Fighting Flower Disease -- Pathogen Attacking Impatiens
By Nancy Bono / April & May 2012
Most Florida gardeners look forward to the early fall, so that they can plant the beautiful impatiens walleriana, members of the Balsaminaceae family. Impatiens adorn many community entrances and colorfully welcome back the snowbirds as they arrive at their homes, condos and town homes in Florida for their winter respite. Impatiens have been the one annual we could depend on to last through the winter holidays into April or May, offering us a cheerful array of colors in our garden beds.
In late 2011, a pathogen known as Plasmopara obducens, or commonly called downy mildew, took its toll on our crops of impatiens. The first detection of this disease was in Western Australia in 2007. The disease has since been found in other countries in Europe and even in the northeast and northwest U.S. in recent seasons. However, this is our first seriously harmful outbreak in South Florida. The disease was probably transported by seeds or small seedlings, purchased by South Florida growers.
Plasmopara obducens is a host-specific, fungus-like organism, also known as an oomycete or water molds. Oomycetes, which include pythium and phytophthora, are some of the most destructive plant pathogens, due to their swimming spores that thrive in wet conditions. They can spread with alarming speed. The incubation period for the infection to the impatiens is from five to fourteen days.
Earliest symptoms are a faint chlorosis or stippling of the leaves that eventually become completely chlorotic, appearing to be almost a neon or lime green. The undersides of the leaves are downy white or light gray with masses of sporangia. In a short time, the leaves fall off, the flowers fall away, and the infection leads to stunting and, eventually, a complete collapse of the plant. Unfortunately, by the time you notice the decline of the plants, it is too late to react.
Spores called oospores may survive in the plant debris and actually be released into the soil as the stems decay. These spores can rest in the soil for a period of years. Experimentation by the Ball Horticultural Company, other university research teams, and researchers in greenhouses have found that the oospores that remain can trigger a new outbreak of downy mildew in subsequent years.
Downy mildew is also spread by water splash and wind. Do not replant impatiens in the same area for several years. The good news is that downy mildew of impatiens does not spread to other plants outside the impatiens group. Fortunately, it has not been found in New Guinea impatiens.
Dispose of any Impatiens that exhibit signs of downy mildew by removing them from the bed, along with their associated debris. Place everything in a sealed bag for trash disposal. Do not compost infected debris.
Sanitize with Clorox any pieces of equipment that you may have used, such as shears or shovels. Consider all infected plants beyond any type of rescue.
If it has been several years since you have planted impatiens and you wish to try them in your garden once again, initially use a fungicide preventively by mixing it with the potting compost that you intend to use for planting. Mancozeb fungicide is easily available over the counter and can be used as a protective barrier. Other commercial products such as Subdue MAXX or Aliette can also be applied to the soil and/or sprayed over the new plantings for preventive controls.
If you do use impatiens as annuals once again, be sure to leave plenty of space between plants, and do not ever water after 4 p.m. Early morning watering is preferred for all your plants and turf grass. Please mulch around your annuals, but leave an eight-inch gap between the mulch and the stems of the plant. Question your supplier as to whether his or her seedlings are guaranteed to be free of the Plasmopara obducens pathogen.
Other annuals that may be used as substitutes in your color spots during the spring and summer are begonias, salvia, pentas, portulaca, purslane, torenia, celosia, marigolds, gomphrena, verbena, and periwinkles. Annuals for winter may include some of the previous and also geraniums and dusty miller. Annuals with the longest lives are begonias, purslane, and periwinkles, if not over-watered. Gallardia or blanket flower, a plant native to South Florida, is also a long-term annual.
Nancy Bono has been the owner of Nuturf Design and Land Works, LLC since 1994 in Margate. She is an internationally certified arborist, a master gardener in Florida since 2003, a certified landscape inspector and member of the Landscape Inspectorsí Association of Florida.
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