Huanglongbing (HLB) is a bacterial disease that is spread by an insect, the Asian citrus psyllid. This two-page fact sheet, which is best viewed as a PDF, http://edis.ifas.ufl.edu/pdffiles/PP/PP32800.pdf, explains how to tell the difference between HLB symptoms and symptoms from nutrient deficiencies. Written by T. Vashisth, M.M. Dewdney, and J.D. Burrow and published by the Plant Pathology Department.
http://edis.ifas.ufl.edu/pp328