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?
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.
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.