A few weeks ago I was called in to look at an apple tree that had "fireblight"... it turned out it only had powdery mildew and the homeowner was relieved but I wondered how an arborist, a neighbor and a homeowner could all be confused about this very distinctive disease so when I saw this Mountain Ash with actual fireblight I had to photograph it.
The symptoms are very distinctive and you never forget them. The tree looks like it has been burned and sometimes only one or two branches are affected. In this case most of the tree in the park was affected. For the safety of the surrounding trees it should be removed. Check other references for more detail about this bacteria and it's control.