Geographic distribution, species spectrum and variation of powdery mildew in the genus Lactuca

Powdery mildews are obligate biotrophic phytopathogenic fungi with a worldwide distribution. Advances in sequencing and molecular analyses have enabled the identification of cryptic species that were previously indistinguishable through morphological methods alone. Consequently, the discovery of new species continues to increase. This study brings together a comprehensive molecular phylogenetic analysis based on the amplification of ITS and IGS regions and a morphological analysis conducted on a broad spectrum of powdery mildew samples collected on various representatives of the genus Lactuca. Our dataset consisted of 200 accessions of herbaria leaves of Lactuca spp. infected with powdery mildew from different habitats worldwide. Microscopic examination revealed the presence of asexual states observable as conidiophores with two types of conidia, with fibrosin bodies (Podosphaera xanthii), and without fibrosin bodies (Golovinomyces bolayi). In some cases, chasmothecia were also recorded. Molecular identifications based on the ITS region confirmed this conclusion. Among 148 concatenated sequences of ITS and IGS regions, we discovered 12 ribotypes in Lactuca samples infected by G. bolayi. A single ribotype was observed for samples with the presence of P. xanthii. The principal component analysis revealed the pathogen’s host-specificity as an important factor determining host-dependent morphological variability in the case of samples from L. sativa and L. serriola.