Questions and answers about impatiens downy mildew.
Q: What is this disease?
A: Impatiens downy mildew is the common name for a disease caused by a fungus-like organism called Plasmopara obducens. It can infect and kill bedding impatiens.
The disease can be spread by wind or water, and it can also live in the soil where infected plants have grown. It first shows up as a downy white growth on the underside of the leaves and can quickly cause the plant to turn yellow, lose its leaves and collapse.
The disease affects mainly the species Impatiens walleriana, the familiar impatiens grown as annuals in our area and planted in gardens and containers. It does not infect New Guinea impatiens or the hybrid SunPatiens.
Q: I grow cucumbers in my garden. Could my impatiens give them downy mildew?
A: No. Downy mildew is a common name given to a number of diseases that affect different plants. It’s a little like the term flu, which is sometimes used for the respiratory ailment influenza and sometimes for an upset stomach.
The various types of downy mildew are caused by different organisms. The organism that causes impatiens downy mildew infects only certain kinds of impatiens.
Q: Is there a way to tell whether my impatiens will get the disease?
A: You should look under the leaves for white growth before you buy or plant impatiens, but even if the plant looks clean, that’s no guarantee it’s unaffected. The organism can be present in a plant long before the first signs of disease show up. The pathogen needs moist conditions to flourish, so a plant that seemed perfectly healthy might suddenly become sickly during a rainy spell.
Q: What can I do to prevent the disease?
A: Some fungicides are available to growers and landscapers that can prevent downy mildew. Unfortunately, the fungicides that researchers consider most effective are not available to home gardeners.
Fungicides can only prevent impatiens downy mildew, not cure it. Once a plant is infected, no treatment will save it.
Q: I heard it’s OK to plant impatiens in containers but not in the ground. Is that true?
A: Impatiens planted in the soil where infected plants have grown are at particular risk, because the disease-causing organism can live in the soil for years. But no bedding impatiens are completely safe. Disease spores can be carried by wind or splashing water and can spread to plants in the ground, in pots or in hanging baskets.
Q: What should I do if one of my plants gets the disease?
A: Remove the plant immediately and throw it away in a closed plastic bag. Avoid planting impatiens in that site for at least a few years.
Don’t yank a plant just because it looks sickly, however. Symptoms such as yellowing and leaf drop can be caused by other factors that are easily remedied, such as too much or too little water. If you suspect infection, look under the leaves for the telltale white growth.
Q: Can I put diseased plants in my compost pile?
A: That’s not a good idea. Most backyard compost piles don’t get hot enough to kill the pathogen.
If you do compost the plants, don’t use the compost on or near healthy impatiens.
Q: I really love impatiens. Should I avoid planting them?
A: Only you can make that call. Just recognize that you’re taking a risk of losing plants.
You’d be wise to keep a close eye on your plants for signs of the disease. Especially if you’re planting impatiens on a large scale, you might also want to have a landscaper treat your plants with fungicide on a regular basis.
Sources: Jim Chatfield, Ohio State University Extension; Michigan State University Extension; American Floral Endowment; Cornell University Cooperative Extension; University of Massachusetts Extension.
Mary Beth Breckenridge can be reached at 330-996-3756 or email@example.com. You can also become a fan on Facebook at http://tinyurl.com/mbbreck, follow her on Twitter @MBBreckenridge and read her blog at www.ohio.com/blogs/mary-beth.