Downy mildew (Plasmopara) of Impatiens
This disease has been found in several states in the U.S. this year (2004). It has been known from the U.S. for many years but generally hasn't been a problem. Last year it was found for the first time in the United Kingdom and caused significant losses. Early detection is important in preventing losses. Another downy mildew found on Impatiens is Bremiella sphaerosperma.
Symptomatic leaves of Impatiens sp. infected with Plasmopara obducens. Typically leaves are mottled with no distinct leaf lesions whereas Bremiella sphaerosperma causes distinct leaf spots.
The conidophores of P. obducens are mainly unbranched, straight, with fairly stiff branches and branchlets while those of Bremiella sphaerosperma are branched, flexuose and with generally curved branches and branchlets.
Reference: Constantinescu, O. 1991. Bremiella sphaerosperma sp. nov. and Plasmopara borreriae comb. nov. Mycologia 83:473-479.
Additional information and photos can be found here:
Suggested citation: Palm, M.E. Systematic Mycology and Microbiology Laboratory, ARS, USDA. 2004. Invasive Fungi. Downy mildew (Plasmopara) of Impatiens. Retrieved March 30, 2017, from /sbmlweb/fungi/index.cfm .