White Blister

Description

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

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