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If you depend upon impatiens for all-season color in containers or the garden, you may have been disappointed this summer. If your impatiens went down fast, it probably wasn’t the fault of the grower, the garden center, the droughty season or your own failing.
The problem was a virulent strain of a water mold called downy mildew that has destroyed impatiens production in Europe and South Africa, and now has been confirmed in 20 states in America. Homeowners and businesses that expected masses of flowers instead saw masses of dying, ugly plants.
Not all impatiens are alike, and this disease only infects some of them. Impatiens walleriana is the victim, known to many people as the old-fashioned impatiens they buy in flats, or as balsam or jewelweed. Newer cultivars of I. walleriana such as double impatiens, Fusion and Spellbound are also susceptible. Fortunately, New Guinea-type impatiens are not affected, including the Sonic and Supersonic series, Sunpatiens and the Divine series. 2013 may be the year of New Guinea impatiens.
The disease story: All diseases require three elements to succeed: the pathogen, the right environment and susceptible organisms. In the case of downy mildew, the organism is the impatiens crop. Its aerial spores settle on the plants. In 2011, downy mildew exploded in Florida, and hurricane winds rapidly dispersed those spores along the East Coast. The environment for the spores to germinate is cool, moist conditions (temperatures from 59 to 73 degrees and either rain or overhead watering, especially late in the day). Since Western New York had a dry summer, and the mold thrived anyway, scientists shudder to imagine what could happen in a season with more cool, wet weather.
Impatiens’ downy mildew also produces oospores that can survive through winter on plant debris above or below ground. So any soil where infected impatiens have grown is probably unsuitable for this crop for many years.
What you saw: In greenhouses and garden centers, most impatiens looked healthy well into summer. In those controlled situations growers have products that manage disease outbreaks in early stages, and wind-blown spores aren’t likely to land on the plants.
When the impatiens flats or 4-inch pots came home – that’s when they were at risk. In two or three weeks, the plants showed pale green or yellowing leaves, some mottled foliage, and eventually wilting, stunted growth, distorted leaves, severe leaf drop and total plant collapse.
Light gray or white fuzz under some leaves is the definitive sign. Until word about this problem came from Cornell and horticulture industry scientists, local CNLP (Certified Nursery & Landscape Professional), garden center experts or master gardener volunteers might have diagnosed several other common problems of impatiens: underwatering, overwatering, lack of fertilizer or spider mites. Now we know.
Prevent and limit diseases: For home gardeners or landscapers, some general principles can minimize the effect of this or another disease:
• Avoid mass planting of anything. Mono-cultures incur risk of a single species problem; diversity is good.
• Water at the base of plants if possible, rather than sprinkle all the foliage.
• Water early in the day if possible, so the garden can dry out before nightfall.
• Thin out crowded plantings; lack of air circulation favors some diseases.
• Do not compost diseased plants; discard them in garbage bags or bury them in a deep hole.
In the impatiens case, plants in brighter areas fared better than plants in deep shade.
The future of impatiens: Wendell Ireland, greenhouse manager for W.D. Henry and Sons, will probably cut his impatiens crop in half for 2013 but isn’t giving up on them completely, since a lot of the gardening public still will want those flats.
“I think impatiens in containers won’t have as many problems as in the ground. And so much depends on weather conditions. Certainly, we’ll focus on some alternative shade garden choices ... In this business it’s always a guessing game,” he said.
Jill Kisker, head grower for Lockwood’s Greenhouses, will also cut back or cut out some of the susceptible impatiens lines in favor of more begonias, more New Guinea impatiens cultivars and some other less-known annuals. (Plan to be surprised next spring.) One industry adviser suggests we all should try using some of the so-called Part Sun plants even in shade, and we may be surprised.
Frank Mischler (Mischler’s Florist and Greenhouse, Amherst) also expects to cut back impatiens production severely, and his growers also have some ideas about great substitutes for color in shady areas. For him, the change isn’t as drastic as it might have been. Mischler explained: “After the October Surprise storm of 2006, many of our customers reported that their trees were gone, and so was their shade. So we stopped growing quite as many shade plants.”
Finally, the public will decide how many Impatiens walleriana are bought and planted. Many people won’t even know what hit the impatiens last season, so there will be some demand.
Cornell University plant pathologist Marge Daughtrey told an industry audience that “Impatiens avoidance behavior will largely be seen for the first time in 2013 ... The amount of inoculum available in the trade and in the garden beds will interact with the spring weather patterns to determine what will happen next year – and those are all unknowns.”
Sally Cunningham is a garden writer, lecturer and consultant.