Podosphaera lini (Ascomycota, Erysiphales) revisited and reunited with Oidium lini

Podosphaera lini (Erysiphaceae) has recently been detected on linseed, Linum usitatissimum var. crepitans, in England and represents the first unequivocal record of this powdery mildew species from Great Britain. The history of powdery mildew on linseed/flax in the UK is critically discussed. DNA sequence data (ITS + 28S rDNA) have been retrieved, from the British material and a specimen from Germany, to be used for phylogenetic analyses. The position of P. lini as a species of its own in the genus Podosphaera, close to P. macularis (hop powdery mildew), was confirmed in the ITS-based phylogeny. The phylogenetic results, in concordance with the morphological traits of the flax powdery mildew (small peridial cells of the chasmothecia), place this species in Podosphaera sect. Sphaerotheca subsect. Sphaerotheca. Oidium lini, described from Croatia by Škorič in 1926, was later erroneously reduced to a synonymy with Golovinomyces orontii. Oidium lini has been reassessed and is now regarded as a synonym of Podosphaera lini. Oidium lini, cited in the literature as a name published by Bondartsev in 1913, is critically discussed and reviewed. The name O. lini was most likely never validly introduced and is doubtful.