We had our first confirmed report of downy mildew on Impatiens walleriana here is south Louisiana this weekend. My wife commented that one of the double impatiens that she’d gotten at one of the big box stores a couple of weeks ago was not looking all that good, and sure enough it has downy mildew caused by Plasmopara obducens. With the onset of warm, drier weather, I wouldn’t expect it to become a major problem, but our nighttime temperatures are still quite favorable and as long as we have 3- to 4-hour periods of dew on a regular basis, the disease will likely continue to develop. None of the fungicides readily available to homeowners are particularly effective in controlling the disease, so it’s probably best to remove and destroy any infected plants as soon as they are found.
Donald M. Ferrin, Ph.D., Louisiana State University Agricultural Center, Baton Rouge, LA