October 15, 2011
Some LO members are reporting that in their many years of business, they have never seen impatiens in such a poor state as this past summer.

No clear explanation was found among those dealing with the problem in the field, but one contributor blames the large amount of rain early in the season, which caused fungus in the soil. This was followed by extreme heat and drought.

Andy DeGroot, of Hensbergen and DeGroot in Markham, says that impatiens did not develop and spread as they have in past years, and there were areas where the plants simply disintegrated into nothing, leaving large bare patches. “We continue to be at a loss in providing our customers with a clear explanation.”

Even the scientists contacted by Horticulture Review are unsure of an answer.

Shannon Shan at the University of Guelph’s Pest Diagnostic Clinic, has had just two or three impatiens samples submitted this year, with a couple of root and crown rot pathogens showing up after culturing them.

Shan's thoughts are that perhaps the cool wet spring provided the perfect growing conditions for Rhizoctonia, a crown rot, and that members should be using crop rotation with ornamentals — just like growers do.

Ontario Ministry of Agriculture Food and Rural Affairs (OMAFRA) specialist Wayne Brown, who works out of Vineland Centre for Research and Innovation, looked at photos of dead impatiens sent by Horticuture Review. He said, “It is very difficult from the photos to say with any degree of certainty the cause of the defoliation. In the one instance, it looks like it might have been caused by Rhizoctonia, because the basal stems looked blackened, but Rhizoctonia does not typically cause defoliation. The defoliation is more consistent with either Alternaria Leaf Spot or downy mildew, but I can’t confirm based on these photos which of the two disease pathogens it might be.”

Brown added that watering the plants during the night, or very early morning would promote development of either disease, and also recommended planting something other than impatiens next year to allow over-wintering inoculum to diminish.

Michael Celetti, a plant pathologist with OMAFRA, thought the problem might be Pythium, a water mold, causing root rot. Celetti notes that Pythium can be managed in the greenhouse, but once the plants are installed in the landscape it is difficult to control as it is spread by water. To help control Pythium, landscape managers are better off to water lightly and frequently – which goes against the usual recommended practice of irrigating infrequently and deeply.

For some, it was puzzling that their customer’s impatiens had huge bare patches, while the garden next door had lush and beautiful plants. “We also planted early in the season on a property with just white impatiens. After a short time, those plants were simply not looking good, so we removed them. I planted another batch, but again the second planting still didn’t develop the way regular impatiens normally do,” said DeGroot.

It is highly recommended that members, who have questions with possible diseases in plants, should contact the scientists to find out answers to those questions.  Plant samples may be sent to the University of Guelph Pest Diagnostic Clinic, 519-767-6299, www.guelphlabservices.com.