comments

Ask the Master Gardener: Putting impatiens in the garden this spring? Be on alert for downy mildew

By: Trish Grenfell, Placer County Master Gardener
-A +A

Question: I recently read in a national gardening blog that impatiens may be hard to find this year due to a disease. Is that true? Has it affected this area? If so, what flowers can I now plant in the shade?
Answer: Yes, an aggressive downy mildew (Plasmopara obducens) specific to ordinary impatiens (Impatiens walleriana) decimated many plantings of impatiens on the East Coast and spread across many states last fall. Washington, Oregon and the central/southern coastal regions of California were also impacted.
This downy mildew is a destructive foliar disease that causes complete impatiens defoliation or plant collapse, especially in landscape plantings under moist conditions and cool nights.  P. obducens only affects I. walleriana and its specific hybrids and is a new pathogen in our gardens.
I can find no reports of the disease in central California and our climate tends not to be conducive to it, but if we experience rainy, cool weather this spring, and folks inadvertently bring home diseased impatiens and plant en mass, we could end up with our own epidemic.
Our normal summer heat should curtail this downy mildew, but if folks overwater and irrigate at night, perhaps summer impatiens could be affected also.  Many states are monitoring their nurseries to help prevent the sale of infected plants and big growers may be pulling back on growing impatiens this year anyway due to last year’s losses
If you do purchase impatiens from a local garden center, ask about the health and origin of the plants. Then watch these plantings carefully, removing any diseased impatiens and sealing them in a plastic bag immediately.
Treat all nearby plants the same, whether they appear infected or not.
Note: Once a plant shows any symptoms of the disease, there is really nothing you can do to save it from complete defoliation and death. Effective fungicides only work for prevention and those are available only to commercial growers.
Early symptoms (in order) include slight yellowing of leaves, downward leaf curling, and white downy-like growth found on the underside of the leaf.
Since the disease is not seedborne, seed-grown impatiens are guaranteed initially to be disease free. Or try these colorful plants to replace your impatiens this year: wax begonias, pansies, lobelia, torenia, caladium, coleus, white alyssum and New Guinea impatiens, which are resistant to this downy mildew.
Resource: Western Plant Diagnostic Network , Richard Hoenisc, rwhoenisch@ucdavis.edu, https://wpdn.org