This month we have a guest post from Dr David Savari at Savari Research Trust:
In our climate of mild, wet, summers, most growers of potatoes are very familiar with the late-blight of potato. As you know, the arrival of the disease from the Americas in the 1840s had a shattering effect on Ireland, Scotland and other potato growing areas in Europe. It is a disease that has followed the potato around the world and is still rife here and in China, Highland Africa and Russia. Its severity varies with the weather but in an average year, it strikes in July but in really wet years can start as early as April.
Infection comes from old blighted potatoes left in the ground or tossed in a rubbish heap. When these sprout they develop spores of the blight on the stem and leaves. These become airborne and if your young, new crop is wet from rain or dew, the spores settle and penetrate the leaves. Three days later, the new leaves develop new spores and these go on to cause more chaos. Rain also takes spores down into the soil to infect the young tubers that were promising a good harvest. The baby potatoes develop brown patches and can then rot rapidly. So what can you do to fight back?
You can grow early varieties that give you a crop before the blight arrives – you may be lucky.
For your heavy-yielding maincrop varieties you will be lucky to escape infection. You can do nothing; some years you will be lucky and other years you will lose the lot. But you can reduce the damage. Plant your crop in an exposed part of your plot. This will allow the foliage to dry off between showers and will reduce infection. Picking off the first diseased leaves you see will help but not a lot. If the foliage is all spotted with blight (and it really is blight) it is best to cut all the stems off at ground level. Whatever you do, if you have blight, always remove the stems at least three weeks before harvest and dig the potatoes in a dry day when they will dry off a bit before you store in paper or hessian bags, in the dark and in a cool place. Look over the potatoes every few weeks to remove any that develop rot.
If your livelihood depends on your spud harvest you need to control the blight. That usually means spraying with a chemical fungicide. Copper has been used for generations and is still by some organic growers but it is a dreadful poison when it hits the soil and should never be used. Commercial growers now use a cocktail of modern chemicals that prevent infection if the plants are kept covered in the stuff. They need to be sprayed every week when blight is present and more often when weather is warm and wet. You can probably guess I am not a big fan of fungicides.
By far the best control is by growing varieties that have been selected to resist the blight. That means, the foliage does not get infected or, if it does, it grows on until the end of the season and the new spuds resist any spores that wash down from the leaves.
Some varieties have tops that get totally blighted but if the tubers have already formed before the foliage dies, they are safe because they have resistance. The popular variety, Charlotte, a second early is one of these.
Other varieties show resistance in the foliage but the tubers have no resistance. So if a few spores wash into the soil, the tubers will get infected. Toluca is one of these.
It is pretty obvious that the best varieties have foliage that rarely gets much blight and the tubers are resistant too. Enter the Sarpo potatoes. Sar-po stands for Sarvari Potato and that tells you that the family of resistant potatoes were produced initially by the Sárvári family in Hungary from wild material collected by the great Russian breeder, Vavilov. In Soviet times, Potatoes that could be grown without chemicals were needed for the USSR and so, the Sárváris developed resistance to viruses and then resistance to blight. The Sarvari Research Trust in North Wales took over the selection of seedlings for growing in Western Europe and the Trust introduced six varieties with a range of maturities and uses in the kitchen.
Our trials in Wales, Scotland and England over the last ten years has shown these new varieties to be able to survive under high blight pressure although some, like Blue Danube and Sarpo Una have resistance that expresses more in the tuber than in the foliage. In most years their more moderate resistance can cope. All our varieties have strong tuber resistance.
But don’t believe all you read about resistant varieties. The cunning blight is busy evolving all the time and has been getting better at causing disease on resistant varieties. A new kind of blight (strain Blue 13) swept through Britain in 2006 and this one knocked out many useful resistant varieties. Lady Balfour, Remarka, Setanta and even Cara were no longer as resistant as they used to be. As expected, the same blight swept through Ireland a few years later. Fortunately our Sarpo varieties retained their resistance, probably because they carry many and different resistance genes. So the amazing thing is that varieties can be resistant in one location but susceptible in another. The breeder has to be aware that resistance can be overcome and that it is impossible to predict if or when this happens. You can check on blight resistance scores in several databases like The British Potato Variety Database www.varieties.potato.org.uk but try to make sure the information is up to date.
Why grow Sarpos? Why grow any variety? GIYers want a good yield of spuds that have a great taste and texture and those in the west of these islands like a dry, floury variety like Kerrs Pink or Queens. Then you should try Sarpo Mira, Axona, Blue Danube or Sarpo Shona with similar properties. If it’s a waxy one you want then the early maincrop, Sarpo Kifli or the early, Sarpo Una will suit you. Many thousands of GIYers in UK grow Sarpo Mira and Axona. Kifli, Blue Danube, Sarpo Una and Sarpo Shona will be available soon.
Other good reasons to grow Sarpos? Their vigour means they smother most weeds and leave the plot clean. Just be careful to cut the tops when the spuds are the size you want. If left to grow on too long, the huge spuds will have hollow heart. Their long natural dormancy means they can be stored without refrigeration until well into the following summer. What more do you want?
Some of our growers in mild parts leave their Sarpo spuds in the ground and harvest as they need them over the winter. Others like to use Axona or Sarpo Mira as second croppers. Just leave some seed in a tray outside and plant them in July to get new potatoes in October and November.
For further information about Sarpo potatoes please contact Dr David Shaw
Sarvari Research Trust www.sarvari-trust.org
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