Severe Outbreak of Downy Mildew Caused by Plasmopara obducens on Impatiens walleriana in Florida.

Impatiens, Impatiens walleriana Hook.f., are grown as an ornamental crop in greenhouse and shade house production in Florida and other regions of the United States. Downy mildew on impatiens was detected from numerous landscapes (Manatee, Hillsborough, Collier, Hendry, Broward, Palm Beach, and Miami-Dade counties) in the winter of 2012. Incidence reached nearly 100% on many affected landscape plantings. Symptoms initially appeared as yellowing on the lower leaves and were typically vein-delineated, although in some cases the entire leaf was affected. Diseased plants later wilted and infected leaves abscised from the stem. A white, downy growth was apparent on the abaxial leaf surface. Microscopic observation revealed coenocytic mycelium with sporangiophores that were hyaline, thin-walled, and had slightly swollen bases. Branches of sporangiophores were monopodial and formed right angles to the supporting branches. Sporangia were hyaline and obvoid with a single pore on the distal ends that was mostly flat. Sporangia measured 19 to 22.5 × 13 to 17 μm. Oospores were observed in stem and leaf tissue. Leaves of 10 potted impatiens plants, I. walleriana 'Super Elfin XP Coral' and 'Super Elfin XP White,' were inoculated with a suspension containing 1 × 10 sporangia/ml and sprayed till runoff (approximately 20 ml per plant) with a handheld pressurized Ulva sprayer. Plants were maintained outside in a shade house under 73% shade where the daytime temperatures averaged 24°C and RH averaged 74% and nighttime temperature averaged 18°C with an average of 91% RH. Ten non-inoculated impatiens plants served as controls. After 10 days, symptoms typical of downy mildew occurred on 100% of the inoculated impatiens plants and sporulation was confirmed microscopically. The non-inoculated control plants remained healthy. The 5' end of the large ribosomal subunit gene (762 bp) from two isolates, one collected in Hillsborough County and one from Miami-Dade County, was amplified by PCR (primers NL1-GCATATCAATAAGCGGAGGAAAAG and NL4-GGTCCGTGTTTCAAGACGG) and sequenced bi-directionally (1,2,3). The consensus sequence from both isolates was identical and it was deposited into GenBank (Accession No. JX217746). Sequence data matched (99% homology) with Plasmopara obducens reported on I. walleriana in Europe and Australia (1,2). To our knowledge, this is the first report of downy mildew on I. walleriana in Florida (4). The disease has made a major impact on impatiens in landscapes throughout Florida and will likely continue to affect future production. References: (1) A. Bulajic et. al. Plant Dis. 95:491, 2011. (2) J. H. Cunnington et. al. Plant Pathol. 57:371, 2008. (3) K. O'Donnell. Curr. Genet. 22:213, 1992. (4) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, 1989.