A new perspective on the evolution of white blister rusts: Albugo s.str. (Albuginales; Oomycota) is not restricted to Brassicales but also present on Fabales

For almost all groups of pathogens, unusual and rare host species have been reported. Often, such associations are based on single or few collections only, which are frequently hard to access. Many of them later prove to be due to misidentification of the host, the pathogen, or both. Therefore, such reports are often disregarded, or treated anecdotally in taxonomic and phylogenetic studies, regardless of their potential importance to unravelling the evolution of the entire group. Concerning oomycete biotrophs there are several reports of unusual and rare hosts for hardly known pathogens. In the order Fabales, for example, a single species of Albugo, A. mauginii, was described as parasitic to Onobrychis crista-galli about 80 years ago, but not recorded again. All other confirmed members of Albugo s.str. are parasitic to representatives of the families Brassicaceae, Capparaceae, Cleomaceae, and Resedaceae in the order Brassicales. In the present study, molecular phylogenetic analysis of cox2 mtDNA sequences and morphological investigations on an original specimen confirmed the occurrence of a member of Albugo on Fabaceae hosts, with the characteristic thin wall of the secondary sporangia, which is almost uniform in thickness. In phylogenetic analyses the species results as embedded within Albugo s.str. Therefore, it is concluded that the natural host range of Albugo s.str. extends from Brassicales to Fabales via host jumping. Our results underscore that unrevised reports of pathogens from unusual hosts should be reconsidered carefully to obtain a more complete picture of pathogen diversity and evolution.