Busy Lizzies: Fungal disease has spread

Question:

I wonder if you could tell me what has happened to all my Busy Lizzie plants. I enclose a picture of the plants looking very unhappy. The leaves have fallen off and the stems have withered. This has also happened to my friends' plants which makes me think it is not my garden or soil. I have dug them up and disposed of them (not in the compost bin) but wonder if I there are any more precautions I should follow. I have grown Busy Lizzies for many years and have never lost them at this time of year. I would appreciate it if you could throw some light on to the problem. (E Aberdeen, via e-mail).

Answer:

Busy Lizzie has been one of the most reliable bedding plants for years and if you had a space to fill you could not go far wrong with impatiens, commonly known as Busy Lizzies, until this year.

The problem is impatiens downy mildew – a fungal disease that causes yellowing leaves, leaf loss and death of bedding impatiens. 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.


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The disease is spread by spores, produced on the underside of infected leaves, and splashed by rain or carried 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 in wet summers.

The airborne spores remain viable a short time, but it is thought the fungus can also produce a second spore type (a resting spore) within the affected plant tissues.

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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.

Impatiens downy mildew was found for the first time in the UK in 2003 and probably arrived on imported commercial propagation material (seed or cuttings). Statutory action was at first undertaken by the Food and Environment Research Agency 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.

This year (2011) the control failed, probably due to resistance to commercial fungicides used.

Infected plants were inadvertently sold widely. This led to 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.

When plants are affected the 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. 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.

Control is difficult as there are no chemicals available to the gardener. Affected plants should be disposed of as soon as possible. Do not compost them. Ideally burn them or bury them deeper than 50cm.

Although the spores should not survive 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 and use a garden disinfectant, as directed by the manufacturer, to cleanse the container if you intend to grow impatiens in it next 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 shaded areas where Busy Lizzies are invaluable. Raising plants from seed will eliminate the risk of buying infected plants.

This type of disease often has a lengthy 'latent period', when plants already infected are not yet showing obvious symptoms. The advent of fungicide-tolerant strains of this disease increases the risk of introducing disease when buying plants.

Growing Impatiens, even ones raised 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.

•This article was first published on September 10, 2011.

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