Complex sentence: Problematic exotic disease introductions in the past century or so include the chestnut blight which has almost eliminated the American chestnut tree from its forest habitat, and Dutch elm disease, which has severely reduced the American elm trees in forests and cities.
Split into simpler sentences: Problematic exotic disease introductions in the past century or so include the chestnut blight which has almost eliminated the American chestnut tree from its forest habitat. Another example is the Dutch elm disease, which has severely reduced the American elm trees in forests and cities.