Documents: Special Interest: In The Yard:

Problems With Impatiens This Summer; You Could Well Have The Same Problem Next Year
by Art Drysdale
by Art Drysdale

email: art@artdrysdale.com

Art Drysdale, a life-long resident of Toronto and a horticulturist well known all across Canada, is now a resident of Parksville, British Columbia on Vancouver Island, just north of Nanaimo. He has reno-vated an old home and has a new garden there. His radio gardening vignettes are heard in south-western Ontario over radio station Easy 101 FM out of Tillsonburg at 2 PM weekdays.

Art also has his own website at http://www.artdrysdale.com


September 11, 2011



Above: Barbara Draffin’s healthy Impatiens in her ‘Picture 2028.” Right below that is a shot of how the majority of her Impatiens look this year; Below: an RHS shot of a typically Downy mildewed Impatiens.

This week Donna Dawson sent me a question from Barbara Draffin of Scarborough regarding problems with Impatiens annuals basically not performing, and many dying off. Here was her original question: “What happened to impatiens this year. They have either rotted at the ground or at the top of stems? Even my sturdy, 12 inch tall impatiens have done this for the first time. Also neighbors have had this trouble as well. Can you help?”

My answer was I have no idea. Ours have done fine here, and I saw plenty of good ones while I was in Toronto and in Niagara. This type of question always needs more info than is supplied; e.g. where did they buy them; did the neighbour buy at the same place; were they planted where impatiens were planted last year; when did this all start; etc. etc.

A few days later Barbara responded as follows: “My small impatiens were purchased at a local store (not a garden centre). The neighbours who have had this problem did not purchase where I did and they also had the problem of rotting. The small impatiens were lovely until the very hot weather hit and when they looked thirsty I watered them. Then I wondered if I watered them too much. The main stems were rotting and one by one the plants died. They also wilted off at the top of stems.

“The large, sturdy, double impatiens were grown over the winter from slips from last summer. I have been doing this for many years without a problem. However, this year, they too developed this problem. Some were near the smaller ones but many were not. Only one very isolated one has grown properly. Picture 2028 shows what the large ones should look like.

“I am hoping to take slips again to grow for next year and hope I don't have this difficulty so I would appreciate any suggestions and explanation you may have.”

In the U.K. growers and gardeners have been reporting a great deal of problems with Impatiens this year, and it is said to be caused by downy mildew (Plasmopara obducens). I should mention at the outset that this is not the same disease that we see so often on phlox, roses, lilacs and grapes--that is known as powdery mildew, which is a common name for a fairly large number of fungal diseases on a large number of genera..

Here is what the U.K.’s respected Royal Horticultural Society has to say about the disease, and I think Barbara is not going to be too happy to read this!

“It was found for the first time in the UK in 2003 and is likely to have arrived on imported commercial propagation material (seed or cuttings). Statutory action was at first undertaken by Fera [a government agency] against confirmed outbreaks of the disease, but this soon ceased. After the wet summer of 2008, damage was much reduced by improved control practices at commercial nurseries. However in 2011 control failed, probably due to resistance to commercial fungicides used. Infected plants were inadvertently sold widely. This led to the most widespread outbreak of the disease so far, with many gardens, nurseries and local authority displays affected.

Outbreaks of downy mildew have been confined to Impatiens walleriana, the common bedding busy Lizzie. No cases have been found on New Guinea Impatiens, Impatiens × hawkeri, or on the few species of Impatiens found growing in the wild in the UK, including Himalayan balsam.

“You may see the following symptoms:

  • On leaves: Affected leaves turn yellow and are rapidly shed from the plant. A fine white fungal growth may be visible on the lower leaf surface, but affected leaves decay rapidly.
     
  • On flowers: Flowers are also commonly shed, and the plant is often reduced to bare branches with a small tuft of yellow leaves and flower buds at the tip. Severely affected plants will eventually die.

“There have been some reports of variations in disease severity between different varieties, but these have yet to be confirmed.

“Affected plants should be disposed of as soon as possible. Do not compost them. Ideally burn them or bury them deeper than 50 cm (20 in). Although the spores should not survive the commercial composting used for [municipal] council green waste collections, it is best to deal with contaminated material within the garden.

“Because of the risk of soil contamination, rest affected areas from Impatiens for at least a year (some species of Plasmopara affecting other plants produce resting spores that can survive for several years).

“Where infected plants have been grown in containers, replace the [soil] compost and wash and use a garden disinfectant, as directed by the manufacturer, to cleanse the container if you intend to grow Impatiens in it again the following year. The disease is specific to Impatiens, so any other bedding plants can be grown without risk. Semperflorens-Cultorum begonias and bedding fuchsias perform well in the shaded areas for which busy Lizzies are invaluable.

“Raising plants from seed will eliminate the risk of purchasing infected plants. This type of disease often has a lengthy ‘latent period’, when plants are already infected but not yet showing obvious symptoms. The advent of fungicide-tolerant strains of this disease increases the risk of introducing disease when buying plants.

“Unfortunately, growing Impatiens, even ones that have been raised at home or in another part of the garden will not guarantee freedom from infection, as the disease may well arrive again as airborne spores from infected plants growing elsewhere. No resistant cultivars are yet available.

“There are no fungicides available to amateur gardeners for the control of this disease.

“Downy mildews are a large group of plant diseases caused by microscopic fungus-like organisms related to the pathogen that causes tomato and potato blight. Despite a similar name and certain similarities in symptoms, they are unrelated to the powdery mildews.

“The disease is spread by spores produced on the underside of infected leaves. These spores are splashed by rain, and are also carried for long distances on the wind. Extended periods of leaf wetness are required for spore production and infection, so severe outbreaks of downy mildew are only likely to occur during wet summers.” [This is the only question I have—did Ontario have a summer that could be described as ‘wet’? I know it was hot but could it have been considered ‘wet’?

“The airborne spores remain viable for just a short time, but it is thought that the fungus can also produce a second spore type (a resting spore) within the affected plant tissues. These resting spores are much more resilient, and are released into the soil as the diseased material rots down. They are likely to survive within the soil for an extended period.”

So, Barbara, it looks as if you better forego Impatiens next year, unless you wish to raise them from seed, and that could by chancy since some disease spores might remain in the soil. If that was the case, it might be the plants from seed could avoid a problem from spores in the soil, at least for the first year, but there is no guarantee of that.

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