tips on how to treat the tree?

spray 2x year around April/August when infestations are at peak with
your product of choice.

If you are unfamiliar with some of the control products here is a
quick guide

http://www.ct.gov/caes/cwp/view.asp?a=2823&q=377508

Dig into info listed below for various products and further reference
leads. Viable Control products/methods are sometimes buried deep in
the documents:

http://edis.ifas.ufl.edu/in668

http://www.aphis.usda.gov/plant_heal...-tx-ea2009.pdf
or
http://tinyurl.com/25d428v

http://www.aphis.usda.gov/plant_heal...greening.shtml
or
http://tinyurl.com/2dp3a39

http://www.aphis.usda.gov/plant_heal...rol2.09.09.pdf
or
http://tinyurl.com/28kcucd

David is correct Psylla & Psyllid but also try googling Citrus
Greening Disease.