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the fight against Blight

Might the remarkable ‘Sárpo’ varieties soon be the only spuds worth growing? John Walker reports from the blight front-line.

Trip wires

The secret of the Sárpos’ success is their ability to go on the defensive when faced with a barrage of airborne blight spores.
David Shaw explains: “When a microscopic blight spore lands on and penetrates a leaf or stem, a ‘trip wire’ effect triggers a reaction in which the plant immediately fights back, isolating the point of infection and preventing most attacks from spreading any further. Although this may cause small dead spots on the leaves late in the season, as the resistance mechanism begins to degenerate, there is barely any effect on yield. ‘Sárpo Mira’ occasionally develops black spotting on its leaves, but this is thought to be caused by nutrient deficiency, not blight.

“The problem in many traditionally ‘resistant’ potato varieties is that a strain of blight evolves that can ‘crawl under’ the trip wire, break into the plant and cause mayhem. Some can still repel light infections, but they all succumb quickly to an early and severe blight outbreak, which results in the rapid and complete death of the plant, and in few if any tubers.

“This is likely to get worse as strains of the blight fungus appear to be becoming ever more aggressive – it’s rather like an ‘arms race’ between blight fungus and potato breeder. So far, the Sárpo trip wire has held firm, even in this last season of record blight pressure.”

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Good things in store

Is there a rather unpleasant whiff coming from your stored spuds this autumn? Or perhaps the first question should really be: did you even manage to get any spuds into store? After what was, in many areas, the wettest May, June and July since records began, potato growers large and small, organic and otherwise, have suffered devastating crop losses. Some of these losses have come from spuds, along with other vegetable crops, lying in waterlogged soil for weeks on end, but by far the greatest losses are the work of Phytophthora infestans – potato blight.

This is the airborne fungus that turns potato tops, temperature and humidity permitting, into a brown, rotting mess within days, and which brings stored tubers out in dark sunken patches of reddish flesh which quickly putrefy into a foul, stinking mess. It’s the fungus disease which organic gardeners probably fear most of all, and this year it’s been on the loose since as early as April, attacking some ‘early’ potatoes, which we can usually harvest before the worst blight attacks appear. It attacks tomatoes too, with outdoor crops suffering badly in many areas.

But things are changing – our climate for one – and that’s likely to fundamentally alter the way the potato blight fungus continues to evolve. But fungi don’t have a monopoly on upping the ante. Switched-on plant breeders are pretty good at it too, as was evident during the open day held in August at the Sárvári Research Trust’s trial of blight-resistant potatoes in North Wales. Other trials were carried out in 2007 elsewhere in Wales, as well as in Cornwall and Suffolk.

“The potato blight fungus is definitely evolving at a rapid rate, and different strains are now widespread,” says Dr David Shaw, Director of Research at the Sárvári Research Trust, who suspects this could have been the “worst ever” year for potato blight. “The fungus now appears to be reproducing sexually, which may give rise to overwintering ‘oospores’. These are a long-lived kind of spore which form in the tissues of the blighted plant as it decomposes, then enter the soil around the infected plant, where they can survive in a dormant state for many years.” This is very bad news for all potato growers.

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Plan crop rotation

Variety performance

We organic gardeners in particular have welcomed the first Sárpos as an answer to our prayers, but we’re hungry for more – and that’s where the news gets even better. After extensive trialling, David Shaw and his team are submitting four new Sárpo varieties for what’s known as National Listing – a process that allows seed tubers to be legally sold to growers and gardeners. We organic gardeners need to keep our fingers crossed that they all get listed. The first seed tubers will, hopefully, be available to buy in winter 2008-2009.

If topsy-turvy weather patterns brought about by global warming mean blight attacking ever-earlier, then new Sárpo ‘Una’ is likely to be a blessing. This pink-skinned first or second early has high blight-resistance, a good yield, and an attractive skin. As well as showing promise as a salad variety, the plants can be left to bulk up as tasty summer bakers.
The skin finish on ‘Adam Blue’ (still a Sárpo, but without the prefix) has already been much admired at a number of gardening shows. Its well-shaped, brightly coloured tubers have excellent tuber blight resistance and a medium dry matter content, making it a good and flavoursome all-rounder in the kitchen. Show-off gardeners are unlikely to show much resistance when thoughts of the show-bench loom...
Sárpo ‘Kifli’ is a white-skinned early maincrop which makes a good salad spud, with high yields and outstanding flavour. Early maincrop Sárpo ‘Eric’ produces high yields of tasty, white-skinned tubers, from plants with more compact growth than those of its siblings.

A word of caution: the names of these four new spuds are, for now, provisional. However, it’s possible that Sárpo ‘Una’ and ‘Adam Blue’ will appear as such when, subject to successful National Listing, they hit the 2009 catalogues.


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Organic Gardening Magazine
Visitors to the trial had the chance to try existing and up-and-coming Sárpo varieties – they make delicious chips!

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