FULL RESEARCH PAPER Characteristics of a Plasmopara angustiterminalis isolate from Xanthium strumarium Hedvig Komja ´ti Æ Ilona Walcz Æ Ferenc Vira ´nyi Æ Reinhard Zipper Æ Marco Thines Æ Otmar Spring Received: 18 December 2006 / Accepted: 21 May 2007 / Published online: 3 July 2007 Ó KNPV 2007 Abstract Leaves of Xanthium strumarium infected with downy mildew were collected in the vicinity of a sunflower field in southern Hungary in 2003. Based on phenotypic characteristics of sporangiophores, sporangia and oospores as well as host preference the pathogen was classified as Plasmopara angusti- terminalis. Additional phenotypic characters were investigated such as the size of sporangia, the number of zoospores per sporangium and the time-course of their release. Infection studies revealed infectivity of the P. angustiterminalis isolate to both X. strumarium and Helianthus annuus. Inoculation of the sunflower inbred line, HA-335 with resistance to all known P. halstedii pathotypes, resulted in profuse sporulation on cotyledons and formation of oospores in the bases of hypocotyls. Infections of sunflower differential lines often led to damping-off. Molecular genetic analysis using simple sequence repeat primers and nuclear rDNA sequences revealed clear differences to Plasmopara halstedii, the downy mildew pathogen of sunflower. Keywords Downy mildew Á Helianthus annuus Á Host specificity Á Internal transcribed spacer Á Peronosporaceae Á Plasmopara halstedii Introduction Novotelnova (1962) had separated Plasmopara an- gustiterminalis, the downy mildew pathogenic to Xanthium strumarium (common cocklebur) from Plasmopara halstedii. The latter has long been considered as a single species complex with a broad host range showing infectivity to > 80 genera of the Asteroideae and Cichorioideae subfamilies of the Asteraceae (Leppik 1966). The first revision on this species complex by Savulescu (1941) was based on detailed investigation of the morphology. It was followed by Novotelnova (1962, 1963, 1966) who, on the basis of morphology and host preference, separated seven new species from the P. halstedii complex. The name P. angustiterminalis f. sp. angustiterminalis was introduced for the Plasmopara isolates from Xanthium sp. (Novotelnova 1962), while isolates from Heliantheae were named as P. helianthi with further divisions as f. sp. helianthi, perennis, and patens. Accordingly, the name P. halstedii remained exclusive for the isolates infective on Eupatorieae, on which this pathogen was first described. However, this H. Komja ´ti Á F. Vira ´nyi (&) Department of Plant Protection, Szent Istva ´n University, Godollo H-2103, Hungary e-mail: viranyi.ferenc@mkk.szie.hu I. Walcz Forage Crops Research Institute, University of Kaposva ´r, Iregszemcse-Bicserd H-7095, Hungary R. Zipper Á M. Thines Á O. Spring Institute of Botany, University of Hohenheim, 70593 Stuttgart, Germany 123 Eur J Plant Pathol (2007) 119:421–428 DOI 10.1007/s10658-007-9178-9