Downy Mildew on Impatiens

Impatiens are the go-to shade plant for color here in the Memphis area.  At the Tennessee Master Gardener Winter School this month, we were alerted to a disease that is making a comeback:  Downy Mildew.

Underside of leaf

Infected plant

Downy Mildew is caused by a fungus, Plasmopara Obducens.  It was first detected in the UK in 2003 and in Tennessee in 2004.  In 2011, however, widespread regional outbreaks were observed in the United States.  Growers are particularly concerned by an outbreak in Palm Beach County this fall and winter. 

Initially, leaves may look yellowish or speckled. As the disease progresses, whitish downy growth, which is the spores that spread the disease, will be visible on the undersides of leaves.   Chemical treatments such as Ortho Max Garden Disease Control can be applied to prevent the disease but are not effective for plants that already have it.  Remove infected plants and bag them. Check impatiens before buying. Don't buy any that have a powder-like substance on the leaves' undersides.

Downy Mildew attacks Impatiens walleriana, the most common species.  SunPatiens and New Guinea impatiens are not susceptible.  Also consider planting other flowering plants such as begonias, lobelia and osteospermum.



Begonia



Lobelia

Osteospermum