With more than half of the country impacted by impatiens downy mildew in 2012, it’s a bit of a relief to hear that – so far – confirmed reports of the disease appear to be much lower in 2013. According to a presentation by Ball Horticultural Company’s Colleen Warfield and Cornell’s Margery Daughtrey at OFA Short Course, only Florida, Louisiana, Texas, California and most recently Missouri and Kentucky have confirmed reports in both greenhouses and in the landscape. A number of other states have reported the disease in greenhouses, but in most cases, it doesn’t seem to have made its way out into the landscape there.
It’s unclear why the disease hasn’t developed as widely this year, but increased awareness among growers, retailers and consumers may have helped blunt the spread of the disease.
One new wrinkle has been the discovery of infection in a species other than Impatiens walleriana: Impatiens balsamina. Balsamina is a plant that’s widely distributed in eastern U.S. and comes back in the spring. Infection causes a yellowing of the leaves, rather than complete plant collapse. There’s both good and not so good news in this discovery. Since plants come back, they can bring back the disease as well. This is a potential reservoir host outdoors that is keeping the disease out there. On the plus side, balsamina does have at least some level of resistance to the pathogen so it will be of interest to breeders and possibly help breed resistance into commercial walleriana varieties.
USDA is looking for a better understanding of impatiens downy mildew and other diseases and has started a program to collect samples from across the country. If you come across a plant infected with impatiens downy mildew (or boxwood blight, rudbeckia downy mildew, sunflower downy mildew or brown rust of mums), visit OrnamentalPathology.com to find out how to share a sample.