How to prevent and treat potato blight with Horkans Garden Centre

How to Prevent & Treat Potato Blight

The current Irish weather conditions are ideal for Potato Blight. Met Éirean have just released an official warning that weather conducive to the spread of potato blight is likely from now until next weekend. Garden expert Paraic Horkan has some invaluable advice on how to prevent and treat this devastating disease.

What are the Causes of Potato Blight:

  • Potato blight is caused by a fungus called Phytophthora infestans
  • This affects members 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.
  • Any period of warm, humid weather increases blight risk.
Phytophthora Infestans photo

Potato plants affected by blight

Potato blight is caused by a fungus called Phytophthora Infestans. This fungus does not just affect potatoes but can also spread to other members of the potato family, including tomatoes, peppers and aubergines.
Potato blight spreads through the air and develops when weather conditions are warm and humid, particularly during what is known as a Smith Period.
A Smith Period is any 48 hour period where the minimum temperature is 10°C or more and 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. This essentially means that infection is dependent on warm, wet weather, much like we’re expected to have.

The disease can be spread by wind and water. Wind can carry spores from plant to plant, and water can wash the spores into the soil where it can infect young tubers. Potato blight is very destructive and can wipe out a full crop overnight. In a sack or a crate, it will travel from potato to potato, ruining them all. The Potato Blight fungus is generally killed by cold weather, but there are some rare resistant strains that survive over the winter.

What are the Symptoms of Potato Blight:

  • Brown freckles of patches on the leaves
  • Yellowish border spreading from the brown patches
  • Dark patches on tubers
  • Inside of the tuber is brown and rotting from the skin down
  • The potatoes will eventually turn into soggy, foul-smelling masses.

The first thing that you will notice if your plants are infected is brown freckles on the leaves of the potato. Also they may have larger brown patches and a sort of yellowish border spreading from this patch.
In a severe attack, you may find that all the potato foliage has become a rotting mass.
The tubers (the potato itself) are shrunken with dark patches on the skin. If you cut the potato in half, you will see brownish rot spreading down from the skin.

How do I Prevent Potato Blight?

  • The best way to prevent potato blight is to plant a good blight-resistant variety
  • Make sure potatoes are well earthed up to prevent spores getting into the tubers
  • Spray leaves and stems with a preventative treatment such as Bayer Garden Blight Control
  • Be aware of weather warnings
  • Check regularly for signs of infection
Earthing up

Earthing up

Prevention is always better than cure and that is definitely true when it comes to potato blight. There are a range of chemical treatments available, but the one Paraic recommends is Bayer Garden Blight Control.
This is a great preventative treatment which is effective against early stages of Phytophthora Infestans. Simply spray all your plants and they will be protected from the blight fungus spores. This is a wonderful product which is rain-fast in just 1 hour. So don’t delay and apply immediately.
For the maximum protection, you should spray your crops four times at intervals of 10 days. This will protect the leaves and stalks, and also the tubers from the risk of late blight infection after harvest.
Each 100ml bottle will treat up to 625sqm.

What is the Treatment for Potato Blight?

  • If only a small number of leaves are affected, you can remove these and dispose of them
  • Continue to spray with Bayer Garden Blight Control. This can be used up to four times per growing season
  • If you have a more serious infection, then cut off all the foliage and stems and either compost or burn
  • 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

Potato_Blight_FoliageIf you have used a blight resistant variety and you have prepared correctly then you should not be affected by potato blight. However if you do notice a small number of infected leaves (with brown patches), you can just remove them. If you have a more serious infection you will need to cut off all the foliage and stems. You should place them in a bucket to stop the infection from spreading and dispose of them in a bin. If it is possible you should burn them. Removing the foliage will also prevent the disease spore getting into the tubers, but this will only be effective if the tubers are well covered with earth. You should cover over any exposed tubers to prevent blight from reaching them. This will also prevent sun and crow damage. Leave the crop for at least two weeks. This will let the blight spores on the surface die and the potatoes develop a thicker skin.

After harvesting, check regularly for signs of blight and remove any suspect tubers at once from your store. Lift the tubers and store in potato sacks in a cool dark area to use over the Autumn and winter period. Ensure you purchase new certified seed potatoes next season as blight can remain active in tubers and can affect next year’s crop.

What are some blight resistant potato varieties?

  • The Sarpo Mira as a wonderful blight resistant variety
  • Orla is another great blight resistant variety
  • Colleen, Golden Wonder, Kerr’s Pink and Record are all blight resistant

The best way to prevent potato blight is to plant a good blight-resistant variety. The Sarpo Hungarian potato varieties have been developed to be extremely blight resistant. The Sarpo Mira as a wonderful blight resistant variety; it is floury, well flavoured and a terrific cropper. It is a nice red skinned potato with lovely white flesh. Other blight resistant varieties include Orla, Colleen, Golden Wonder, Kerr’s Pink and Record.

Please remember to follow instructions carefully when using chemicals.

Photo by Scot Nelson

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