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Late Blight

Field of growing potatoes

What is late blight?

Late blight is a disease caused by a fungus-like organism (oomycete) called Phytophthora infestans. Late blight is the most serious and costly disease affecting potatoes and can quickly develop and spread throughout the potato foliage and tubers at any stage of crop development.

Leaf blight can be identified by a necrotic patch (lesion) surrounded by a pale green ring. The area appears as an irregular shape with dark spots that enlarges as the disease develops. On the leaf upper surface, a lighter green halo often surrounds the lesion and, on the underside, a white spore–bearing mould develops around the lesions.

Stem lesions are elongated, grey brown to black in colour, can encircle the stem and are often found at leaf axils and the apex of stems. Late blight can be misdiagnosed for other ailments, such as wind damage and fertiliser scorch, so for confirmation, leaf samples should be sent for testing.



Conditions favouring and promoting late blight

Conditions favouring late blight are a combination of temperature and relative humidity. The James Hutton Institute has shown that conditions favouring late blight development have changed, and as such, has reviewed the guidance outlining these conditions. 

Now defined as “The Hutton criteria”, an alert is a period of 2 consecutive days with each day having a minimum temperature of 10 degrees Celsius and each day has a relative humidity of more than 90% for at least 6 hours. The change from The Smith Period is the shortening of the duration of high humidity required to trigger an alert – from 11 hours to 6 hours.

Late blight can quickly develop if left untreated. It will cause foliage to die and infection can spread to the tubers. The late blight spores can easily spread by water and wind to neighbouring plants and crops.

Management Approaches

A range of cultural and chemical control methods exist and all should be followed. These include:

Focusing on farm hygiene; destroy all out-grade piles, killing any growth before emergence.

Control all potato volunteers in other crops 

Use foliar fungicides, such as Valbon in a preventative manner

Use products with multisite activity and alternate use of chemical groups and modes of action for best practice resistance management.

Products you may be interested in



A potato blight fungicide, containing 17.5 g/kg (1.75% w/w) benthiavalicarb-isopropyl and 700 g/kg (70% w/w) mancozeb.

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Further Information


AHDB potatoes logo

AHDB Blight Report

The AHDB blight report, as part of the Fight Againt Blight (FAB) initiative from AHBD potatoes. Reporting began in 2008, collating information from voluntary 'blight scouts' routinely walking potato fields. Part of this interactive tool allows you to moitor blight occurance by area.

FRAG Blight Resistance Management

AHDB Potatoes report on late blight fungicide resistance management