Plasmopara obducensoccurring onImpatiens wallerianahybrids and species in Hungary: Morphological and molecular characterization of the pathogen

Abstract

In May 2011, young Impatiens walleriana Hook. f. plants showing downy mildew symptoms were collected from a greenhouse (Kecskemet, Hungary). The upper surface of affected leaves turned yellow and white fungal-like growth was observed on the underside. Diseased plants wilted and rapidly collapsed, resulting in losses of nearly 100%. Downy mildew of impatiens can be caused by two pathogens, Plasmopara obducens or Plasmopara constantinescui (syn: Bremiella sphaerosperma), which can be distinguished by the symptoms and by morphology of sporangiophores and sporangia. For molecular identification the 5′-end of the large subunit of ribosomal RNA gene was amplified by PCR using NL1 and NL4 primers and cloned and sequenced. The nucleotide sequence of the Hungarian isolate showed highest identity (99.72%) with two isolates of Plasmopara obducens from the United Kingdom and Serbia. On the basis of the symptoms, molecular and morphological characters, the pathogen was identified as P. obducens. The appearance of the pathogen in Hungary seriously endangers the production of I. walleriana, which is a popular ornamental in gardens and city parks. The chemical plant protection is not effective against the pathogen, by the occurrence of the first symptoms the diseased plants should be removed from the plantlet and must be destroyed.