Phylogenetic relationships among Phytophthora species were investigated by sequence analysis of the internal transcribed spacer
region I of the ribosomal DNA repeat unit. The extensive collection of isolates included taxa from all six morphological groups
recognized by Waterhouse (1963) including molecular groups previously identified using isozymes and mtDNA restriction fragment
length polymorphisms. Similar to previous studies, the inferred relationships indicated that molecular groups of P. cryptogea/drechsleri-like and P. megasperma-like taxa are polyphyletic. Morphological groups V and VI, which are differentiated by the presence of
amphigynous or paragynous antheridia, are not monophyletic: species of the two groups are interspersed in the tree. Species with
papillate and semi-papillate sporangia (groups I–IV) clustered together and this cluster was distinct from those of species with non-papillate sporangia. There was no congruence between the mode of antheridial attachment, sporangial caducity, or homo- or
heterothallic habit and the molecular grouping of the species. Our study provides evidence that the antheridial position together
with homo- or heterothallic habit does not reflect phylogenetic relationships within Phytophthora. Consequently, confirming studies
done previously (Cooke & Duncan 1997), this study provides evidence that the morphological characters used in Phytophthora
taxonomy are of limited value for deducing phylogenetic relationships, because they exhibit convergent evolution.