MSU Extension

Editor's note: On Jan. 21, 2013, Part II of this series has been removed from the MSU Extension website.

This is the first article in a series about impatiens downy mildew for the commercial grower. In this first article, we ask Michigan State University Extension’s ornamental plant pathology expert, Mary Hausbeck, some questions about the biology of this disease.  The other article asks Dr. Hausbeck questions about how impatiens will perform in the landscape and what homeowner or landscaper options for impatiens downy mildew may be available.

Question #1: Could you give us a brief background on the biology of impatiens downy mildew?

Answer: Impatiens downy mildew is caused by the pathogen Plasmopara obducens. This pathogen only infects Impatiens walleriana and garden balsam (Impatiens balsamina). Impatiens downy mildew produces sporangia, or the white fluffy material, on the underside of impatiens leaves that can spread through the air or water. Cool, moist conditions (59 73 degrees Fahrenheit) are the conditions most likely for sporangia formation. Newly infected plants may harbor the pathogen without showing sporulation on the undersides of leaves for a period of time, usually five to 14 days. Impatiens downy mildew disease symptoms include yellowing of foliage, white sporulation on underside of leaves, plant stunting, plant collapse and severe defoliation. In addition, impatiens downy mildew also creates oospores (pronounced oh-oh-spores) in the plant tissue, which serve as dormant structures that allow the disease to survive outside the plant for long periods of time.

For more detailed information on the biology, symptoms and preventive program for impatiens downy mildew disease, see the following MSU Extension articles:

• Downy mildew on impatiens becomes widespread in Michigan landscapes
• Impatiens downy mildew prevention and management
• How to manage impatiens downy mildew in the landscape

Question #2: Is impatiens downy mildew the same as basil downy mildew?

Answer: No. Downy mildew is a common name that is used for a whole host of pathogens. Impatiens downy mildew’s formal name is Plasmopara obducens. This is not the same as basil downy mildew, whose formal name is Peronospora belbahrii, nor is impatiens downy mildew the same as cucumber downy mildew or sunflower downy mildew. Impatiens downy mildew is specific to Impatiens walleriana and Impatiens balsamina (garden balsam). New Guinea impatiens are not susceptible to impatiens downy mildew.

Question #3: Is impatiens downy mildew seedborne?

Answer: Impatiens downy mildew is not considered to be carried on or in the seed coat.

Question #4: How long can impatiens downy mildew remain latent in a plant? In other words, how long can the disease exist in the plant, but not express any disease symptoms whatsoever?

Answer: It entirely depends on the environmental conditions. Downy mildews can infect the plant tissue on a microscopic level and then lay quiet in the tissue. The infection can then increase rapidly in the tissue (e.g., explode) when the conditions are favorable. The weather conditions that favor downy mildew include moist or wet weather and cool or moderate temperatures.

From a grower’s standpoint, their plants can look healthy and then when there is a cool, wet period in the greenhouse, the white sporulation, which is the visible part of the disease, will appear overnight. It looks like the disease came on suddenly, when in fact the infection had occurred earlier but had not progressed and was quiet in the tissue until favorable environmental conditions occurred.

Question #5: Are there any genetic differences among impatiens lines in regards to resistance to impatiens downy mildew?

Answer: No differences have been found yet. Companies who breed impatiens have noted that at this point in time, all lines of bedding impatiens appear to be equally susceptible to impatiens downy mildew.

Question #6: Is there a strain of impatiens downy mildew that is more virulent (i.e., that is more likely causing the disease)? Is that why we are seeing a larger outbreak this year (2012) compared to previous years?

Answer: It is possible that some strains of impatiens downy mildew are more virulent than other strains. At this point, this type of information is not known. In general, downy mildews are very good at adapting to fungicides and they produce a lot of spores. Anytime that you have any organism producing a large number of spores, there are increased odds that a mutation may occur, resulting in a more virulent strain or one that is able to overcome a particular fungicide.

Question #7: I had a trial bed of impatiens last summer that were infected with impatiens downy mildew. The plants defoliated and then grew back. Was the pathogen still in the plant?

Answer: Yes. Impatiens downy mildew may not kill the plant immediately, but first affects the leaves that will drop from the plant. If the weather becomes hot and dry, the impatiens downy mildew may not progress further for a period of time, thereby allowing the plant to temporarily recover and produce new growth.

Question #8: Since impatiens downy mildew can spread via airborne spores or water-splashed spores, which one is more problematic?

Answer: It depends. You need to think like a pathogen. If I’m an airborne downy mildew sporangia and I’m traveling via air currents and it is really hot and dry, I’m not likely to live very long while I travel in search of new impatiens plantings. It is likely that I will shrivel up in these unfavorable conditions and won’t have the energy needed to infect a new impatiens should I land on one. In this scenario, I’m not likely to cause an outbreak.

However, if I’m an airborne spore and the environmental conditions are foggy, maybe with some light rain and cool temperatures, I’m going to live longer and while I’m being carried on the wind I will have plenty of energy to initiate a germination tube when I land on a new impatiens host. I may cause an outbreak in this scenario.

If I’m a swimming zoospore, I cannot travel far because I can move only via splash droplets. So I must look for nearby impatiens plant to infect (i.e., one that is within splash droplet distance). However, since zoospores are produced in high numbers when the conditions are wet, disease can increase dramatically and quickly.

Related articles

• Ask the plant pathologist about impatiens downy mildew: Part III – Landscaping

Dr. Hausbeck's work is funded in part by MSU's AgBioResearch.