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2009 Irish summer conditions have encouraged ideal potato blight conditions – Humid moist weather helps to spread potato blight on early and maincrop varieties.

Potato blight is one of the worst disease problems when growing Potatoes. It can wipe out the plants almost overnight and, worse still, it can infect the tubers causing them to rot in storage. In a sack or crate it will travel from potato to potato ruining the lot.

What Causes Potato Blight?

Potato Blight is caused by a fungus Phytophthora infestans. This can also infect other members of the potato family, Solanaceae such as tomatoes. It spreads through the air and develops when the weather conditions are warm and humid.      The Irish summer, as we laughingly call it.

Smith Periods – Weather conditions that favour blight conditions in the west of Ireland.

Blight forecasting has often been based on the occurrence of “Smith periods”. A “Smith period” is a 48 hour period in which the minimum temperature is 10°C or more and the relative humidity exceeds 90% for at least 11 hours during the first 24 hours and for at least 11 hours again during the final 24 hours. However, any period of warm, humid weather increases blight risk.

Tuber Blight – Prevent the foliage blight affecting your developing tubers

Potatoes infected with late blight are shrunken, on the outside, corky and rotted inside

Symptoms of Potato Blight

The first thing you may notice are brown freckles on the leaves or sections of leaves with brown patches and a sort of yellowish border spreading from the brown patch. In a severe attack you may walk onto your patch to find all the potato foliage a rotting mass.

Tubers (the actual potato) affected by potato blight can be told by dark patches on the skin. Cutting the potato in half will reveal brownish rot spreading down from the skin. Later the entire potato will turn into a soggy, foul smelling mass.

Prevention of Potato Blight

Potato blight fungus is generally killed by cold weather, although there are some rare resistant crossbred strains that over winter. Otherwise, the disease reservoir is infected tubers in the ground or your sack. Wherever it comes from, it can travel miles on the wind and there is little you can do if the weather is right (above 10deg C and 75% humidity) and there has been recent rain leaving wet foliage.

There are a range of chemical treatments available to gardeners Dithane 945 is good as long as it is applied before blight takes hold.

Your best preventative is to grow a resistant variety of potato that is less affected by the blight Horky recommends Sarpo Mira as the best blight resistant variety for west of Ireland conditions.

Other actions to help prevent potato blight

Try to get all the potatoes out from the ground when you harvest and so you won’t leave a reservoir on your plot. Ensure potatoes are well earthed up to protect tubers even if you get it in the foliage

Treatment of Potato Blight

If you notice a small number of affected leaves with patches, you can try removing those and disposing of them. Burn if possible. This is a good time to make sure the potatoes are well earthed up to prevent spores getting into the tubers unless you have already done this. Spray with copper fungicide or Dithane 945 may prevent spread if applied early enough.

If you have a more serious infection, then you need to cut off all the foliage and stems and either compost or burn it.

Removing the foliage prevents the disease getting into the tubers, as long as they are well covered with earth. Leave the crop alone for at least two weeks to let the blight spores on the surface die and the potatoes develop a thicker skin.

After harvest, check regularly for signs of blight and remove any suspect tubers at once from your store.

For next Year – Horky recommends you grow blight resistant varieties.

The recent development of Sarpo Hungarian varieties of potato are extremely blight resistant and these are available from Horkans. Sarpo Mira is a wonderful blight resistant variety, its floury, well flavoured and a terrific cropper – nice red skinned potato with lovely white flesh.

There are other varieties of potato with varying degrees of blight resistance listed below. The Sarpo types are exclusive to Horkans.

Best Potatoes for Foliage Blight Resistance
First Earlies Orla Premiere
Second Earlies Colleen Nadine
Main Crop Cara
Golden Wonder
Kerrs Pink
Sarpo Mira
Lady Balfour
Pentland Dell
Pentland Squire


Best Potatoes for Tuber Blight Resistance
First Earlies Orla Colleen
Second Earlies Cosmos Nicola
Main Crop Cara
Golden Wonder
Sarpo Axona
Sarpo Mira
Lady Balfour


Remember if potato blight is a problem in your existing crop.

  1. Firstly remove the foliage and stems and remove & dump or burn.
  2. Cover over any exposed tubers to prevent blight from reaching the tubers and to prevent sun damage and crow damage.
  3. Leave the potato tubers in the ground for a further two weeks ( you may use some of the tubers straight away)
  4. Lift the tubers and store in Potato sacks in a cool dark area and use over the Autumn and winter period.
  5. Ensure you purchase new certified seed potatoes next season as blight can remain active in tubers and ma effect next year’s crop.

Horkans open 7 days Sunday 12 to 6pm call us on 0949031435 for more information.