Downy Mildew - a Problem for Impatiens Plants

By Marie Iannotti, About.com Guide

There's been an increase in downy mildew affecting Impatiens plants recently. The particular mildew affecting impatiens is Plasmopara obducens. It spreads rapidly, under the right conditions, and once infected, there is no cure. However there are some preventative measures you can take. But first, here's some background to help you understand the disease.

What is Downy Mildew?

Downy mildew diseases are caused by oomycetes or water molds. They are fungus-like, but more closely related to algae.

There are 2 types of downy mildew spores. One type, zoospores, can be splashed up by water or spread by wind. The other type, oospores, reside inside the plant tissue and can spread rapidly and over-winter. At this point, there is no evidence that this particular mildew affecting impatiens is doing that, but why take chances.

Downy mildew is more prevalent in the spring and fall, when the cool, wet or humid weather provides ideal conditions.

Which Impatiens Varieties are Affected?

According to an e-Grow Alert by Nora Catlin, Floriculture Specialist, Cornell Cooperative Extension of Suffolk County, the following types of Impatiens are susceptible to downy mildew.

• All Impatiens walleriana, including the double and mini types. These are the most common bedding impatiens sold in garden centers. (I. walleriana hybrids, like the Fusion® series are less prone, but still susceptible)

• Balsam Impatiens / Garden Balsam (I. balsamina) These are taller plants than common Impatiens, with elongated, pointed leaves.

• Jewel Weed (I. capensis) and Yellow Jewel Weed (I. pallida). Generally considered weeds, but if they are in the area, they can spread the disease to your landscape Impatiens.

Impatiens Not Affected

• Himalayan balsam / policeman’s helmet (Impatiens glandulifera), a wild and sometimes invasive species that can reach heights of 6 - 10 ft. (2 - 3 m), is tolerant of the disease, but can act as a host.

• New Guinea impatiens (Impatiens hawkeri) and its hybrids, like SunPatiens®, have so far shown resistance.

There may be more plants affected or that act as hosts, but this is the information so far.

Symptoms of Downy Mildew on Impatiens

Early symptoms can be hard to spot and might be mistaken for a nutrient problem.

1. Leaves stippled or chlorotic (yellowing) and looking like they need fertilizer.
2. Floppy or downward curling leaves.
3. Sometimes white fuzzy spots will form on the undersides of the leaves. These are spores and usually appear in cool, damp weather. (See photo on page 2.)
4. As the disease progresses the plants may stop growing and look stunted, drop their leaves or completely collapse.

What to do about downy mildew on impatiens.

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