Why is Potato Blight So Dangerous?

It can wipe out the plants almost overnight
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.
Potato blight is one of the worst disease problems when growing potatoes.
The humid moist weather helps to spread potato blight on early and maincrop varieties; weather found during Irish summers.

Potato blight foliage damage showing blight spores

What Causes Potato Blight?

Potato Blight is caused by a fungus called Phytophthora infestans, which can spread to other member of the potato family, including tomatoes. It spreads through the air and develops when the weather conditions are warm and humid, particularly during a Smith Period. 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 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!

Symptoms of Potato Blight :

The first thing you may notice are brown freckles on the leaves or sections of leaves with brown patches
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 in a rotting mass.
Tubers affected by potato blight can be seen by dark patches on the skin.
Inside will reveal brownish rot spreading down from the skin.
Later the entire potato will turn into a soggy, foul smelling mess.

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

Prevention of Potato Blight

Potato blight fungus is generally killed by cold weather, although there are some rare resistant crossbred strains that survive over winter.
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 10° C and 75% humidity) and there has been recent rain leaving wet foliage.
There are a range of chemical treatments available to gardeners. Murphy’s Copper Fungicide is good as long as it is applied before blight takes hold.
It is an organic treatment for the prevention of potato blight.
Your best preventative is to grow a variety of potato that is less affected by the blight.

Horky recommends:
Sarpo Mira and Sapro Axona as the best blight resistant varieties for west of Ireland conditions.
Click here to read our Growing Guide for Blight Resistant Seed Potatoes

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. An excellent product to try is Proxanil. Follow the instructions on the packaging carefully.

Treatment of Potato Blight

If you notice a small number of affected leaves with patches, you can try removing those and dispose of them.
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 as this 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 is a real breakthrough in potato cropping. They are extremely blight resistant and are available from Horkans. Sarpo Mira is a wonderful variety; it is floury, well flavoured and a terrific cropper. It is a 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 Kerr's Pink	Romano Sante Kondor	Sapro Mira Sapro Axona 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	Record Sante	Sarpo Axona Sarpo Mira	Kondor Lady Balfour	Valor Picasso

Shop our entire range of seed potatoes here.

If potato blight is a problem in your existing crop this summer follow these steps

Step 1
Remove the foliage and stems and dump or burn.

Step 2
Cover over any exposed tubers to prevent blight from reaching the tubers and to prevent sun damage and crow damage.

Step 3
Leave the potato tubers in the ground for a further 2 weeks (you may use some of the tubers straight away).

Step 4
Lift the tubers and store in potato sacks in a cool dark area and use over the autumn and winter period.

Step 5
Ensure you purchase new certified seed potatoes next season as blight can remain active in tubers and may effect next year’s crop.