Plasmopara obducens: A new threat to the production of Impatiens Walleriana in Serbia

Abstract

During 2010, Impatiens walleriana plants with symptoms of downy mildew were collected in a greenhouse in the vicinity of Mionica, Kolubara District. Disease incidence was extremely high, approaching 100%, and wilting and collapse of affected plants was very rapid, resulting in losses of more than 90%. White downy growth produced on the lower leaf surface consisted of hyaline, thin-walled sporangiophores with monopodial branching and numerous, ovoid and hyaline sporangia. Apical branchlets of sporangiophores were at right angles to the main axis, with no apical thickening. Pathogenicity tests included inoculation of young I. walleriana plants by spraying with a sporangial suspension, and downy mildew symptoms were observed after 13 to 15 days. The absence of well-defined spots on the infected impatiens leaves and straight sporangiophores indicated that the pathogen was P. obducens, which was further supported by molecular identification, the 5'-end of the nuclear DNA coding for the large ribosomal subunit (LSU rDNA) was amplified by PCR, using primers NL1 and NL4. A representative isolate, 28-10, was sequenced and phylogenetic analysis showed its grouping with other P. obducens isolates of different origin. Considering that impatiens downy mildew in Serbia is proved to be caused by P. obducens it is necessary to employ adequate phytosanitary measures to prevent further spread of the pathogen.

Tokom 2010. godine biljke Impatiens walleriana sa simptomima plamenjače sakupljene su u stakleniku u okolini Mionice, Kolubarski okrug. Učestalost oboljenja bila je izuzetno visoka, blizu 100%, a sušenje i propadanje zaraženih biljaka veoma brzo, što je dovelo do šteta većih od 90%. Bela prevlaka patogena koja se razvijala na naličju listova sastojala se od hijalinskih monopodijalno razgranatih sporangiofora sa tankim zidovima i brojnih, ovoidnih i hijalinskih sporangija. Vrhovi grana sporangiofora granali su se pod pravim uglom u odnosu na glavnu osu, bez vršnog zadebljanja. Testovi patogenosti uključili su inokulacije mladih biljaka I. walleriana prskanjem suspenzijom sporangija, a simptomi plamenjače razvili su se nakon 13-15 dana. Odsustvo definisanih pega na zaraženim listovima impatiensa i prisustvo pravih sporangiofora ukazalo je da je prouzrokovač P. obducens, što je dalje potvrđeno molekularnom identifikacijom. Primenom lančane reakcije polimeraze (polymerase chain reaction, PCR), pomoću prajmera NL1 i NL4, umnožen je kodirajući 5'-kraj DNA velike ribozomalne podjedinice (LSU rDNA). Amplifikovani produkt odabranog izolata 28-10 je sekvencioniran, a filogenetske analize pokazale su njegovo grupisanje sa ostalim izolatima P. obducens različitog porekla. Uzimajući u obzir da je ovim istraživanjima dokazano da je prouzrokovač plamenjače na Impatiens walleriana u Srbiji P. obducens, neophodno je preduzimanje odgovarajućih fitosanitarnih mera u cilju sprečavanja daljeg širenja patogena u našoj zemlji.