Ornamental Plant Downy Mildews

An overview of the research on ornamental plant downy mildews in the lab.

Impatiens Downy Mildew

The recent emergence of impatiens downy mildew disease (IDM) in the U.S. is causing major economic losses on a national scale. Impatiens (Impatiens walleriana) are extremely popular flowering annuals, with sales ranked 1st among bedding plants in the U.S. ($130 million annually). Millions of dollars have been lost to this destructive new disease.  Plants affected by IDM cannot be cured, and death occurs within weeks of infection. The emergence of the IDM epidemic is perplexing. Prior to 2003, IDM was not found on cultivated impatiens. However, the causal agent (Plasmopara obducens) is documented in North America since the 1880s on native wild relatives (I. capensis, I. pallida).

Rubeckia (black-eyed Susan) Downy Mildew

Downy mildew disease of Rudbeckia (Rudbeckia fulgida 'Goldstrum'; perennial black eyed susan; ) is caused by the destructive oomycete Plasmopara halstedii.  This organism has been described as the cause of downy mildew disease from at least 100 host plants in the Asteraceae family based on shared morphological traits.  The primary economic host of P. halstedii is cultivated sunflower (Helianthus sp.).  Recent destructive outbreaks of downy mildew disease on Rudbeckia attributed to P. halstedii have resulted in substantial losses of this popular flowering perennial in nurseries and landscapes. 

Septoria Leaf Spot on Rudbeckia

At first glance, downy mildew on Rudbeckia may resemble Septoria leaf spot. 
But Septoria leaf spot will not exhibit the diagnostic downy white masses on the underside of the leaf. 

Sunflower Downy Mildew

Downy mildew disease of sunflower is caused by the oomycete Plasmopara halstedii.  This disease is of increasing interest to specialty crop producers, since sunflowers  are frequently sold as annual ornamental plants to consumers.

Dr. Jo Anne Crouch



Mycology & Nematology Genetic Diversity & Biology Lab


10300 Baltimore Ave

Beltsville, MD 20705

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