Since 2003, Torenia fournieri plants grown for experimental purposes were repeatedly infected by powdery mildew in a laboratory in Hungary. Based on morphological characteristics, the pathogen belonged to the mitosporic genus Oidium subgen. Reticuloidium, the anamorph stage of Golovinomyces. The rDNA ITS sequence was identical to that of two other powdery mildew fungi, infecting Arabidopsis and Veronica, respectively, in different parts of the world. According to a previous phylogenetic analysis of ITS and 28S rDNA sequences, those two powdery mildews belong to a recently evolved group of Golovinomyces characterized by multiple host range expansions during their evolution. Both the ITS sequence and the morphological data indicate that the powdery mildew anamorph infecting Torenia also belongs to this group. It is likely that the powdery mildew infections of the experimental T. fournieri plants, native to south-east Asia, were the result of a very recent host range expansion of a polyphagous Golovinomyces because (i) T. fournieri is absent from our region, except as an experimental plant grown in the laboratory, (ii) the powdery mildew fungus infecting this exotic plant belongs to a group of Golovinomyces where host range expansion is a frequent evolutionary scenario, (iii) cross-inoculation tests showed that this pathogen is also able to infect other plant species, notably A. thaliana and tobacco, and (iv) no Golovinomyces species are known to infect T. fournieri anywhere in the world. Although host range expansion has often been proposed as a common evolutionary process in the Erysiphales, and also in other biotrophic plant pathogens, this has not been clearly demonstrated in any case studies so far. To our knowledge, this is the first convincing case of a host range expansion event in the Erysiphales.Torenia fournieri (Scrophulariaceae) has been used widely in plant reproductive biology research and has long been grown from seeds for this purpose at our department at Eo¨tvo¨s University (e.g., Kristo´f and Imre, 1996;Imre and Kristo´f, 1999;Kristo´f et al., 1999;Va´gi et al., 2004). It is native to southeast Asia and it is not known to occur in the field in Hungary and neighbouring countries. Since 2003, severe powdery mildew infections were repeatedly observed on our potted T. fournieri plants in a laboratory. Intensively sporulating powdery mildew mycelia covered the stems, petioles, leaf lamina, and sepals of the plants (Figures 1 and 2). The infected parts withered and eventually died 3-4 weeks after the first symptoms appeared. As the disease was unwanted, we destroyed all the infected plants whenever the infection occurred, and produced new ones from seeds in different locations. In spite of these measures, severe infections occurred four times during the past 3 years.