Doesn't appear to be a Mg problem. Looks more like verticillium wilt
but that is a " looks like", not any confirmation.
Again don't go dumping a bunch of Epsom salts until you can accurately
diagnose the problem Hopefully its not silver leaf as mentioned.
You can prune a small branch to see If a brown core is running through
it... if so, you got silver leaf and a whole set of new problems. If
not, you are in the clear.

Don't know about false silver leaf, that is an environmental problem
and could be an issue. Yet again, I recommend you talk to your local
Master Gardener or go directly to WSU's diagnosis lab to get an
assessment on your own:
http://www.puyallup.wsu.edu/plantcli...mples/ppd.html.

We have seen an increase in mildews, fungi up here the last few years
with the present Pacific Decadal Oscillation, So yes we are heading
into winter and the leaves will fall but many of these problems do not
go away and will reappear next year, so watch for early signs and
accurately treat .

good luck