Downy mildew of impatiens has been observed in the United States since 2004, but became widespread and highly destructive in 2011. Cool wet weather likely contributed to the disease epidemic of 2011 since the downy mildew pathogen thrives under these conditions.
Plasmopara obducens produces tough survival spores called oospores that can overwinter in soil and plant debris. As a result if downy mildew was present in your garden last year, it is likely to show up again in 2012. Downy mildew can also move in on wind blown spores. Although it is not known how far the downy mildew pathogen can travel by wind, a close relative, downy mildew of cucurbits, has been shown to move over 600 miles in 48 hrs. Plasmopara obducens does not move on seed, but it could move into a garden on infected transplants. Inspect all new impatiens for yellowing of leaves, stunting and white fluffy growth on the lower leaf surface. Nurseries producing impatiens are well aware of the threat of downy mildew and are scouting regularly to find and control the pathogen.
M. Grabowski, UMN Extension
Photo 2: Downy like growth on the lower surface of an infected impatiens leaf
Once the impatiens are in the ground for 2012, inspect the plants weekly for symptoms of downy mildew. Yellowing and stunting of the plants are typically the first symptoms observed. Downy white growth on the lower leaf surface confirms the disease. If downy mildew shows up, promptly bag and remove infected plants to reduce spread to neighboring plants. Space plants to allow air movement between plants. This will help reduce humidity and leaf moisture. If beds were infected in 2011, consider choosing a different annual plant this year. Downy mildew of impatiens only infects Impatiens walleriana, the standard impatiens. New Guinea impatiens, impatiens hawkerii are highly tolerant of the disease. No other plants are infected by this pathogen, so Begonias, Caladiums and other shade tolerant ornamentals are good choices as well.