Potato blight and how to beat it

RHS Chief Horticulturist Guy Barter looks at one of the vegetable gardener's worst enemies

Potato blight is the most important potato disease in Britain and in fact is of global significance.  While blight is often considered a 'fungal' disease, the blight organism itself is not strictly a fungus but is more closely akin to an algae.

Like algae it must have a wet environment to survive. Blight attacks usually follow warm rainy weather and are commonest in mild, wet western regions. Tomatoes are affected too, although greenhouse tomatoes often escape damage because the plants remain dry.

No pesticides to control blight are available to UK gardeners. Happily, resistant potato and tomato cultivars are available,  and better ones are being bred. These include; potatoes ‘Coquine’, ‘Sarpo Axona’, ‘Sarpo Mira’, tomatoes ‘Crimson Crush’, ‘Mountain Magic’ and ‘Losetto’ (bush)

How to pick out potato blight

Signs of potato blight are unfortunately common in summer. Potato  leaves commonly darken at the edges with white mould occurring at the paler edges of the dark patches followed by destruction of the entire leaf. 

Tubers can become infected and rot in the ground (or later in storage) unless foliage is removed promptly when blight has been spotted. Wait two weeks to allow the spores to die out, then gather in your potatoes.

Tomato blight shows itself as dark marks on stems and brown blotches on fruits with leaf rots.  The plants can quickly collapse.

Once a crop is infected the disease quickly spreads in wet weather but can dry up and become dormant in hot dry spells. Blight infections stop once colder autumn weather arrives.
Blight overwinters in tubers left in the soil ('volunteers') or stored tubers discarded in spring. Resting spores that can survive the winter and infect crops in subsequent years are also reported but it is unclear how important they are in Britain. Any infected crop remains should be burnt or buried at least 40cm deep.

When does blight strike?

Blight prediction services based on weather parameters are available which text or email subscribers when there two consecutive days over 10C, and each days has at least ten hours of  relative humidity of 90% or over occur or are expected (a so-called 'Smith period'). This is mainly of use to farmers to time their sprays to catch the infections before they get established.
Commercial non-organic crops of potatoes are sprayed every 7-14 days with powerful fungicides, which are a huge expense, a risk to the environment and also  commonly lead to fungicide resistance. Although potatoes genetically modified to be immune to blight have been developed, none are allowed to be grown in British farms or gardens.

See also

RHS advice

RHS advice on potato and tomato blight

Gardeners' calendar

Find out what to do this month with our gardeners' calendar

Advice from the RHS

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