First Report of Downy Mildew Caused by Plasmopara obducens on Impatiens in Taiwan

In February 2013, single and double flowered impatiens (Impatiens walleriana Hook. f.) affected by downy mildew were observed in nurseries (cv. Accent) and in the wild in central Taiwan. More than 90% of the plants were infected in areas where the disease broke out. Symptomatic leaves showed yellowing, with white, fungal-like structure covering the lower leaf surfaces, causing the plants to become wilted and defoliated. Under microscopic observation, hyaline, thin-walled sporangiophores branched monopodially and had slightly swollen bases. Three apical branchlets were at right angles to the main axis, measuring 4.3-15.0 μm (average 8.5 μm). Sporangia were hyaline, ovoid, with an average length and width of 14.2 (10.0 to 18.0) × 12.1 (9.3 to 15.0) μm. For molecular categorization, PCR amplification of the 5' end of the large ribosomal subunit gene was performed with primers NL1 and NL4 (2). The amplicons were cloned, sequenced, and deposited in GenBank (Accession Nos. KC905620 and KC905621). The sequence similarities were 99% compared with that of Plasmopara obducens (J. Schröt.) J. Schröt from Florida (JX217746) (3). Based on morphological and molecular characters, the pathogen was identified as P. obducens. Three voucher specimens (TNM Nos. F0026644, F0026645, and F0026646) were deposited in the herbarium of the National Museum of Natural Science, Taichung, Taiwan. Pathogenicity was confirmed by inoculation of five young, potted impatiens plants with a suspension containing 1 × 10⁵ sporangia/ml in 0.05% Tween 20 (approximately 8 ml/plant). An additional five plants sprayed with 0.05% Tween 20 served as negative controls. The plants were maintained in an outdoor ambient environment. After 2 weeks incubation at an average temperature of 20°C and approximately 80% relative humidity, the inoculated plants exhibited typical downy mildew symptoms, while the control plants remained healthy. The pathogenicity test was repeated in a dew chamber under 20°C with similar results. In the Asia-Pacific region, impatiens downy mildew was recently confirmed in Korea and Japan (1,4). To our knowledge, this is the first report of downy mildew on impatiens in Taiwan. Our further surveys indicated the disease has spread to other parts of the island and will become a potential problem requiring prevention. References: (1) Y. J. Choi et. al. Plant Pathol. J. 25:433, 2009. (2) K. O'Donnell. Curr. Genet. 22:213, 1992. (3) A. J. Palmateer et. al. Plant Dis. 97:687, 2013. (4) M. Satou et. al. J. Gen. Plant Pathol. 79:205, 2013.