New insights into the identity and origin of the causal agent of oak powdery mildew in Europe

Powdery mildew is the most common disease on oaks in Europe where it was first recorded at the beginning of the 20th century. Yet, little is known about the origin of the causal agent. In this study, we analysed the variability of the ribosomal DNA (rDNA) of the pathogen. The internal transcribed spacer (ITS) region, the 5.8S rRNA coding gene and the intergenic spacer (IGS) of the rDNA of 33 European (mostly French) samples of oak powdery mildew were amplified using polymerase chain reaction (PCR) and PCR products were subsequently sequenced. Four different haplotypes were obtained for ITS among the various samples (ITSA, ITSB, ITSC and ITSD). Each ITS sequence corresponded to a different IGS sequence. The comparison of ITS sequences obtained with sequences accessible in the GenBank database revealed very high homologies with different taxa. Of these, three taxa had already been described on oaks in Europe, i.e. Erysiphe alphitoides (100% homology with ITSA), Erysiphe hypophylla (99.4% homology with ITSC) and Phyllactinia guttata (97.64% homology with ITSD). Our data also confirmed the 100% homology between ITSA and the sequence described for Oïdium mangiferae, the agent of mango powdery mildew. The fourth haplotype, i.e. ITSB, represented by nearly 25% samples, showed 100% homology with the recently described Erysiphe quercicola from Quercus spp. in Asia, and several tropical and sub-tropical powdery mildew species, including Oïdium heveae, a major pathogen of rubber trees worldwide. Our results suggest that oak powdery mildew might originate from host shifts of tropical Erysiphe species introduced to Europe through infected exotic host plants.