First Report of Downy Mildew Caused by Plasmopara muralis on Boston Ivy (Parthenocissus tricuspidata) in Poland

Abstract

Given its decorative foliage and leaf discoloration in autumn, Boston ivy (Parthenocissus tricuspidata Siebold & Zucc.) is mainly used in garden compositions. The climber is commonly grown in Poland, because it is characterized by rapid growth and low soil and climatic requirements. ‘Veitchii’ is the most common variety cultivated in Poland. In May 2017, the symptoms of downy mildew on Boston ivy plants were observed in one of the ornamental plant nurseries in Mazowieckie Province (52°06′42″N, 20°59′13″E), where disease incidence reached 80%. The disease symptoms on the plants were noted first on lower and then on upper leaves. On the upper surface of the lamina, there were reddish lesions on older leaves and light green lesions on young leaves. In both cases, the injuries on leaves were delimited by veins, and on the lower surface monopodially branched colorless, straight or slightly curved sporangiophores with sporangia developed. The causal agent of downy mildew in Boston ivy was identified using morphological and molecular methods. The sporangia were hyaline, ovoid to shortly ellipsoidal, with a protruding pedicel. They measured (13.1 to) 14.7 to 22.1 (to 38.1) × (11.2 to) 12.3 to 16.3 (to 20.5) µm (n = 100). The length of ultimate branchlets of sporangiophores was (3.7 to) 5.1 to 8.9 (to 11.8) µm (n = 100). These measurements were similar to those obtained by Thines (2011). Koch’s postulates were fulfilled. The experiment was carried out twice. Five 1-year-old healthy P. tricuspidata seedlings (approximate height of 50 cm) growing in pots (diameter 13 cm) and one infected seedling were placed in a foil tunnel. The plants were sprayed with water to induce transfer of pathogen sporangia from the diseased onto healthy plants. Next, the tunnel was closed. In the control variant, six healthy plants were sprayed with water. The field experiment was carried out with three replications at a temperature 20 to 26°C and relative humidity of approximately 80%. After 10 days, the first symptoms of downy mildew were observed on the inoculated plants, whereas the control plants remained symptomless. Cytochrome c oxidase of the subunit II gene of mtDNA for the oomycetes isolated from P. tricuspidata was amplified with the use of oomycete-specific primers COX2-F (Hudspeth et al. 2000) and COX2-RC4 (Choi et al. 2015). It turned out that the downy mildew on the Boston ivy was caused by Plasmopara muralis. The sequence from the oomycete exhibited 100% similarity to a reference sequence for this pathogen (Thines 2011) deposited in GenBank (accession no. FJ394346.1). The obtained sequence was deposited in GenBank under the number MH492326. Downy mildew may seriously affect Boston ivy commercial production in nurseries. That is why further research is needed to develop recommendations for disease control. To our knowledge, this is the first report on the occurrence of P. muralis on Boston ivy in Poland.