Peronospora spp.        Family: Peronosporaceae
(per-oh-NES-spore-ah)
Common name: Aquilegia Downy Mildew (ADM)

Plant affected: Aquilegia (columbine)
Main symptoms: yellow leaves, whitish-purple fungal growth, flower and leaf loss
Caused by: fungus-like oomycete organism
Timing: spring and autumn        

Description & Life Cycle: Affected columbine plants will show yellow patches on the upper side of their leaves. These patches are distinguishable from natural leaf variegation by their straight lines. They also tend to initially run alongside the main leaf vein before the disease spreads to the rest of the leaf. You may see a whitish-purple fungal growth on the underside of the leaves on affected plants. Eventually, with the progression of the disease, leaves will curl and turn brown before falling off.

Flowers are not immune either. Affected blossoms will look as if they have been soaked by rain and become distorted. You may see purple spots developing on flower stalks. Eventually, flowers drop off. If the plant becomes infected after flowering and as seed pods are forming, you will see brown spots on the pods. These will not set viable seeds.

Aquilegia downy mildew (ADM) is spread by air- and water-borne spores. Can be carried long distances by the wind. Outbreaks typically occur when there are prolonged periods of rain in spring and/or late summer. Ideal conditions for production of spores and infection.

While the airborne spores are only viable for a short time, the fungus is capable of producing a second type, called a resting spore, between the leaf tissues. These resting spores are more resilient. Released into the soil from decaying diseased material they can survive for several months, and perhaps for several years although this fact has not been confirmed.

Special Notes: This virulent disease was first recorded in UK gardens in 2013. However, the Food and Environment Agency of Great Britain was aware of Aquilegia downy mildew prior to this date. It appears to be restricted solely to the UK, at this time.

The disease is so new, scientists have yet to assign a species name to the genus. While it has been placed in the same genus (Peronospora) as powdery mildew which also affects aquilegias, it is not related…despite certain similarities in symptoms.

Remedial Action: There is no chemical control for this disease. However, strict attention to hygiene in the prompt removal of any diseased parts or whole plants, as a non-chemical control method, will reduce the risk of resting spores contaminating the soil. Immediately burn diseased plants, or bag them for garbage removal. DO NOT COMPOST.

The Royal Horticultural Society (RHS) has recommended not re-planting aquilegias in an affected area of the garden for at least a year. This advice may not have merit as there other species of Peronospora affecting other plant species which produce resting spores capable of surviving several years in the soil.

Read more about Aquilegia Downy Mildew (ADM) in my Blog dated March 1, 2017. (Quick link here.)

Posted on February 14, 2018