In May 2011, young impatiens plants (Impatiens walleriana Hook.f.) showing downy mildew symptoms were collected from a greenhouse in Kecskemét, Hungary. The disease was later reported from different parts of the country from a number of different cultivars. 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%. Appearance of the disease caused a loss of approximately 2,000 euros for the growers in Kecskemét. In Hungary, losses for growers and consumers could have reached half a million euros. Downy mildew of impatiens can be caused by two pathogens, Plasmopara obducens or Bremiella sphaerosperma (1). P. obducens differs from B. sphaerosperma by monopodial sporangiophores with straight branches and the lack of apical thickening on the branchlets. Sporangia of B. sphaerosperma are spherical, while those of P. obducens are subspherical. Sporangiophores of the fungus were colorless with straight trunks and monopodially branched four to seven times. Sporangiophores ended with three apical branchlets at right angles to the main axis, measuring 4.6 to 16.4 μm (average 9.6 μm). The ovoid and hyaline sporangia measured 11.04 (7.7 to 13.8) × 13.9 (9.9 to 17.4) μm. For molecular identification, total DNA was extracted from the pathogen scraped from the leaves using a cetyltrimethylammoniumbromide (CTAB) extraction method. The 5′-end of the large subunit of ribosomal RNA gene was amplified by PCR using NL1 and NL4 primers (3) and cloned and sequenced. The nucleotide sequence of IWPO-H1 Hungarian isolate (GenBank Accession No. HE577169) showed highest identity with Accession Nos. AY587558 and HQ246451 from the United Kingdom and Serbia (2), respectively, with 99.72% identity. On the basis of the symptoms and molecular and morphological characters, the pathogen was identified as P. obducens. Pathogenicity was confirmed by spraying young I. walleriana plants with a sporangial suspension (1 × 104 sporangiospores ml–1). Control plants were sprayed with sterile water. Plants were incubated at room temperature under glass vessels at approximately 90% relative humidity. Symptoms of downy mildew occurred 12 to 16 days after inoculation, while control plants remained healthy. Presence of P. obducens was reported from Austria, Australia, Bulgaria, China, the Czech Republic, Denmark, Germany, Finland, Korea, Lithuania, Norway, Pakistan, Poland, Romania, Russia, the United Kingdom, the United States, and recently from Serbia (2). To our knowledge, this is the first report of downy mildew of I. walleriana caused by P. obducens in Hungary. The appearance of P. obducens in Hungary seriously endangers the production of I. walleriana, which is an important and popular ornamental in gardens and city parks.
References: (1) O. Constantinescu. Mycologia 83:473, 1991. (2) D. F. Farr and A. Y. Rossman. Fungal Databases. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/, September 1, 2011. (3) K. L. O'Donnell. The Fungal Holomorph: Mitotic, Meiotic and Pleomorphic Speciation in Fungal Systematics. CAB International, Wallingford, UK, 1993.