White blister

White blister diseases attack plants in the Asteraceae (daisy) family and ornamental and edible Brassicaceae plants, such as brassicas and aubrieta and honesty. It initially produces pale spots and blotches that develop into white blisters. These contain and then release huge numbers of powdery spores.

When the infection takes hold, it results in distorted growth, reduced plant vigour and stunted growth and badly affected plant parts can shrivel up and die. Examples of distorted growth include sunken pits and/or bulges, curved and buckled stems and abnormal seedpods.

White blister is caused by several different fungus-like organisms, which are closely related to downy mildews. On some plants, downy mildew and white blister can be present at the same time.

The infection and spread of white blister is worse in wet weather – the same conditions that encourage downy mildews.

Because of the white pustules, white blister diseases are sometimes referred to as white rusts, but they are unrelated to true rust diseases.

• Edible brassicas, including Brussels sprouts, broccoli, cabbage and cauliflower
• Aubrieta
• Honesty
• Wallflowers
• Members of the daisy (Asteraceae) family, including gerbera, pericallis, senecio and sunflower

White blister is caused by a number of different fungus-like organisms. They spread throughout the plant and from plant to plant by spores, which are released from the white pustules. They are dispersed by several different mechanisms, including rain splash, or when watering, in air currents and by insects.

Wet or very damp weather and leaves, particularly at night, are needed for infection to occur, and severe outbreaks are only very likely during a wet summer.

On wet leaves, the spores germinate and produce a second type of spore, which goes on to infect the plants further.

A third kind of spore is then produced, which falls to the ground when the plants die or leaves fall and can remain dormant in the soil for several months, ready to infect new plantings.

The disease can also be present in the seeds and can be seed borne.

Try to keep plants growing as strongly as possible to help them fend off disease attacks. Avoid the use of high nitrogen fertilisers, which produce soft growth, using high potash ones instead, which help to harden the growth. However, when heavy outbreaks of white blister occur, this may not prevent or control them.

Carefully pick off and dispose of affected leaves when the disease first strikes to reduce or slow down its spread. This is highly unlikely to actually eradicate the disease. Try not to disturb the spores, or you will be spreading the disease further.

Prune out badly infected growth, or carefully dig up badly affected plants, and dispose of it by either burning it or put it in the council’s green waste bin. Don’t compost it.

Similarly, carefully pick up or rake up affected fallen leaves – especially in autumn – to try and reduce carry-over through the winter.

Crop rotation may help to break the cycle of re-infection from year to year.

Some weeds act as a secondary host and so should be controlled and removed. These include creeping thistle, groundsel, ragwort and shepherd’s purse.

Unfortunately, there are no fungicides available to home gardeners to control white blister.

Avoid watering plants over their leaves, watering the soil or compost instead. And water early in the morning or during the day, rather than in the evening, so that wet leaves dry out quickly. Watering in the evening means that the leaves remain wet over night.

In greenhouses, water plants carefully to reduce excessive water increasing humidity, and open doors and vents whenever possible to help reduce humidity further.

Don’t plant too densely, to help ensure there is good air circulation around the plants.

Some plant varieties are listed as being downy mildew resistant. These include Brussels sprout ‘Bridge’, ‘Exodus’ and ‘Maximus’ and broccoli ‘Green Magic’. Bear in mind that over time such resistance can break down.

• Secateurs
• Leaf rakes