I had never heard of the muratic acid before so I started some research. Below is a link to a
Horticulturist at NDSU. Having planted some 25 colorado spruce, 25 junipers, 100 black hills spruce, 100 arborvitae, 25 lilac, 10 little leaf lindens, and another 100+ others in the last 2 years, I am looking for anything to get them to grow faster. I am also leery of doing anything to make me re-plant them now. He doesn't say it works, but he doesn't say it does either.
Q: Some of my evergreen’s leaves have turned brown and are falling off. My neighbor suggested I use muriatic acid because the plants are being infected by insects. Would muriatic acid work? (E-mail reference)
A: I don't know where people ever got the idea that hydrochloric acid is good for evergreens. I have never seen any research to support this claim. It is a little early for evergreens to be infested by insects. Even if they are, muriatic acid is not an insecticide. I don't mean to sound like I am picking on you - I'm not, but I am writing this for you as well as others who may be thinking of asking the same question. There are lots of maladies that can mimic the symptoms you describe. I’m guessing from your e-mail address that you are in Ontario, Canada. I would suggest that you contact the horticulture department at the university in Guleph. Send them a sample or have someone in extension come out and check the tree to accurately determine what the cause could be. The problem could be winter desiccation, drought response from last summer, needle cast, canker or (hopefully not!) more than one of these maladies.
http://www.ext.nodak.edu/extnews/hortiscope/tree/evrgreen.htm