Why Your Impatiens Die - Downy Mildew Kills Them
Downy Mildew is currently wiping out Impatiens plantings and breeding work across North America and this article is intended to give home gardeners enough information to make informed decisions about this disease and the alternatives they have.
First of All, What is Down Mildew
The pathogen Plasmopara obducens is specific only to Impatiens walleriana (pack impatiens) and Impatiens balsamina (Garden Balsam) . It does not infect other impatiens species such as New Guinea impatiens (Impatiens hawkeri)
It produces a white fluffy coating (sporangia) on the underside of the leaf and this spreads through air or water droplets. It is not thought to be spread on the seed.
It does best in cool, moist conditions (59-73 F) and can spread very quickly. Overcrowding of impatiens in beds creates perfect conditions for this disease to spread.
How Does It Develop?
Plants that are newly infected have the disease but don't show it for 5-14 days. After this time, the underside of leaves will start turning white as well.
Is That All?
Unfortunately, no. Once the white coating is established, the plant develops other symptoms including wilting leaves, plant collapse, severe defoliation and death.
Do All Varieties of Impatiens Get This?
All common varieties of the bedding plant Impatiens get it. There are no resistant varieties.
Part of the problem is that Downy Mildew is very good at adapting to fungicides and does so quickly.
How Does It Overwinter
In the soil and it can live there dormant for 5 years. So as long as there are impatiens around, it will re-emerge to infect those and continue the cycle.
Can Plants Recover?
If the weather was cool and damp and the plants stunt and defoliate - then the weather turns hot and dry, it is possible impatiens will regrow. They will still be infected but the plants will outgrow the mildew (that doesn't like hot and dry) . Once the weather returns to cool and damp, the impatiens will collapse again.
Is There Something I Can Spray?
Commercial growers have a wide variety of sprays they can use to "prevent" mildews (of all kinds) from becoming established.
But once the problem is established, there are no really effective treatments known at this time for home gardeners. You can try a copper based fungicide such as Bordeaux
or even Neem Oil
but don't bet the farm on these. Neither will be a "solution" although they might delay the inevitable.
Can I grow Impatiens in Containers?
Sure you can - until the plants are infected by blown-in spores. If that happens do not save the soil but toss everything into the compost pile or garbage.
Fresh soil every year may let you grow a small crop of this plant if you don't have surrounding infected gardens or soils.
How Long Does It Live in the Soil
Reports say it lives between 3 to 5 years in the soil.
So What You're Saying Is?
What I'm saying is that once you've had it in your garden, you can't grow Impatiens any more. Or at least not for 3-5 years or if anybody near you has it as well.
Once your neighbor has it - or anybody within your neighborhood has it, you won't be able to grow this plant for much longer.
One way to slow the speed of infection is to grow your own impatiens from seed. If you don't purchase infected plants then you may be fine for some time.
A video from a Plant Pathologist on Youtube.
(I don't have one to share with you)
Also - do not water at night. Night temperatures are cooler than high-heat day temps so avoid combining cool temperatures and damp conditions - the two things Downy Mildew needs to grow.
As a second note, should you see a plant that appears to be suffering (it will have that white coating on the underside of the leaves) immediately pull it up and discard into the trash. Cross your fingers you got it early enough.
Alternatives to Impatiens
You can use these plants to replace impatiens
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