Late Blight

Late blight remains the single most important potato disease, costing the industry an estimated £50M annually in crop protection chemicals in a typical blight pressure season.

Genotypes or 'strains' of blight evolve over time and require monitoring. New 'aggressive' strains can go through a reproductive cycle quicker than your spray programme, while occaisonally strains can develop resistance to recognised fungicides (see FRAG UK, in tools section).

AHDB funds the British involvement in Euro Blight in order to track strains as they develop in the continent and share research and information on treatment and resistance. It is worth signing up to our technical newsletters to be alerted to information from the network.

Outgrade potatoes piles can be a source of blight. Read our best practice guide for managing outgrade piles.

The Hutton Criteria

The Hutton Criteria occurs when there have been two consecutive days with a minimum temperature of 10°C, and at least six hours of relative humidity (90%). Arising from research funded by AHDB Potatoes and undertaken by the James Hutton Institute, the Hutton Criteria is a significant advancement on the 60-year old Smith period that used wider ranges of temperature and longer periods of humidity. The new criteria has been built in and will improve the reliability and reach of blight risk reporting across the blight pressure season. The new systems will now work with the new criteria in order to help to support decisions, refine action plans and re-empower growers against blight.

A large body of work was undertaken by JHI, funded by AHDB Potatoes, to identify precisely what new criteria might produce the optimum Blight risk prediction results.

Experimental investigations showed that:

  • Reducing the minimum temperature did not improve alert performance markedly
  • The relative humidity threshold should not change
  • Reducing both temperature and relative humidity criteria resulted in an overly sensitive alert system
  • Shortening of the duration of high relative humidity proved most effective

Now: The Hutton Criteria

Two consecutive days:

  • Each day has a minimum temperature of 10°C
  • Each day has at least six hours with relative humidity ≥ 90%
  • Performance up from 41 to 69% of > 2000 historic potato late blight outbreaks receiving an alert
  • Increased proportion of days receiving an alert from 7 to 16% for a 28% gain in performance
  • Improved consistency in alert performance across all regions of Great Britain
  • The future for Blight modelling and alert systems


Foliar late blight can be identified by a necrotic patch surrounded by a pale green ring. Lesions on leaves generally appear as irregular shaped dark spots which enlarge as the disease develops. On the upper surface a lighter green halo often surrounds the necrotic area and, on the lower surface, whitish spore–bearing mould which looks like fine cotton wool develops around the lesions under moist conditions. Stem lesions are elongated, grey brown to black in colour and can encircle the stem. These lesions are often found at leaf axils and the apex of stems.

Other causes of necrotic patches can be misdiagnosed as late blight. These include Alternaria (early blight), Botrytis, wind damage and occasionally fertiliser scorch. To confirm late blight send a sample to the Fight Against Blight campaign.

Tuber infection symptoms are a darker brown sometimes purplish area on the tuber surface. The internal rot is a reddish brown granular rot which can remain close to the surface or progress to the centre of the tuber. Rot development is irregular and sometimes threadlike through the tuber flesh. Late blight causes skin damage which allows opportunistic bacteria to invade and cause soft rotting.

Blight and fungicide tools and resources

Fungicide Resistance Action Group (FRAG) Fight Against Blight BlightSpy Tool - weather forecasts for the predicted occurrence of Hutton Criteria