The linkage of morphological, genetic, and molecular characters of "Armillaria" over the past few decades has led to the recognition of intersterile groups designated as “biological species”. Data from such studies, especially those using molecular diagnostic tools, have removed much uncertainty for mycologists and forest pathologists. New questions remain unanswered regarding the phylogeny of North American "Armillaria" species and their relationships to their European counterparts, particularly within the “"Armillaria mellea" complex”. Some data suggest that North American and European "A. gallica" isolates are not monophyletic. Although North American and European isolates of "A. gallica" may be interfertile, some North American isolates of "A. gallica" are more closely related to the North American taxon "A. calvescens" than to European isolates of "A. gallica". The increase in genetic divergence has not necessarily barred inter-sterility between isolated populations of "A. gallica". Although the relationships among some groups in the genus seem clearer, the investigation of geographically diverse isolates has revealed that the relationship between some North American species is still unclear (Hughes et al. 2003).