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last updated Mar 26, 2014
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White blister

White blister is a foliar disease that may be found in the garden on a limited range of ornamentals, vegetables and weeds. It can reduce plant vigour, and sometimes also causes distorted growth.

What is white blister? Back to top

White blister is a foliar disease caused by a fungus-like organism called Albugo. It is closely related to the organisms causing downy mildew diseases, and the two sometimes occur together. Like downy mildews, infection and spread of white blister is favoured by wet weather.

Only three Albugo species are likely to be found in gardens. Each affects a different plant family, namely the Brassicaceae, the Asteraceae and the Aizoaceae. Due to the pustules that they produce they are sometimes also known as white rusts, although they are unrelated to the fungi causing true rust diseases.

Symptoms Back to top

You may see the following symptoms:

  • Pale leaf spots form, which eventually develop into blister-like white pustules releasing huge numbers of tiny, powdery spores
  • The pustules are usually found on the underside of the leaf, and may develop singly or in concentric rings
  • There is often abnormal pigmentation (e.g. purpling) surrounding the affected area
  • Other aerial parts, such as stems, flowers and seed pods can also be attacked
  • Affected plant parts are often distorted, resulting in bulges or sunken pits on leaves, curved stems and abnormally-shaped flower heads or seed pods. Brussels sprout buttons develop conspicuous protrusions where white blister is present
  • Vigour is reduced and severely-affected plant parts can shrivel and die

Control Back to top

Non-chemical control

  • Remove affected plant parts, and dispose of severely infected plant material
  • Avoid close spacing of plants, which encourages high humidity and infection
  • Remove weed hosts of the disease (although see the comment under ‘Biology’ regarding races of Albugo)
  • Crop rotation may help to reduce the incidence of the disease
  • Seed catalogues list cultivars with resistance or tolerance to white blister including Brussels sprout ‘Bridge F1’ listed as resistant, and 'Doric F1', ‘Exodus F1’,  ‘Maximus F1’ and the broccoli cultivar ‘Green Magic’  noted as having good tolerance

Chemical control

There are no fungicides available to amateur gardeners for the control of white blister diseases.

Biology Back to top

Three species of Albugo may be found in gardens:

Albugo candida affects many members of the Brassicaceae, including vegetable brassicas and ornamentals such as honesty, aubretia and Aurinia saxatilis. It is also often seen on the cruciferous weed shepherd’s purse. On many of these plants it can sometimes be found in close association with downy mildew.

Albugo tragopogonis affects members of the Asteraceae. It is the most important disease of salsify and scorzonera, and is found on ornamentals such as Senecio cineraria, gerbera, florist’s cineraria (Pericallis × hybrida) and sunflower. Weeds that can be affected include ragwort, groundsel, goat’s-beard and creeping thistle.

Albugo trianthemae was recorded for the first time in the UK in 2007. It affects the Aizoaceae, and has been found on Delosperma and Lampranthus.

The life-cycles of the various Albugo species are very similar. The microscopic spores released from the white pustules are dispersed by rain splash, insects and in air currents. They require leaf wetness in order to germinate and produce a second spore type, which infects the plant through its air pores or stomata.

A third type of spore is formed within the affected plant tissues. This is a resilient resting spore, which can contaminate the soil for several months and germinate to infect new plantings of susceptible hosts. The disease can also be seed-borne.

Some Albugo species are known to exist as a number of genetically-distinct ‘races’, each with a limited host range. For example, the race of Albugo candida found on shepherd’s purse is thought to be incapable of affecting vegetable brassicas.

Quick facts

Common name White blister
Scientific name Albugo species
Plants affected Members of the BrassicaceaeAsteraceae and Aizoaceae
Main symptoms White pustules producing powdery spores on leaves and other aerial parts. Growth is often distorted
Caused by Fungus-like organism
Timing Seen most commonly in spring and summer, although it can be found year-round on biennials/perennials, vegetable brassicas and indoor plants
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