Blight

We seem to be losing all the battle against blight on Keith's plot he has been hit by it, now mine had got it as well as some other people's allotments. We are just hoping it doesn't find it's way into the greenhouse and attack the tomato plants,fingers crossed and legs crossed it doesn't go any further. I've copied a section on blight from the net for furtive reference.



Potato Blight

Potato blight is one of the worst disease problems for the potato grower. 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 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 British summer, as we laughingly call it.

Smith Periods

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.

Some History of Potato Blight


Potatoes infected with late blight are shrunken on the outside, corky and rotted inside
If we look into history a little, there was a terrible famine in Ireland between 1845 and 1849. This was known as The Irish Potato Famine or The Great Hunger.

The biological cause of this was that the potato blight struck and the Irish basically grew a very limited number of varieties of potatoes, which was their staple food. If they had had more varieties, chances are that one would have been resistant to the blight and the famine’s effects would have been lessened.

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 overwinter. 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 farmers but gardeners are more limited. The traditional spray was Bordeaux mixture. This isn’t such a good idea as it contains copper and is hardly good for your diet although it is organically approved. It is worth checking in horticultural suppliers what anti-fungals are currently available and approved for use on potatoes.

Your best preventative is to grow a resistant variety of potato like Sarpo 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. Spraying with Bordeaux mixture or fungicide 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. I think composting, especially hot heaps, will be safe but burning is 100%.

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 are extremely blight resistant and these are coming onto the market now. Sarpo Mira was the first to be generally available but this is not an all round potato. I’ve tried the Sarpo Axona variety and found it not as blight resistant as the Mira and little different in taste or type. Still looking for a Sarpo that will make a good chip.

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

Best Potatoes for Foliage Blight Resistance

First Earlies Orla Premiere
Second Earlies Cosmos Nadine