Downy mildew on impatiens

Impatiens downy mildew changed what many gardeners planted in their shade beds starting around 2011–2012, when it spread rapidly through commercial production and established itself in gardens across the US and Europe. The disease can destroy an entire planting of standard impatiens (Impatiens.

Impatiens downy mildew changed what many gardeners planted in their shade beds starting around 2011–2012, when it spread rapidly through commercial production and established itself in gardens across the US and Europe. The disease can destroy an entire planting of standard impatiens (Impatiens walleriana) within weeks. It persists in soil as oospores for years. And unlike many plant diseases, the main advice from plant pathologists is not "treat it" but "stop planting the susceptible species."

I don't grow impatiens at my Long Island property, but I've watched this disease flatten the shade-annual plantings at a number of properties in my neighborhood. This guide draws on Cornell Cooperative Extension, Penn State Extension, and University of Massachusetts Amherst research.

The pathogen

Impatiens downy mildew is caused by Plasmopara obducens, an oomycete specific to the genus Impatiens. Per Cornell Cooperative Extension, the disease was known in Europe from the mid-20th century but became epidemic in North American commercial greenhouse production around 2011–2012, and spread widely into home gardens through infected transplants.

Per UMass Extension, P. obducens produces oospores (thick-walled resting structures) in infected plant tissue; when infected plants die and decompose in the soil, oospores are released and persist for 5 or more years. This long soil persistence means that once a bed is infested, replanting I. walleriana without some form of soil treatment is essentially guaranteed to fail.