DID YOUR IMPATIENS appear yellowed, weak, or even give up and collapse early this summer? (Thankfully, the pot above did not.) If there were any issues, don’t compost the plants’ remains this fall; bag and discard them–root system and all–in the trash. Impatiens downy mildew, a relatively recent but very serious fungus-like disease affecting plants in seven states in 2011, spread in 2012 from Florida up through the mid-Atlantic and all the way to the Northeast. Because the spores can overwinter in the soil even in Northern gardens, proper fall-cleanup sanitation practices are key to avoiding a repeat next year.
All Impatiens walleriana types are potentially affected. That’s the species that the common garden impatiens is bred from, as are the double-flowered types and miniatures and the Fusion series I like so much (that’s variegated Fusion Peach Frost, above). Other impatiens species may likewise be susceptible–even the native jewelweed, Impatiens capensis.
The updated Impatiens Downy Mildew factsheet from Cornell Cooperative Extension explains that early signs of infection may have been leaves that looked yellowish, as if the plant needed feeding, or foliage that curled under or seemed to wilt. Sometimes, you can see a white material (the fungus) on the undersides of leaves. Eventually, plants may defoliate, drop their flowers, and basically collapse. The American Floral Endowment website has detailed photos of symptomatic plants, and other links (most geared to growers) about this threat to a favorite bedding plant.