. They can affect other parts of tree but this discussion is limited to disease on citrus trunk bases.
Upon initial inspection I thought deer had damaged the trunk (and they may very well have wounded the tree) but excessive rains are probably more of a factor, encouraging zoospore travel. Once I noticed orangey sap oozing from the base I knew this needed further attention. It is a contagious disease so prompt attention is important.
Here are two pics of what the base of my Satsuma looked like:
