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.