Carl, 
I've seen the rust colored bark on isolated Hemlock trees for decades, well before HWA showed up. I figured it was some environmental or soil born problem, no obvious signs.  As I read this article, I recalled that the browning of the Hemlock was on drier sites. I think this article explains the conditions creating environmental stress. Environmental stress increases susceptable to a variety of environmental conditions. pathogens, and inset infestations.  I also read a little in Diseases of Trees and Shrubs By Wayne Sinclair and Howard Lyon, an esential text for anyone growing woody plants. I noticed that  Phytopthora can cause browning in some species. 

https://www.torontomastergardeners.ca/askagardener/browning-hemlock...