Downy Mildew: A New Garden Threat
A new disease is threatening garden impatiens. Downy mildew, caused by the fungus-like
pathogen Plasmopara obducens, begins as a subtle yellowing of the leaves. This is then followed by flagging (greenhouse
speak for wilting and curling downward), sometime giving the appearance that
the plants need to be watered. In humid
conditions like we have been experiencing recently, you will see a white
coating of fungus sporangia on the underside of the leaves. Impatiens with downy mildew will gradually drop
their flowers and leaves, and eventually even the stems will die.
Downy mildew affects Impatiens
walleriana, the standard bedding plant, as well as double impatiens. However, New Guinea Impatiens are not
affected by this disease and no other bedding plants are hosts of this downy
mildew. So you shouldn’t worry about
downy mildew spreading to other plants in your garden like begonias, roses,
etc.
Impatiens plants can become infected by downy mildew either
by spores that overwinter in the garden soil or by spores spread from nearby
infected plants via water splash (short distances) or wind (potentially
miles!). Since the disease can
overwinter in the soil, impatiens should not be planted in beds where the
disease has previously been found.
Impatiens planted in containers or in unaffected beds are not risk free
because they can easily be infected by airborne spores. Consider alternative shade plants if you live
in an affected area.
The disease thrives in moist, humid conditions, and new
infections will occur when there is a thin film of moisture of the leaves for
at least a few hours. Once an impatiens
plant is infected it will not recover, though how fast the disease kills the
plant will depend on environmental conditions.
Fungicides labeled for downy mildew may offer some protection for
healthy plants, but would need frequent reapplication (as often as every 7
days) and this still might not effectively protect plants.
If impatiens are planted in your garden, watch for symptoms
like yellowing foliage, stunted growth, and white sporulation on the undersides
of leaves. If found, check with your
county extension agent for confirmation.
Once infected the only recommended action is to entirely remove and
dispose of infected plants and replace them with another plant.
Our Response to The Downy Mildew Threat
Here at Bob’s we have been working with our suppliers, other
growers, and some of the top brains in the horticultural industry to develop a
treatment program. Luckily, we are in a
region where the outside air temperature is too cold to allow airborne
infection during the bulk of our growing season (January to early May). This means that the plants leaving Bob’s are
disease free, but all bets are off once they get planted outside. They are then susceptible to airborne
infection during the warm summer months.
We have seen a shift away from Impatiens walleriana in our own wholesale production as our
wholesale customers focus more on alternative plants to offer their
customers. This year we saw a boom in
many other varieties production, and a drop in Impatiens walleriana, as growers look for alternatives.
As home gardeners, I would suggest looking at
downy mildew as we are here at Bob’s. It
is an opportunity to branch out and try new things! One day, if climate pattern change, some
experts have speculated that downy mildew could all but disappear.
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