Back in 2010, I wrote a column warning gardeners about a potentially threatening disease moving up from the south and threatening to attack one of our most beloved herbs, basil.
Little did I know that another form of downy mildew was even more dastardly and would move into Ohio and wipe out one of the most planted summer bedding flowers, impatiens.
Plasmopara obduces, or impatiens downy mildew, was still a new threat last year when symptoms of the disease in those plants were reported from California to Indiana. But this year, the disease has become widespread across the country, traveling as far south as Florida and all through the mid-Atlantic and the northeast. And it isn't a pretty sight.
What were once lovely mounds of impatiens plants and flowers soon became sickly looking sticks with no leaves or flowers at all. Early symptoms showed yellow, wilted and curled leaves. Spores of the disease were evident on the undersides of leaves making them look as though they were covered with a white, fuzzy coat. Within days the leaves simply drop away. These spores are airborne and can easily travel from your neighbor's yard to your yard and vice versa. Not only do the spores travel, but they can be splashed from infected plants onto healthy plants. It doesn't seem to matter if you buy your plants already grown and blooming or if you start them yourself from seed. Downy mildew isn't particular.
Landscape impatiens, those hybrids we buy by the flats at local garden centers in spring, are the most affected, but horticulturists have noticed the disease on wild, native impatiens as well.
Impatiens are often used in shady areas that get a bit of filtered sun throughout the day. They are popular annuals for landscape designs because they bloom prolifically from spring through summer well into fall and they come in a wide variety of colors.
If your impatiens were infected this year, pull out the dead plants. Don't toss them onto the compost pile as the downy mildew fungus is believed to be quite adept at surviving the winter. Some experts say to ''bag them, bury them or burn them,'' but get them out of the garden as soon as symptoms appear. It is also important to remember not to plant impatiens in the same place next year.
According to Cornell University's Cooperative Extension Service, the best way to manage downy mildew on impatiens is with good management and prevention. Fungicides offer only short-term protection and if plants are infected, continual application is required making the fight against the disease a battle few gardeners would be anxious to fight.
Some ways to avoid infection, in addition to cleanliness, is do not water plants from the top, especially at night and don't plant too close together making sure the plants have adequate air circulation. Downy mildew thrives when the temperatures are cool and moist, which is what we have in the spring and the fall. During the hottest months of the year, and particularly this year when we had a virtual heatwave, once the disease is present, hot, dry conditions don't seem to slow it down.
Gardeners also are encouraged to grow something else instead of impatiens. Flowers that grow under similar conditions but are not susceptible to downy mildew include begonia, coleus and New Guinea impatiens, which don't seem to be bothered by downy mildew. New Guinea impatiens can even be planted in the same beds that once grew infected plants.
Don't confuse downy mildew with powdery mildew, a totally different fungal disease. Powdery mildew usually forms on the tops of leaves while downy mildew is underneath. Powdery mildew causes leaves to turn yellow, but usually not until after the white spores have been present on the leaf surface for a while. With downy mildew, the leaves can turn yellow even before the spores are evident.
It is possible for a plant to have both powdery mildew and downy mildew at the same time.