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Impatiens downy mildew

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last updated Apr 2, 2013
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Impatiens downy mildew. Image: RHS, Horticultural Science

Impatiens downy mildew is a fungal disease that causes yellowing leaves, leaf loss, and death of bedding Impatiens, commonly called busy Lizzies, during wet weather and damp conditions. Attacks are most likely in summer or spring in greenhouses.

What is impatiens downy mildew? Back to top

Impatiens downy mildew is a disease caused by the fungus-like organism Plasmopara obducens.

It was found for the first time in the UK in 2003 and is likely to have arrived on imported commercial propagation material (seed or cuttings). Statutory action was at first undertaken by FERA against confirmed outbreaks of the disease, but this soon ceased. After the wet summer of 2008, damage was much reduced by improved control practices at commercial nurseries. However in 2011 control failed, probably due to resistance to the commercial fungicides used. Infected plants were inadvertently sold widely. This led to the the most widespread outbreak of the disease so far, with many gardens, nurseries and local authority displays affected.

Outbreaks of downy mildew have been confined to Impatiens walleriana, the common bedding busy Lizzie. No cases have been found on New Guinea impatiens, Impatiens × hawkeri, or on the few species of Impatiens found growing in the wild in the UK including Himalayan balsam (Impatiens glandulifera).

 

Symptoms Back to top

You may see the following symptoms:

  • On leaves: Affected leaves turn yellow and are rapidly shed from the plant. A fine white fungal growth may be visible on the lower leaf surface, but affected leaves decay rapidly
  • On flowers: Flowers are also commonly shed, and the plant is often reduced to bare branches with a small tuft of yellow leaves and flower buds at the tip. Severely affected plants will eventually die

There have been some reports of variations in disease severity between different varieties, but these have yet to be confirmed.

Control Back to top

Non-chemical control

Affected plants should be disposed of as soon as possible. Do not compost them. Ideally burn them or bury them deeper than 50cm (20in).  Although the spores should not survive the commercial composting used for council green waste collections, it is best to deal with contaminated material within the garden.

Because of the risk of soil contamination, rest affected areas from Impatiens for at least a year (some species of Plasmopara affecting other plants produce resting spores that can survive for several years).

Where infected plants have been grown in containers, replace the compost and wash using a garden disinfectant, as directed by the manufacturer, to cleanse the container if you intend to grow Impatiens in it again the following year. The disease is specific to Impatiens, so any other bedding plants can be grown without risk.  Semperflorens-Cultorum begonias and bedding fuchsias perform well in the shaded areas for which busy Lizzies are invaluable.

Raising plants from seed will eliminate the risk of purchasing infected plants.  This type of disease often has a lengthy ‘latent period’, when plants are already infected but not yet showing obvious symptoms. The advent of fungicide-tolerant strains of this disease increases the risk of introducing disease when buying plants.

Unfortunately, growing Impatiens, even ones that have rasied at home, in another part of the garden will not guarantee freedom from infection, as the disease may well arrive again as airborne spores from infected plants growing elsewhere. No resistant cultivars are yet available.

Chemical control

There are no fungicides available to gardeners for the control of this disease.

Biology Back to top

Downy mildews are a large group of plant diseases caused by microscopic fungus-like organisms related to the pathogen that causes tomato and potato blight. Despite a similar name and certain similarities in symptoms, they are unrelated to the powdery mildews.

The disease is spread by spores produced on the underside of infected leaves. These spores are splashed by rain, and are also carried for long distances on the wind. Extended periods of leaf wetness are required for spore production and infection, so severe outbreaks of downy mildew are only likely to occur during wet summers.

The airborne spores remain viable for just a short time, but it is thought that the fungus can also produce a second spore type (a resting spore) within the affected plant tissues. These resting spores are much more resilient, and are released into the soil as the diseased material rots down. They are likely to survive within the soil for an extended period.

Quick facts

Common name Impatiens downy mildew
Scientific name Plasmopara obducens
Plants affected Impatiens walleriana (busy Lizzies)
Main symptoms Yellow leaves, leaf and flower loss, white fungal growth
Caused by Fungus
Timing Summer, or spring in greenhouses
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