Stealthy Disease of Impatiens May Go Unnoticed
Tom Creswell, Plant & Pest Diagnostic Lab Director, Purdue University
These impatiens plants (Impatiens walleriana) appear fairly normal for late September in northern Indiana (Fig. 1). They look a little scruffy but that's to be expected this time of year. But if you turn over the leaves you find the lower surfaces (Fig. 2,3,4) are covered with a white fuzzy growth, known as downy mildew. The fungus responsible is Plasmopara obducens, a fungus that has was first reported in the U.S. in 1942 but until recent years has shown up only sporadically.
It seems to cause fairly little damage in the landscape in the mid-West until conditions are right: cool nights and humid, rainy weather. Infected plants may show a slight stippled appearance on the upper surface of the leaf. The leaves may be lighter green in color and the whole plant may be stunted. Spores are spread long distances in the wind.
The downy mildew fungus on Impatiens can also infect garden balsam (Impatiens balsamina) but doesn't cause disease on other commonly grown flowers in the landscape. Fungicides are not required since the problem tends to show up late in the growing season after hot dry weather has ended so damage to planting beds doesn't usually justify spraying. You can reduce spread by irrigating with a drip system or by hand watering on the soil around the plants instead of using a sprinkler.
The downy mildew fungi are easy to distinguish microscopically from many other pathogenic fungi because they produce spores on a tree-like structure with multiple branches and spores present at the tips of each branch (Fig. 5 and 6).
Click image to enlarge
Fig 1. Late season impatiens
Fig 2. Downy mildew on the lower leaf surface
Fig 3. Downy mildew on the lower leaf surface
Fig 4. Close-up view of downy mildew covering the lower leaf surface
Fig 5. Dissecting microscope view of individual spore-bearing structures (sporangiophores)
Fig 6. Compound microscope view of spore-bearing structures and individual spores