Impatiens walleriana (Balsaminaceae), popularly known as Impatiens, is an African succulent and a popular ornamental plant worldwide (GBIF, 2019). In Brazil it is broadly grown indoors and outdoors, including in public parks of Curitiba, State of Paraná (Viezzer et al. 2018). In September 2018, I. walleriana plants showing typical downy mildew symptoms were observed in wastelands and gardens in Curitiba. The symptoms included adaxial chlorotic leaf spots with abundant white sporulation on abaxial side (Supplementary figure 1). The disease led to severe defoliation of the plants and the incidence of the plant disease varied from 20 to 80% of plants in an area ranging from 400 to 40,000 m2. A representative sample was deposited in herbarium of the Museu Botânico Municipal de Curitiba (MBM 331601). The following morphology was observed: Sporangiophores (n = 30), hyaline, thin walled, emerging through stomata, 407.3 to 551.1 μm long, slightly swollen base, first branch at 165.8 to 324.7 μm from base, end branches 5.1 to 13.1 μm long, sporangia (n = 50) hyaline, thin-walled subglobose to ovoid, from 12.8 to 21.9 μm x 12.5 to 17.9 μm, slightly papillate. Due to morphological and genetic variations within the species Plasmopara obducens, Görg et al. (2017) proposed the new species P. velutina and P. destructor. The morphology of the Curitiba specimen was equivalent to that described for P. destructor (Görg et al. 2017). DNA was extracted from LEMIDPRTf-19-02 isolate and the ITS1 and cox2 regions were PCR amplified as described in Görg et al. (2017). The resulting sequences were deposited in GenBank (ITS1, MT680628; cox2, MT952335). A BLASTn analysis of the sequences revealed 100% homology with ITS (MF372742) and cox2 (MF372728) sequences of type strain of P. destructor (GLM-F107554). A Bayesian phylogenetic analysis was performed to compare the sequences from this study with reference sequences for P. obducens, P. destructor and P. velutina (Görg et al. 2017; Salgado-Salazar et al. 2018). The oomycete from Curitiba grouped in a reliable clade with P. destructor (Supplementary figure 2). Pathogenicity was carried out by ex vivo and in vivo tests. For ex vivo, stems with approximately four healthy leaves of I. walleriana (n = 10) were embedded in aluminum grid inside of gerbox with the stem bases immersed in distilled water. The inoculation of five stems was carried out by spraying a suspension with 6 x 104 sporangia mL-1 on the abaxial side of the leaves. Five stems with leaves inoculated with sterile water were used as controls. They were incubated in a growth chamber in the dark for 48 h at 20 °C and another 12 days in a 12 h light photoperiod. The confirmation of pathogenicity in plants (in vivo) was obtained with the inoculation of I. walleriana seedlings (one-month old) grown in 2 dm3 aluminum pots. The inoculation methodology and number of plants were the same as the stems test. After the inoculation, plants were incubated in a growth chamber for 48 h in the dark at 20 °C with 100% RH with nebulization, and another 10 days at a photoperiod of 12 hours of light. For both tests, abundant sporulation was observedwith morphology equivalent to Plasmopara destructor described by Görg et al. (2017). No disease developed on control plants. To our knowledge, this is the first report of P. destructor on I. walleriana in Brazil (Farr and Rossman 2019, Silva et al. 2019) representing a potential loss to flower production and a reduction in flowering period in public gardens and parks.
Keywords: Causal Agent; Crop Type; Etiology; Fungi; Ornamentals; Subject Areas; herbaceous/flowering plants.