Ed Stover and crew at USDA, Fort Pierce in Florida have been studying the response of different scions and rootstocks that have been inoculated with the HLB bacteria. As reported in the Proceedings of the International Citrus Conference (http://www.icc2016.com/images/icc2016/downloads/Abstract_Book_ICC_2016.pdf), they have found some scion/rootstock combinations more tolerant or less susceptible to the disease six years after they have been inoculated.  It turns out scions with a citron pedigree have more resistance.  Also those with a Poncirus (trifoliate) background also had lower populations of the bacterium. 

What this means is that there is resistance and possible immunity out there and there is breeding/genetic engineering material out there that can be used to improve the whole of the citrus family to HLB. Of course, this will take time, but there is hope.

Photos:

HLB symptoms and trifoliate leaves