In pathogenic fungi and oomycetes, interspecific hybridization may lead to the formation of new species having a greater impact on natural ecosystems than the parental species. From the early 1990s, a severe alder (Alnus spp.) decline due to an unknown Phytophthora species was observed in several European countries. Genetic analyses revealed that the disease was caused by the triploid hybrid P. × alni, which originated in Europe from the hybridization of P. uniformis and P. × multiformis. Here, we investigated the population structure of P. × alni (158 isolates) and P. uniformis (85 isolates) in several European countries using microsatellite markers. Our analyses confirmed the genetic structure previously observed in other European populations, with P. uniformis populations consisting of at most two multilocus genotypes (MLGs) and P. × alni populations dominated by MLG Pxa-1. The genetic structure of P. × alni populations in the Czech Republic, Hungary and Sweden seemed to reflect the physical isolation of river systems. Most rare P. × alni MLGs showed a loss of heterozygosity (LOH) at one or a few microsatellite loci compared with other MLGs. This LOH may allow a stabilization within the P. × alni genome or a rapid adaptation to stress situations. Alternatively, alleles may be lost because of random genetic drift in small, isolated populations, with no effect on fitness of P. × alni. Additional studies would be necessary to confirm these patterns of population diversification and to better understand the factors driving it.

Department Crop and Forest Sciences University of Lleida Alcalde Rovira Roure 191 25198 Lleida Spain

Department of Biology CESAM University of Aveiro Campus Universitario de Santiago 3810 193 Aveiro Portugal

Department of Forest Mycology and Plant Pathology Swedish University of Agricultural Sciences Box 7026 750 07 Uppsala Sweden

Department of Plant Production and Forest Resources University of Valladolid Avenida de Madrid 44 34071 Palencia Spain

Federal Research and Training Centre for Forests Natural Hazards and Landscape Seckendorff Gudent Weg 8 1131 Vienna Austria

Institute of Botany at The Lithuanian State Research Institute Nature Research Centre Žaliųjų Ežerų Str 49 LT 08406 Vilnius Lithuania

Joint Research Unit CTFC Agrotecnio Alcalde Rovira Roure 191 Lleida 25198 Spain

Phytophthora Research Centre Department of Forest Protection and Wildlife Management Faculty of Forestry and Wood Technology Mendel University in Brno Zemědělská 3 61300 Brno Czech Republic

Plant Protection Institute Centre for Agricultural Research Herman Ottó Str 15 H 1022 Budapest Hungary

Swiss Federal Research Institute WSL Zuercherstrasse 111 CH 8903 Birmensdorf Switzerland

The Silva Tarouca Research Institute for Landscape and Ornamental Gardening Květnové náměstí 391 Průhonice 252 43 The Czech Republic

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MALDI-TOF MS as a method for rapid identification of Phytophthora de Bary, 1876