Impatiens Downy Mildew Update

Chris Beytes

As journalists, we often cover the big story and then, easily distracted by shiny objects, get lured away to the next big thing. To counter that, I try to drag myself back for regular peeks at the last story, to see what’s changed.
 
That’s the case with Impatiens downy mildew, which has knocked an estimated 50% to 60% out of the worldwide Impatiens walleriana business (as much as 80% in the UK and 40% in the U.S., according to one seed company expert). Yet I forget to talk about it!
 
So I asked Ball’s in-house pathologist, Dr. Colleen Warfield, for an update on IDM in the U.S. She replied back with notes from various sources:
 

• In Florida, they usually see IDM starting in late December. Pathologist Aaron Palmateer of the University of Florida didn’t find any disease until March, coinciding with a strong storm that moved through with cool temperatures and lots of wind and moisture, he told her.
• In Maryland, two pathologists who had planted sentinel plots (specifically to watch for IDM) reported seeing the disease in June. Also in June, it was spotted in Delaware on Impatiens balsamina (which re-seed in the bed and come up every year).
•  At Michigan State University field trials, sporulation was noted [in September] in one bed, but not at all in another bed at a different location.
•  In New York, pathologist Margery Daughtrey mentioned some downy mildew had been spotted in a landscape around the middle of August.
• In Tennessee, there was a report of IDM in the landscape also in mid-August, but a sentinel plot planted by a pathology professor was not infected as of this week.
• Ball Seed sales rep Craig Yoshida reports not seeing any signs of the disease in the Pacific Northwest.
• In Illinois, Colleen didn’t see any IDM sporulation on the impatiens in her own yard until September 5. Ball’s Will Healy reported seeing IDM in his yard in mid-August. One bed had heavy sporulation; another, next to the house, had no symptoms.

 
So as Colleen has illustrated, IDM is still present and plants did become infected in at least three regions of the country, but there were very few reports of the disease. Says Colleen in late September: “It’s hard to say whether that is due to fewer impatiens in the landscape, whether people now recognize what it is and don’t send [samples] to labs anymore or the later occurrence this year means that the plants are just now starting to show signs (in another two to three weeks, I expect the plants in my yard will start to lose their leaves). So, like previous seasons (except 2012), this is more of a late-summer disease in the Midwest and a mid-summer disease in the Northeast.”
 
Meanwhile, we asked growers and retailers to report in on their own sightings and views on the topic:
 

“I keep the coral Impatiens walleriana in my greenhouse and now treat it like the exotics that grow with them. I take cuttings and plant seeds from those plants and place in several areas outdoors here in the summer, under many conditions. This year, I found downy mildew in one shaded area around July 20 … then lots of it in mid to late August in adjacent areas receiving more light and air flow. I lost most of my plants (except for a few seedlings) in those areas, but still have them thriving in other areas with lots of sun and air flow, specifically fans.”
 —Michael Bowell, Create A Scene, Malvern, Pennsylvania

 

“We no longer grow them and recommend to our customers not to grow them. I don’t see any in landscapes around here in North Carolina. There are too many other plants to grow without growing and selling a problem plant.”
 —Judy Mitchell, Mitchell’s Nursery & Greenhouse, King, North Carolina

 
“Coastal New Hampshire, no sign of IDM in the beds and window boxes I see on my ride to work. Surprising, but it has been very warm and dry.”
 —Beth Simpson, Rolling Green Nursery, Greenland, New Hampshire

 

 “My impatiens at home in Lynden, Washington, just started showing IDM last week—about a month later than I saw two years ago when it first started to show up in our area. Last year, I didn’t plant any impatiens in my landscape to see if a year off would have any benefit. Whether the oospores lasted the two years or my plants were infected from a neighbor is hard to tell, but given that my plants looked beautiful until mid-September, I’m not complaining!”
 —David Vos, Vander Giessen Nursery, Lynden, Washington GP