Downy mildew on impatiens has been reported in Michigan for the first time, based on studies at the University of Michigan. Downy mildew is a pathogen caused by a fungal-like microscopic organism called Plasmopara obducens. It has been reported in the United States, Canada, Asia, Europe and India. It has not been a persistent problem in the United States because of its sporadic reports. The last time it was reported on impatiens in the United States was in 1982. It was also reported in Canada in 1997 and most recently in the United Kingdom. In the United States, the disease has been found in Montana, Minnesota, Wisconsin, Mississippi, Missouri, Indians, West Virginia and the Northeast.
Crops most commonly susceptible to downy mildew are snapdragon, rose, alyssum, pansy and salvia. Other crops such as cineraria, gerbera, lisianthus, ranunculus, larkspur, anemone, garden balsam and impatiens can also become infected but are less frequent. The downy mildew on snapdragons cannot damage roses, nor can the downy mildew on impatiens spread to any plant outside of the Impatiens family. Plants in the Impatiens family include bedding plant impatiens, New Guinea impatiens, vegetatively propagated impatiens and garden balsam.
Downy mildew can be very destructive if not detected early. Disease symptoms may be localized within the plant without causing severe injury. Leaf spots are "local infections" and have mild symptoms because the downy mildew is limited within the plant. However, if downy mildew advances and invades the internal or vascular system of the plant, the disease becomes "systemic" and more destructive.
Mary Hausbeck, professor and extension specialist for the department of plant pathology at the University of Michigan, says, "Given the widespread losses that the growers in the U.K. suffered just a few months ago, I think we must assume that losses from this disease have the potential to be large. If growers don’t catch it at its early stages, it could spread and make plants unsaleable."
The most obvious signs of downy mildew are clumps of spores that form white, grayish fuzz on the underside of leaves. The symptoms often go unnoticed because growers are not accustomed to turning over leaves to check for plant problems. Plants become stunted, and leaves will curl downward and become a discolored yellow or pale green. When young plants and seedlings are infected, they most likely will not survive.
A small amount of downy mildew can form over several days or weeks and remain unnoticed until the environment is favorable for rapid production of spores. The spread of downy mildew is most rapid in wet, humid weather. A collection of water on the plant’s surface for more than six hours allows the downy mildew pathogen to germinate and infect. Humidity should stay below 85 percent to prevent condensation from forming.
Downy mildew has a thick-walled survival structure that allows it to persist in soil, growing media, or diseased plants for years. The disease can spread from one greenhouse to another by air current or splashing water. All diseased plants should be disposed of immediately, including the growing media and the pot. Even adjacent plants should be disposed of, even if they appear healthy. Hausbeck says that this is difficult for growers to do, but "They must believe that if they have found plants with that white mildew underneath, it has already spread to nearby plants, and it’s just a matter of time until those plants are infected."
Upon first receiving impatiens, the plant should be scouted immediately by fully examining the leaves, paying close attention to the undersides of leaves. Since plants may be infected with downy mildew but won’t show any symptoms, they should be scouted weekly. Hausebeck recommends examining impatiens very closely every week because the symptoms might not show up for three weeks, and because of weather conditions, growers would see the white mildew on the underside of leaves when they wouldn’t have noticed it before.
Hausbeck wants growers to implement a preventative spray program this year, even if they don’t find the mildew, until the threat is gone. "You have to take drastic action early on, and apply fungicide to all impatiens on the premises. Even if they are in a different greenhouse or on a different range, any impatiens on the premises should be treated with the fungicide for the duration that the greenhouse has them on site."
Systemic fungicides are the most successful way to manage the downy mildew pathogen because these products are absorbed by the plant and can help fight newly established infections. Fungicide sprays are best started prior to disease outbreak, acting as a preventative barrier. Mancozeb (Protect T/O, Cleary Chemical) is the best protectant fungicide for this disease. Systemic fungicides include mefenoxam (Subdue Maxx, Syngenta) and dimethomorph (Stature DM 50 WP, SePRO). Hausbeck says, "There are not that many products that would be helpful to downy mildew, so if growers don’t stick with the product that’s most helpful, they’re going to have problems controlling it, and that would be unfortunate."
According to the Department for Environment, Food and Rural Affairs (DEFRA), the actions required to reduce the outbreak of the downy mildew pathogen are: destruction of severely infected plants; treatment of the remainder with chlorothalonil; adoption of hygiene precautions regarding movement of potentially infected plants between and within greenhouses; and measures to reduce humidity, increase ventilation and reduce irrigation.
Dr. Hausbeck emphasizes, "Early detection and a rapid response can avert the devastation that some U.K. growers experienced as a result of downy mildew. U.S. growers must be vigilant and be prepared to take quick action…This is where early scouting, vigorous scouting, vigilance, and consistent spraying of plants when it’s found is so important."
For more information on this outbreak, see the March issue of GPN.