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New impatiens resist mildew

New impatiens resist mildew

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Impatiens are a bright, cheerful annual that gardeners use to light up dark and shady areas of the yard and garden. But there has been concern about planting them since 2012 when a pathogen called downy mildew began turning some plantings to mush in the Midwest.

Here are some questions about impatiens with answers from horticulturists at Iowa State University Extension and Outreach. To have more questions answered, contact Hortline at hortline@iastate.edu or 515-294-3108.

Q: Should I be concerned about impatiens downy mildew if I plant impatiens?

A: In 2012, impatiens downy mildew appeared on a number of garden impatiens plantings in Iowa and Illinois. Downy mildew is caused by the water mold Plasmopara obducens.

Symptoms of impatiens downy mildew initially appear as yellowing of infected leaves. The yellow-green foliage may initially be confused with a nutritional deficiency. As the infection progresses, leaves may curl downward and a white, fuzzy growth can be seen on the undersides of the leaves.

Severe infections lead to defoliation and blossom drop, leaving bare stems with a few, tiny yellow leaves at the tips of the shoots. Widespread, severe outbreaks of impatiens downy mildew have occurred in recent years.

Previously, all cultivars of the common garden impatiens were susceptible to impatiens downy mildew. However, that has now changed. Imara XDR is a new impatiens downy mildew tolerant series of garden impatiens introduced by Syngenta. Several colors and a mix will be available this year (2019).

An additional line of downy mildew-resistant impatiens (called Beacon) will likely be available from Pan American in 2020. New Guinea impatiens and SunPatiens are highly tolerant to impatiens downy mildew.

Q: Which other impatiens can be successfully grown in home gardens?

A: The common garden impatiens, New Guinea impatiens and SunPatiens are the three most widely planted impatiens in home landscapes.

The common garden impatiens (Impatiens walleriana) have been a popular bedding plant in the United States for many years. Garden impatiens are easy to grow and bloom continuously from spring until the first fall frost. Garden impatiens have glossy, medium green leaves. Flowers are 1 to 2 inches in diameter, may be single or double and come in a wide variety of colors. Plants commonly grow 12 to 18 inches tall.

New Guinea impatiens (Impatiens hawkeri) are native to New Guinea and the Solomon Islands. They were introduced to the United States in the early 1970s. A large number of cultivars have been introduced in the last 25 to 30 years. New Guinea impatiens have green, bronze or variegated leaves and large, showy flowers. Flowers can be up to 3 inches in diameter. Flower colors vary from white and pale pink to bright pink, red, violet and orange.

SunPatiens are hybrid crosses between New Guinea impatiens and wild impatiens. Plants in the Compact Series grow approximately 18 to 30 inches tall, while those in the Vigorous Series grow 24 to 36 inches tall. Flowers are up to 3 inches in diameter and available in shades of orange, salmon, coral, pink, red, orchid, lavender and white. Leaves vary from light to dark green, with a few cultivars having variegated foliage.

Q: What are good planting locations for impatiens?

A: Impatiens perform best in moist, well-drained soils. Light requirements vary among the different types.

Garden impatiens prefer sites that receive two to four hours of sun each day. Garden impatiens can also be grown in full shade. However, plants will be taller and bloom less profusely in fully shaded locations.

New Guinea impatiens perform best in locations that receive morning sun and afternoon shade. Eastern exposures that receive four to five hours of sun are usually good sites. The foliage of New Guinea impatiens often scorches in full sun.

SunPatiens are more versatile. They can be successfully grown in full sun to part shade. Avoid sites in full shade.

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