The work is aimed to study of morphological variability of Phytophthora alni complex in the area of the Czech Republic with use of numerical phenetics. The morphological analysis of 29 Phytophthora alni strains acquired from the area of the pathogen in the Czech Republic was carried out. In total there were studied 50 cultural and morphological characteristics: 14 cultural characteristics, 16 characteristics describing asexual and 20 characteristics describing sexual structures. The all morphological measurements were repeated twenty times at magnification 1600× with use of Olympus BH-2 microscope, Olympus E-510 camera and Quick Photocamera 2.3 measuring software. The data were processed in Statistica 8.0 in modules of non-parametric statistics, descriptive statistics, cluster analysis, analysis of principal components and discriminant analysis. In the area there was confirmed the presence of the two taxa belonging to the P. alni complex: Phytophthora alni subsp. alni (Paa) a Phytophthora alni subsp. uniformis (Pau). The third taxon (P. alni subsp. multiformis) was not found. The nominate subspecies Paa prevailed - more than 4/5 of studied isolates belonged to this taxon. The classification of isolates was in full agreement with molecular study of identical set of isolates which was parallely performed (Štěpánková et al. 2013). The morphological analysis showed, that majority of Czech isolates of P. alni complex can be successfully determined on the basis of standard morphological characters -- oogonial wall ornamentation, antheridial length, rate of aborted oogonia etc. Ca 10 % of isolates were found to be intermediate. All studied isolates were divided with series of cluster and PCA analysis in two relevant clusters and a discriminant function dividing the isolates in the two taxa on basis of morphological characters was generated and validated. The rate of standard oogonia, high of oogonial ornamentation, rate of two-celled antheridia with central septum and antheridial length were used as discriminators. The two different Paa morphotypes were found in the area of the Czech Republic. The discriminant function dividing the Paa isolates in these two morphotypes was also constructed and tested. The discriminators were oogonia diameter, high of oogonial ornamentation and width of antheridium. The most reliable character was the antheridial width: the north-western morphotype has antheridia wider by 3 micrometres than the south-eastern one. The Paa taxon was continuously distributed in the western part of the area, its frequency diminshes eastward. The distribution of the second taxon - Pau - was scattered and insular, but it was found out in the whole area of the Czech Republic. It could be supposed, that the parental taxon Pau had spread before Paa invasion in the area. Later the new and more combative Paa replaced the older weakly pathogenic Pau. The recent area of Pau is apparently insular and probably relict. The two Paa morphotypes importantly differ in their distribution - the first morphotype is more frequent in north-western part of the area (and predominates in western, central and northern Bohemia) while the second one is frequent in the south-eastern part (southern Bohemia and Moravia). Likely, these two subpopulations can represent the remains of colonisation waves of two allopatric Paa lineages. The lineages can originated by repeated independent hybridisation between parental Pau and Pam taxa somewhere in western or central Europe.

Depuis les années 1990, l'aulne glutineux, espèce clé des ripisylves, est affecté par un oomycète qui cause son dépérissement : Phytophthora alni subsp. alni (Paa). La genèse de Paa est liée à un événement d'hybridation interspécifique entre deux espèces proches, improprement nommées P. alni subsp. uniformis (Pau) et P. alni subsp. multiformis (Pam), car initialement considérées comme des variants de Paa. L'objectif de cette thèse était d'identifier les facteurs ayant pu jouer un rôle dans l'émergence de la maladie en Europe. Par une approche de génétique des populations, nous avons montré que Pau est une espèce invasive en Europe, probablement originaire d'Amérique du Nord. Après son introduction, l'hybridation de Pau avec Pam serait l'un des facteurs essentiels de l'apparition de Paa. Nos résultats confirment que Paa aurait été généré suite à des hybridations récurrentes, qui ont structuré géographiquement les populations en Europe. L'analyse de la variabilité génétique de Paa, révélée par des marqueurs microsatellites, a toutefois montré un faible polymorphisme, avec un génotype dominant largement répandu en Europe. Par ailleurs grâce à la modélisation, nos résultats ont établi que le dépérissement du houppier des aulnes est lié à la température. En particulier l'incidence de la maladie augmente lors des hivers doux, qui pourraient favoriser la survie du mycélium de Paa, celui-ci ne présentant pas de structure de survie hivernale (chlamydospores ou oospores). La température estivale joue également un rôle, plus complexe à interpréter. On constate en effet que l'incidence de la maladie diminue avec l'augmentation des températures estivales, mais ce phénomène pourrait dépendre d'autres facteurs tels que l'état physiologique des arbres ou le type de communautés microbiologiques présentes dans les sols. Inversement, le phénomène de guérison des arbres est favorisé par des températures hivernales basses et par des températures estivales élevées. L'émergence de la maladie ne peut pas être expliquée par le changement climatique. Cependant, une augmentation des températures hivernales dans le futur dans le cadre du changement climatique aggraverait très probablement l'épidémie / Since the early 1990's alder decline caused by the oomycete Phytophthora alni subsp. alni (Paa) is one of the most important threats to riparian ecosystems in Europe. The emergence of Paa is related to an interspecific hybridization event between two related species -initially considered as Paa variants- misnamed as Phytophthora alni subsp. uniformis (Pau) and Phytophthora alni subsp. multiformis (Pam). The objective of this thesis was to identify the factors that may have contributed to the emergence of the disease in Europe. Following a population genetics approach we showed that Pau is likely to be an invasive species in Europe, probably native to North America. Its introduction would have enabled hybridization with Pam and, consequently be a major cause on the emergence of Paa. Our results confirm that Paa has arisen from several hybridization events, which have geographically structured its European populations. Paa's genetic variability, revealed by microsatellite markers, showed low levels of polymorphism, with a dominant genotype scattered throughout Europe. In addition, a modelling approach revealed that alders' crown decline is linked to temperature. In particular, the disease incidence increases during mild winters which favours mycelium survival as Paa does not produce resistant spores (chlamydospores or oospores). The effect of summer temperatures is more complex to explain. Disease incidence decreases when summer temperatures are higher, but this phenomenon can also be linked to the physiological conditions of trees or changes in soil microbiological communities. Conversely, tree recovery is favoured by lower winter and higher summer temperatures. Climate change does not explain the emergence of the disease. However, increases in winter temperatures du to climate change may strengthen the epidemic