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Potato Blight

Potato Blight

Potato blight is one of the worst disease problems that affect the potato family. It can wipe out potatoes almost overnight and can also infect the tubers in storage causing them to rot.

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 by spores that are spread by wind and rain and infect plants under favourable conditions. These conditions are high humidity and a minimum temperature of 10C (50F). Infected tubers from the previous year may be discarded at the edge of the vegetable patch or left in the ground and if conditions are favourable can act as a source of the following year’s outbreak.

Symptoms.
These first appear as brown or black patches at the tips and margins of leaflets. Under suitable conditions the patches enlarge and the leaflets curl and wither. In moist weather the fungus is sometimes visible as a delicate white growth on the margins of the patches, particularly on the underside of the leaves. Brown patches may also develop on the stems and the disease can spread rapidly to all aerial parts of the plants, resulting in collapse. 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 10 deg C and 75% humidity) and there has been recent rain leaving wet foliage.

Non Chemical Control.
A hot dry spell may slow down the infection as blight is dependant on the weather. Tuber infection can be limited by earthing up the soil around the stems. Prompt removal of infected foliage before the disease spreads to the tubers may save the crop. Ensure that the tops of the plants are removed and destroyed - do not leave them in the soil or at the edge of the plot. Remove any diseased tubers before planting.

Chemical Control.
There are a range of chemical treatments available to farmers but gardeners are more limited. Murphy Traditional Copper Fungicide or Dithane 945 may be used as a routine spray to protect plants before they become infected. Regular applications are required as the fungicides are ineffective once infection has occurred.

Your best preventative is to grow resistant variety of potato that is less affected by the blight.

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 Water from the base rather than spraying potatoes.
If the weather is right but no rain, your artificial rain will give it a hold.
Leaky hoses are a good way to water potatoes, unless you have hosepipe restrictions.

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 Bordeaux mixture 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 haulm 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.

Good news.
The recent development of Sarpo Hungarian varieties of potato is extremely blight resistant and these are coming onto the market now although not widely available as yet.