Three new phylogenetic lineages are the closest relatives of the widespread species Albugo candida

White blister rust caused by the obligate biotroph Albugo candida (Albuginaceae; Oomycota) is one of the most notorious and common diseases of Brassicaceae. During the past 5 y, A. candida specimens collected from about 30 host genera were phylogenetically and morphologically investigated in several studies. These not only revealed that A. candida s.str. has a broad host range, encompassing a large number of host plants belonging to Brassicales, but also the presence of previously overlooked species of Albugo with hosts in this order. In this study, we examined specimens from Alyssum, Barbarea, and Rorippa, of which many species were commonly recorded as host plants of A. candida but could not be included in previous works due to the paucity of specimens available. It was revealed that Albugo specimens from Alyssum montanum, Barbarea vulgaris, and various Rorippa species, were placed in three phylogenetically distinct clades, but closer to A. candida s.str. than any previously reported species. Oospores were observed from Albugo specimens parasitic to Rorippa and could be distinguished morphologically from A. candida. Therefore, Albugo rorippae sp. nov. is described and illustrated here. In addition, a key of Albugo species described previously from Brassicales is given. The present study reveals that a large number of Albugo species remain still undiscovered, and that species close to A. candida exist. This could help elucidating the basis of the broad host range of A. candida as opposed to the narrow specialisation that is seemingly present in other species of Albugo on the Brassicaceae. (C) 2011 The British Mycological Society. Published by Elsevier Ltd. All rights reserved.