Q: Should I stop planting impatiens because of the disease everybody is talking about that kills the plant?
A: This is an important question to consider, since downy mildew infestations of impatiens are increasing across the Midwest, with two reports of this disease in Ohio filed with The Ohio State University Extension the first half of April. The pathogen that causes the disease, Plasmopara obducens, is extremely aggressive on bedding plant impatiens. Ohio’s impatiens producers have been put on alert to scout for this disease.
Generally, impatiens downy mildew is a destructive parasitic foliar disease of garden impatiens that begins with spotting or stippling of the leaves, curling of leaves in a downward fashion and leaf yellowing. A white, fuzzy growth develops on the underside of both yellow leaves and healthy green leaves. Eventually, leaf drop takes place, leaving bare stems behind.
This particular downy mildew has been around for a while, but just in the past few years it has become increasingly widespread in North America.
The disease is spread by two different types of spores. One type is easily airborne but luckily remains viable for just a short time. The other moves through a film of water.
It develops in cold or warm weather, when plant foliage remains wet for prolonged periods and relative humidity is high. On the other hand, hot or dry conditions discourage spore growth and infection won’t occur.
To control impatiens downy mildew in your garden, don’t use fungicide sprays or washes; they are of little help to the home gardener. Rather, water only in the mornings, and only on the surface of the soil; do not wet the foliage unless it’s a very sunny day so the leaves will dry quickly.
If you spot downy mildew on your impatiens, remove and dispose of the entire infected plant, including roots, immediately. Toss it in the trash, not on the compost pile. Only replant New Guinea impatiens or other species of flowers in the infected areas.
While downy mildew can adversely affect many types of impatiens, even native impatiens known commonly as jewelweed, thankfully it does not threaten or affect New Guinea impatiens or other flower or vegetable plants.
The extra-large flowers of New Guinea impatiens can provide brilliant color in spots with some sun, but also where there is little or no sun. Depending on the cultivar you choose for your garden, New Guinea impatiens have foliage that ranges from solid dark green to an almost kaleidoscope of variegated choices.
If you’d like to try New Guinea impatiens in your garden, here is a partial list of cultivars and hybrids to choose among. Notice how the plant name seems to help you anticipate intensity of color:
‘Celebrette Hot Pink,’ ‘Celebrette Orchid Star,’ Fiesta™ ‘Appleblossom, ’ Fiesta™ ‘Burgundy Rose Double,’ ‘Applause Orange Blaze,’ ‘Celebration Blush Pink,’ ‘Celebration Bright Salmon,’ ‘Celebration Lavender Glow,’ ‘Celebration Orange,’ ‘Celebration Raspberry Rose,’ ‘Celebrette Deep Red,’ ‘Celebrette Purple Stripe,’ ‘Infinity Lavender,’ ‘Infinity Cherry Red,’ ‘Painted Paradise Lilac,’ ‘Painted Paradise Pink,’ ‘Painted Paradise Red,’ ‘Painted ‘Paradise White,’ ‘Paradise Cherry Rose,’ ‘Paradise Lavender on Fuchsia,’ ‘Paradise Mango Orange,’ ‘Paradise Rose on Violet,’ ‘Paradise Salmon Pink’ and ‘Sonic Magic Pink.’
All impatiens require a good, well-drained soil, adequate moisture and afternoon shade. New Guinea impatiens grow best with about four to six hours of afternoon shade. Choose plants that are mounded and well-branched with healthy root systems, shiny leaves and lots of flower buds. Plant them immediately if you can; don’t let them dry out. And don’t store the plants in a dark, dry location. Provide bright light but no direct sun.
For container planting, first be sure the receptacles are thoroughly cleaned and disinfected. The container size should be approximately one-and-a-half times the diameter of the potted plant purchased. For container gardening, use only high-quality commercial potting soil — do not reuse soil left in the pot from last year and do not use soil you have dug from your garden. Set the plant into the container at the same level it grew in its pot.
For garden beds, loosen the soil and mix in compost or peat moss along with an all purpose fertilizer. Top-dress both the soil bed or container with slow-release fertilizer, if desired. Set the plant into the soil at the same level it grew in its pot.
New Guinea impatiens prefer soil that is consistently moist but not soggy. Water at ground level to avoid wetting the foliage.
For more horticultural information, call The Ohio State University Summit County Extension Hotline Tuesdays from 9 a.m. to noon at 330-928-4769, ext. 3, and request Fact Sheet HYG-1236-98, “General Maintenance of Herbaceous Ornamentals.”
Please note fact sheets are sent out free; there is a fee for bulletins. Many bulletins are available online at ohioline.osu.edu and can be printed from home or accessed at the public library.
Dayle Davis is a freelance writer and avid perennial gardener, with a B.A. in communications and course work in botany, geology and wildflowers. Davis is a master gardener emeritus under The Ohio State University’s Horticultural Extension. Readers can send in questions regarding lawn and garden issues, which could be featured in a future edition, as well as about the OSU Master Gardener program. Questions can be emailed to firstname.lastname@example.org, faxed to 330-665-9590 or sent to Leader Publications, 3075 Smith Road, Suite 204, Akron, OH 44333. Please do not send any leaves, seeds or other organic material. Inquiries about area garden clubs or groups should be sent directly to the particular organization in question.
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