HLB is a bacterial disease that is spread by the leaf sucking pest, ACP.
The Disease causes citrus fruit to become bitter and disfigured, it will also cause the leaves to yellow.
Unfortunately, once a tree is infected with HLB, the disease is irreversible and will eventually kill the tree.
Your only line of defense is preventative treatment!
If you suspect you have an infected tree please contact the CA Dept of Food and Agriculture at (800) 491-1899 for a free inspection.
Get all the information you need on this threat at the Citrus Pest & Disease Prevention Program website, CaliforniaCitrusThreat.org hosted by the California Citrus Growers and the CA Dept. of Food & Agriculture.