Aquilegia downy mildew

Aquilegia downy mildew is a fungus-like organism that initially causes yellowing of leaves and purple blotching on the flowering stalk. The disease can progress rapidly under damp conditions, leading to wilting and eventually death of the whole plant.

Aquilegia downy mildew symptoms on upper leaf surface

Quick facts

Common name Aquilegia downy mildew
Scientific name Peronospora species
Plants affected Aquilegia
Main symptoms Yellow leaves, leaf and flower loss, white fungal growth
Caused by Fungus-like (Oomycete) organism
Timing Spring and Autumn

What it is

Aquilegia downy mildew is a disease caused by a species of Peronospora, which is a fungus-like organism. Being closely related to algae, downy mildews thrive in cool, damp conditions. Downy mildews can spread from plant to plant by airborne or rain-splashed spores and may be seed-borne, but bought seed is likely to be disease-free.

Outbreaks of Aquilegia downy mildew in UK gardens began to be reported to RHS Gardening Advice in 2013 and at that time Fera (The Food and Environment Research Agency) were aware of the disease. Currently the host range seems to be restricted to Aquilegia.

Symptoms

You may see the following symptoms:

  • On leaves: Affected leaves develop angular, yellow patches which spread and eventually the whole leaf curls and turns brown. A fine whitish/purple fungal growth may be visible on the lower leaf surface
  • On flowers: Flowers become distorted and look water-soaked. Flower stalks develop purple blotches. If infection occurs after flowering, the seed pods can develop brown patches and fail to set seed

Images to aid diagnosis can be found on the National Aquilegia Collection Holder's website.

    Aquilegia downy mildew on lower leaf surfacePlant badly affected by aquilegia downy mildewSymptoms on Aquilegia plant of systemic infection

    Control

    Non-chemical control

    Hygiene measures can greatly reduce the risk of resting spores contaminating soil, from where they can infect subsequent aquilegia plantings. 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 aquilegia for at least a year (some species of Peronospora affecting other plants produce resting spores that can survive for several years but this information is unclear for Aquilegia downy mildew).

    Where infected plants have been grown in containers, replace the compost (bury contaminated compost, ideally in a vegetable plot) and wash using a garden disinfectant, as directed by the manufacturer, to cleanse the container if you intend to grow aquilegia in it again the following year.

    It is not yet known if Aquilegia downy mildew can be seed-borne. The host range of the disease is not yet known, but downy mildews usually only affect a limited range of plant species.

    Seek assurances from suppliers that plants offered are disease free. Growing from seed is a lower risk source of plants than buying plants from sources that are unable to offer suitable assurances.

    No resistant cultivars (varieties) are currently known.

    Do not use excess nitrogen rich fertiliser that can lead to soft susceptible growth. Avoid crowded plantings where air movement is limited.

    Chemical control

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

    Biology

    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, which also affect aquilegia.

    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 springs and 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. Resting spores are much more resilient, and are released into the soil as the diseased material rots down. These resting spores could survive within the soil for several months to several years.

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    • Simon Park avatar

      By Simon Park on 18/05/2015

      Hi, I am studying downy mildew on Aquilegia. I know it has been spotted down in the south of England, but I live in Leeds (West Yorkshire) and have noticed the mildew on our Aquilegia. Was wanting to spread the word that it is in Yorkshire now, so other gardeners in the area can be on the lookout for it.... Also, it looks a lot like the mildew that can sometimes attack apple trees, could it be related? Any info is helpful at this point, cheers!