Hi, have you checked your LODs for the foliage meshes? Doubleclick on the mesh, and go to the lowest LOD, if the mesh disappears then the problem is that your LOD is not working properly, you can just remove that LOD level and it should work! If all the LODs are corrupt just delete them all, however, performance may suffer if you dont have LODs, so make sure you set them up properly in the future.
Cheers!