Show breadcrumb on forum only
I recommend you try the Disable Breadcrumbs module.


  Disable breadcrumbs on a per node, per content type, or by path (any path) basis; Omitting the need to write additional code in the theme layer (template.php file etc..) to determine the visibilty of breadcrumbs on specific pages. This can be administered on node edit forms and from the content administration page (node operations) to select/unselect multiple nodes for enabled types.