Control Potato Blight
Potato blight foliage damage showing blight spores - Phytophthora infestans
Irish summers tend to encourage ideal potato blight conditions. Humid moist weather helps to spread potato blight on early and main crop 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, including tomatoes. It spreads through the air and develops when the weather conditions are warm and humid. The typical Irish summer, as we laughingly call it.
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 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 and 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 noted 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 can last 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. Bordeaux Mixture is good as long as it is applied before blight takes hold.
Your best preventative is to grow a blight resistant variety of potto. I recommend 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 you reduce the damage to the tubers if covered sufficiently with a layer of top soil.
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.
If you have a more serious infection, then use a treatment called Proxanil which is favoured by the growers as it prevents the spread of blight and will also reverse the effects on early attacks of blight. Apply Proxanil every two weeks but do not exceed four applications in one growing year.
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 – I recommend you grow blight resistant varieties.
The Sarpo Hungarian varieties of potato are extremely blight resistant and are available from Horkans Lifestyle & Garden Centres. Sarpo Mira is a wonderful blight resistant variety; it is floury, well flavoured and a terrific cropper with a nice red skin and lovely white flesh.
There are other varieties of potato with varying degrees of blight resistance listed below. The Sarpo types are exclusive to Horkans.
Remember if potato blight is a problem in your existing crop apply a dressing of Proxanil to the entire foliage
- Cover over any exposed tubers to prevent blight from reaching the tubers and to prevent sun damage and crow damage.
- Leave the potato tubers in the ground for a further two weeks (you may use some of the tubers straight away)
- Lift the tubers and store in potato sacks in a cool dark area and use over the autumn and winter period.
- Ensure you purchase new certified seed potatoes next season as blight can remain active in tubers and may effect next years crop.
For more ways to protect your crops, see our pest control range.