As i noted here elsewhere before: Straight from the horse's mouth, the Dutch bowyers have found out (Jan Van der Veen, book is Houtkoorts, called Woodfever in English) and it's confirmed that Dutch Elm Disease does NOT affect the bowmaking ability of the wood, bows can be made with it. Go for it, bowmakers can use it and people who conserve the forest or places where the diseased trees are will be happy to give you it most likely.