SUMMARY -The early phases of basidiospore-derived infection of Me/ampsora pulcherrima (Bub.) Maire on the leaves of Mercurialis annua L. were studied by light microscopy, SEM and TEM. The fine morphology of the basidiospore germling penetration and intraepidermal infection structures is discussed in comparison with that of other rusts recently described. The direct penetration through the epidermal cell wall, characteristic of the rust basidiospore-derived germlings, is confirmed. The absence of an extrahyphal matrix around the intraepidermal vesicle and the presence of a collar around the vesicle neck are pointed out.
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-tremulae could indicate that the two rust fungi are still closely related entities, however diverging, which evolve from «formae speciales» to «species».
Melampsora pinitorqua on Populus tremula from Tuscany and M. lariei-tremulae on /'. tremula from central Alps were artificially inoculated on the same pycnio-aecial hosts in the genera Pinus and Larix, and were compared on the grounds of their pathogenicity and of some morphological aspects. The results indicate that the two rusts can be effectively considered .is two separate species as they are different both in their pathogenicity, notwithstanding the partially overlapping host range, and their morphology, at least in a few chamcters.
The presence of Melampsora lariei-tremulae Kleb. and M. pinitorqua Rostr. in the saine area (Pizzorne, Lucca) was proved by studying differences between periods of teliospore germinability and by inoculations on pycnio-aecial hosts., , . The life cycles and pathogenicity of these two rusts were separately studied in a controlled environment and^ compared to those of M. lariei-tremulae from Bressanone and M. pinitorqua from Monticiano. observations on some Italian provenances of M. populnea 73 '•"•• -H 38 -•=•' -(i", + = infection on seedling stem present. -= in--1-epicotyl" in herbaceous seedlings and "shoot axis" in seedlings 132 B. Naldini Longo, N. Longo, F. Moriondo and F. Drovandi Receipt ofms.: J. 2. 1987
In some Italian provenances of Melarnpsora populnea on Popuh trernula from mountain zones, the presence of Melampsora initorqua was tested through teliospore germinability tests and subsequent basidios ore mocufations. In this study the risk of twist rust epidemics in new plantations is discussed?
Receipt of ms.: 221991
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