Stinky Tree - Dutch Elm Disease?
My neighbors have a few very large elm trees in their yard. There is an awful smell of natural gas / rotting animal in my backyard, that has never gone away in 20 years, and is even present in the winter. Is it possible that their elm trees are causing this odor, diseased or not?
As far as I know, Dutch elm disease does not cause a foul smell. Furthermore, Dutch elm disease kills a tree in one or two seasons. You said the odor has been around for 20 years.
One disorder that can cause a stink is slime flux, which won't harm the tree:
Elms are also somewhat known by hardwood workers to be "smelly".
I don't think either of these things smells like natural gas, so please consider that you might have a natural gas leak somewhere and call your utility company.