Diameter and treewidth in minor-closed graph families

It is known that any planar graph with diameter D has treewidth O(D), and t his fact has been used as the basis for several planar graph algorithms. We investigate the extent to which similar relations hold in other graph fami lies. We show that treewidth is bounded by a function of the diameter in a minor-closed family, if and only if some apex graph does not belong to the family. In particular, the O(D) bound above can be extended to bounded-genu s graphs. As a consequence, we extend several approximation algorithms and exact subgraph isomorphism algorithms from planar graphs to other graph fam ilies.