As reported in this post, ashes (Fraxinus sp. pl.) are being seriously threatened by ash dieback, also known as Chalara, a vicious disease caused by the fungus Hymenoscyphus fraxineus, which kills the leaves, then the branches, trunk and eventually the whole tree.
On top of that, the emerald ash borer (Agrilus planipennis), an invasive beetle, is also causing serious damages to ashes, because its larvae bore under the bark and in to the wood, killing the tree.
Ex situ conservation appears to be a very good strategy to contrast such threats, but it is not mentioned in the post.