I am going to stick my neck out on this one. It is based
purely on a few observations and a few situations of elm stands. One
case in particular is a american elm
dutch elm free and where there are no maples in the vicinity.

And where there are alot of maples, especially silver maple, there is
a die-off of elms from dutch elm disease.

Now I looking in the literature for any reports of a connection
between maples and elms as vector of
dutch elm disease of either the beetles or the fungus,
and I found none.

So let me make this Hypothesis or Speculation that Maples provide a
host affect or a vector affect for Dutch Elm Disease and when maples
are absent from a region with elms that the elms will not have Dutch
Elm Disease. However, some of the elms will be attacked by the disease
but not killed for the beetle or fungus have their life cycles
interrupted due to the absence of maples.

Maybe it is the maple sap that is required by the beetles or the
fungus, or it could be something else.

I have noticed that where dense elms, and live elms grow there are few
if no maples around. And if a few catch Dutch Elm where no maples are
around, they can recover and grow back to normal.

I have noticed that where maples are well established
nearby to elms that all the elms disappear due to dutch elm disease.

So, if anyone cares to prove or disprove the above Conjecture? Feel
free to jump into it.

And perhaps one clue is offered by the fact that Siberian Elm is very
tolerant to dutch elm diseases DED and maples are native to Asia where
Siberian elm is native. So that the nativity of both over geological
time formed a elm resistant to DED but on other continents of elms, no
resistance was developed. And in North America and Europe, the elms
are dying in large percent leaving behind, over time, resistant elms,
much like what played out already in Asia with the Siberian Elm
resistance.

Archimedes Plutonium
http://www.iw.net/~a_plutonium/
whole entire Universe is just one big atom
where dots of the electron-dot-cloud are galaxies