Massachusetts,
United States 
why is my established forsythia wilthing
12 Aug, 2010
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Unless there has been a severe drought, I would suspect root damage. Things to look for:
Digging nearby, as for pipelines or wall foundations.
Signs of gophers, moles, or voles nearby.
Heavy infestation of grubs--usually found by digging a small test hole at the drip line.
A change in soil grade around the base--possibly more than a year ago.
12 Aug, 2010