Pests and Diseases

Blight

Blight is a fungus Phytophthora infestans. It needs very high humidity and mild temperatures, day and night, to grow on the potato plants.

Initial signs are that of small dark spots on the leaf margins, in moist conditions spots start to be surrounded by white mould.  The stems may also develop dark brown patches at areas where leaves join them.  Tubers will have an appearance of brown / purplish discoloration on the skin.

Treatment: If blight starts to develop, then the haulm (foliage) should be cut and removed from the garden to avoid infecting other crops. Join the “Fight Against Blight”, an informative campaign run by The Potato Council to help gardeners. For details see www.potato.org.uk/blight.

PCN (Potato Cyst Nematodes)

PCN cause cysts of approx. 0.05mm in diameter will appear on the roots and potatoes.  There are two species of PCN which cause symptoms to occur during mid-July and mid-August.  The species are best deciphered by their colour, Golden Yellow cysts = Globodera rostochiensis, Creamy White cysts = Globodera pallida.  These cysts in both strains will turn a reddish brown colour at maturity. Plants will appear stunted and weak with the foliage having a dull wilting appearance. 

Treatment: Make sure you rotate the crops in your garden. Potatoes ideally need a rotation of 7 years but a 4 year rotation will help. If your garden is affect by PCN, try growing earlies, as these are usually harvested before the PCN mature, thus reducing the problem and the PCN population in your garden.

Blackleg

Blackleg is a common bacterial soft rot, generally Erwinia carotovora subsp atroseptica which predominately occurs during periods of prolonged cold, wet weather.  The stem of the potato plant is infected, turning black and decomposing, followed quickly by the potatoes.  The plant leaves will turn yellow and the plant will appear slumped.

Treatment: Pull out the affected plant(s) so that the remaining plants not affected. The potatoes can be consumed (provided they have reached a reasonable size and have not started to decompose).

Common Scab

Common Scab is caused by a fungus, generally Streptomyces scabiei.  Cases are worst in dry conditions and in soils with a high pH. This scab will not affect the eating quality of the potatoes but their appearance will be unattractive.

Treatment: Watering during dry spells, particularly at the time the potatoes are developing will decrease the level of infection in tubers. 

Slugs

Slug damage appears as odd shaped holes on the surface and interior of the potatoes. More common in potatoes planted in heavy wet soil or in crops planted beside grass.  A late harvest may mean tubers are more at risk as slugs are more active around the late autumn period.

Treatment: Try a border of sharp gravel or similar around the potato patch, as slugs don’t like to cross sharp textures.

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