The initial symptom of blight on potatoes is a rapidly spreading, watery rot of leaves which soon collapse, shrivel and turn brown. During humid conditions, a fine white fungal growth may be seen around the edge of the lesions on the underside of the leaves

Brown lesions may develop on the stems

If allowed to spread unchecked, the disease will reach the tubers. Affected tubers have a reddish-brown decay below the skin, firm at first but soon developing into a soft rot as the tissues are invaded by bacteria. Early attacks of blight may not be visible on tubers, but any infected tubers will rot in store.

Non-chemical control

Infected material should be deeply buried (more than 45cm deep), consigned to the green waste collection or, ideally, burned rather than composted

Earthing up potatoes provides some protection to tubers

Early-harvested potatoes are more likely to escape infection

Picking off leaves or leaflets when just a few are affected may slow down the progress of the disease very slightly, but will not eradicate the problem

When infection levels reach about 25 percent of leaves affected or marks appear on stems cut off the foliage (haulm), severing the stalks near soil level and raking up debris. When the skin on tubers has hardened, after about two weeks, the tubers can be dug up.

To prevent slug damage avoid leaving tubers in the soil after this time

Use the tubers from blighted crops as soon as possible, checking any stored tubers regularly for deca

Operate a rotation to reduce the risk of infection, ideally of at least four years

Destroy all potatoes left in the soil, and any waste from storage, before the following spring

Blightwatch

What are Hutton Periods

A Hutton Criteria occurs when 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 in time for the 2017 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.

Potato Blight