That's a great question, and I really don't know the answer. Certainly the exposure to foot traffic and human vandalism is a major factor in opening weaknesses for fungus to invade.

Common sense would seem to dictate that pollution and/or the artificial environments for street trees probably aren't a net positive - but that would just be a guess.

As to whether the lack of biodiversity hurts things, again I can't say anything concrete, but perhaps it makes it more likely for spores to land on target rather than on nearby, more inhospitable life. Again though, that's pure guesswork.

Whatever is urging it on, there's no doubt the problem is real. And of course the information is applicable for hardwoods everywhere - its just particularly visible in the city