Morphological aspects of early host-parasite interactions in infections of Melampsora pinitorqua and Melampsora larici-tremulae on Pinus sylvestris. Implications in the taxonomical relationship of the two rust fungi

A cyto-histological study was carried out on Pinus sylvestris seedlings artificially infected with basidiospores of Melampsora pinitorqua and Melampsora laricitremulae, in order to differentiate between the two rust fungi and delineate their taxonomical relationships. The general morphology of the early phases of the majority of infections of Melampsora larici-tremulae on P. sylvestris show similar characteristics to those of M, pinitorqua. Moreover, in M, larici-tremulae /P. sylvestris interactions, the following were the specific characteristics: abnormal germlings and secondary basidiospore production, ''cell wall-like appositions,, encasing the fungal structures in the host, necrosis of the infection structures, necrosis of the host tissue colonized by mycelium from epidermal to parenchymal cells. Such reaction types reveal the coexistence of pre-and posthaustorial defence mechanisms, typical, respectively, of a nonhost and host resistance. This coexistence which is useful in understanding the evolution of this host-rust interaction, was examined. The described defence mechanisms of P. sylvestris vs. M. larici-tuemulae could indicate that the two rust fungi are still closely related entities, however diverging, which evolve from ''formae speciales'' to ''species''.