The Purdue Plant and Pest Diagnostic Laboratory

What's Hot at the P&PDL
March 28, 2012

Downy Mildew - A Stealthy Disease on Impatiens

Gail Ruhl, Sr. Plant Disease Diagnostician, Purdue University

High humidity paired with cool nights creates the perfect conditions for the spread of Downy Mildew infections on Impatiens (Impatiens walleriana). Downy mildew sporulates on the UNDERSIDES of leaves and thus may go undetected until it is too late to save the plants. Infected plants exhibit 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 and may 'melt' down quickly. Spores are airborne and spread easily from plant to plant.

If your impatiens appear somewhat unthrifty in appearance, turn over the leaves to check the the lower surfaces (Figure 1) for the white fuzzy growth of downy mildew. This fungus, Plasmopara obducens, was first reported in the U.S. in 1942 and occurred only sporadically on impatiens. In recent years it has become an annual occurrence and cause for concern in bedding plants in both the greenhouse and landscape environments.

For more information on the disease, read MSU Extension News article Downy mildew on impatiens: disease detected in Florida. For recommendations on fungicide programs, read Impatiens downy mildew prevention and management

Click image to enlarge

Closeup of underside of leaf

Figure 1

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service