Where did Plasmopara obducens come from?
This fungal-like defoliating disease, also known as downy mildew, was known to experts in the 1880s, and was observed in a few places starting in 2003 in Europe and the UK and in the US in 2004. P. obducens did not show up in more than an occasional, isolated outbreak until 2011, when it began to devastate landscapes and nurseries across the continent. In summer 2011 it was recorded in landscape plantings for the first time. By January 2012 there was a large outbreak in the greenhouses of southern Florida, affecting plants which were destined for Big Box stores all over the eastern seaboard. As July 2012 came to a close, P. obducens or downy mildew was reported in 25 states and Washington DC, and by winter 2012-13, it was present in 35 states in the continental US. as well as a few Canadian provinces.
We don't know whether P. obducens mutated
Impatiens Downy Mildew may have changed into a more virulent form or the epidemic could just be a combination of bad weather and lack of awareness on the part of growers. It is a water mold, meaning water is necessary for it to complete its life cycle. P. obducens can reproduce in only five days. The best prevention is keeping your Impatiens plants dry, with good air circulation.
Does it over-winter in the soil?
Other mildews and funguses live in the soil from year to year. We don't know how long P. obducens can live in the soil. Some researchers advise that you avoid replanting I. walleriana in a location with infected plants for two to five years. Others point out that the downy mildew that affects sunflowers lives for 8-10 years in the soil. Will freezing temperatures kill P. oducens? Unknown.
How is it spread?
P. oducens can be transmitted in three basic ways.
- Through infected plant material. Plants grown from cuttings which are infected will also be infected themselves. Destroy all plant material contaminated with P. oducens.
- Through water/droplet. If P. oducens is in the water, it can spread to healthy plants. If I. walleriana are growing in very moist or humid environments, the pathogen can spread through water condensation. Don't water at night and it's preferable to water the roots instead of the leaves so there isn't water resting on the leaves.
- By transmission of oospores. The fungus-like P. oducens reproduces via tiny spores (called oospores, oh-oh-spores). Oospores are thick-walled spores from sexual reproduction, meaning this pathogen can mutate. The oospores don't just travel by water, they travel on the wind; one source says to "nearby plants" while another says "for hundreds of miles.'' They can be viable at least three years in soil, even though frozen, and their true longevity is unknown.
What should I do if I see signs of P. obducens?
Click here for excellent pictures of Impatiens Downy Mildew. As with Late Blight (which affects tomatoes and potatoes), once a plant is infected with P. obducens, it must be destroyed. Do not compost plant material, and thorougly disinfect contaminated tools. You cannot save the plant by pruning it or treating it with anything. It is better to be vigilant with preventive measures, like ensuring that relative humidity does not stay above 85%.
how can Downy Mildew be prevented?
There are indications that P. obducen is less active during hot, dry summers. It is not transmitted by seed. So impatiens that you start yourself, from seed, are the best choice. You may still have time to plant seeds for displays this year. However just because you start with healthy plants does not mean that they will not become infected later on. And considering that it is a host plant for an epidemic....use your judgement.
What else should we do?
There are treatments with heavy-duty fungicide drenches which home gardeners do not (and should not) attempt, but which have had success in experimental trials. But even if you leave the store with I. walleriana plants which escaped infection, they may be contaminated by the time you plant them. Downy mildew does not respond to treatment with the usual home gardener's remedies like copper sulfate. if you do buy plants, be sure the grower followed a regimen of fungicide application.
Other Downy Mildews
Other species can be infected by organisms referrred to as "Downy Mildews," but these pathogens do not jump from species to species. Downy mildews are parasites; they cannot exist without a host plant. In other words, I. walleriana is the only host for P. obducens. Wheat Downy Mildew, Sunflower Downy Mildew, Hops Downy Mildew and Grape Downy Mildew are each caused by its own species-specific parasite.
The yearly crop of I. wallierana has been valued at $130 million. Depending on their experiences with the devastation of 2012, greenhouses will now probably offer fewer I. walleriana and more of other plants that are suited for similar conditions, such as coleus, browallia, nemophila, torenia, mimulus, perilla and caladium. Furthermore, not all varieties of Impatiens are vulnerable! New Guinea Impatiens (Impatiens x hawkeri), including the SunPatiens® series, have not been reported to be susceptible to the P. subducens pathogen.
Annuals for shade: look on the bright side
If you find that you were in an impatiens-growing rut, this is your OPPORTUNITY to experiment with the large variety of summer annuals which can brighten your shady corners! If and when impatiens become available as widely as they have been for the past 50 years, your new combinations will be unbelievably gorgeous. If you have never grown annuals from seed, this is your chance. Click on this link which will take you to all 60 vendors of seed for annuals listed in the Garden Watchdog. And this article gives tips on starting annuals from seeds; many of the plants listed above can be successfully grown from seed.
The thumbnail photograph of an Impatiens walleriana devastated by Downy Mildew is made available anonymously. Photograph of New Guinea Impatiens 'Sunpatiens Compact Magenta' c. LadyAsleyR.
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