The Fight Against Blight

I report from the potato blight front-line, and ask whether the remarkable ‘Sárpo’ varieties might soon be the only spuds worth growing.

By John Walker. Published in Organic Gardening, December 2007.

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 – late potato blight.

Checking out tuber development on plants of ‘Sárpo Mira’, which has proved a hit with organic gardeners. Here it’s still growing and flowering in early August, while blight-susceptible varieties growing alongside have been wiped out.

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.

Classic symptoms of blight attack on a susceptible variety, with entire leaves and stalks being killed by the fungal infection, which is spreading rapidly through the haulms.

“Blight is 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”

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.

‘Sárpo Mira’ foliage showing the black spotting which often appears in late summer. This is thought to be caused by nutrient deficiency, rather than blight.

“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.

“In the garden, blight infection usually arrives during summer as spores on the wind, or spreads from infected ‘volunteers’ which grow from tubers you missed when lifting the crop the previous autumn. So there’s always a chance you might be lucky and not get blight, but with oospores it’s different. When you plant a seed potato in ground infested with oospores, the roots of the plant trigger the spores into action and the plant becomes infected directly from the soil.”

Freshly dug tubers of Sárpo ‘Una’, which makes a good early ‘salad’ potato as well as a tasty summer baker.

The potato tops or haulms then succumb to the disease, which can quickly spread to neighbouring gardens or allotments. On top of that, spores wash down and infect the tubers, causing them to rot in store – which is what many of us will be finding this winter. Meanwhile, more oospores find their way into the soil, storing up future trouble.

To add to organic gardeners’ woes, the only recourse available to combat a blight outbreak is to spray with Bordeaux mixture – which most organic experts suggest should be a ‘last resort’. Copper sulphate, the active chemical in Bordeaux mixture, is a highly toxic material in the environment, and its use is controversial even among commercial organic growers (it’s likely to be phased out soon anyway). Non-organic gardeners can turn to other chemicals for help, but they suffer the same fate as their organic counterparts; for any chemical to work against blight infection, it has to remain on the leaf surface and be reapplied frequently. Has anyone forgotten those incessant, drenching summer downpours?

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’ [‘Blue Danube’*] (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’ (right) 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, but it’s possible that Sárpo ‘Una’ and ‘Adam Blue’ [‘Blue Danube’*] will appear as such when, subject to successful National Listing, they hit the 2009 catalogues.

Axona’ foliage, showing the very limited damage done by blight attack towards the end of the growing season.

But don’t despair. There’s good news, and even better news. You might already have grown and harvested the ‘good news’ in the form of the two most blight-resistant potato varieties currently available to gardeners: ‘Sárpo Mira’ and ‘Axona’. It was no surprise to find the top veg expert from seed company Thompson & Morgan among those attending the Sárvári trials; T&M were the first to introduce the remarkable blight-resistant ‘Sarpo’ potatoes to gardeners. The name derives from sárvári and potato, being pronounced ‘sharpo’. In recent years these red-skinned maincrops have helped to make blight a fading memory in many an organic garden and allotment.

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

“Both ‘Sárpo Mira’ and ‘Axona’ are high- yielding maincrops, with high dry matter content, making them ideal for mashing, baking and for chips,” says David Shaw. “Like all potatoes, their taste and cooking quality depends on climate, soil type, and when you harvest them. You get the best results by letting them grow on into September before removing the tops three weeks before lifting. This lets the skins ‘set’ and reduces the chance of blight spores invading them during harvesting. Drying the tubers thoroughly and quickly, and leaving a coating of soil on them, is the best way to avoid blighted tubers. You should never wash the soil off before drying and storing them.”

All these potato leaves have been deliberately infected with blight. See how the three Sárpos – ‘Mira’, ‘Una’ and ‘Kifli’ – show only limited damage at the point where the blight spores were placed on the leaf.

The Sárpo potatoes are the result of a conventional plant-breeding programme stretching back over forty years, masterminded by the Hungarian Sárvári family. Their aim has always been to produce potato varieties that thrive in low-input, organic growing systems. The Sárpos are high-yielding, with dense, weed-smothering haulms. As well as resisting blight, they resist common viruses, wireworm and slugs, and store well for a long period with no special temperature requirements (stored tubers will keep until April). ‘Sárpo Mira’ shows such good blight-resistance that it’s now the European industry standard against which other varieties bred for resistance are measured.

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 (above) 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.”

Plants of ‘Sárpo Mira’ in early August, alongside heavily blighted ‘Bintje’.

“‘Sárpo Mira’ shows such good blight-resistance that it’s now the European industry standard against which other varieties bred for resistance are measured”

A walk through the Sárvári Research Trust’s North Wales trial field is both a revelation and an education. The potatoes are grown in neat, ordered blocks according to scientific protocols, the whole field containing a mixture of established and ascendent Sárpo varieties, some still known only by code numbers, alongside more ‘traditional’ blight-resistant varieties such as ‘Lady Balfour’. As this extraordinary blight season progressed, the mettle of the Sárpos was evident; whole blocks of blight-susceptible varieties were wiped out many moons ago, leaving the Sárpos still going strong. Being ‘indeterminate’, many Sárpos never actually stop growing – there have even been reports of plants still flowering in a cold frame in February.

The tall haulms of ‘Axona’ suppress weed growth beneath the dense canopy of foliage.

Although the future looks bright for the Sárpos, the outlook for other traditionally ‘blight-resistant’ potato varieties appears bleak. With climate change tipping our weather patterns into unknown territory, and with summers like the one just past likely to become more frequent, Phytophthora infestans will be in its element. Add in an ever more rapidly evolving blight population, a window that’s gradually closing on the organic gardener’s last line of chemical defence, and the Sárpos look set, now more than ever, to be the only logical choice for the organic gardener – or any gardener – who’s banking on a reliable and worthwhile harvest in what could be even worse blight years ahead.

“Although the future looks bright for the Sárpos, the outlook for other traditionally ‘blight-resistant’ potato varieties appears bleak”

It might seem counter-intuitive to consider putting all your eggs in the basket marked ‘Sárpo’. But the performance of these disease-busting ‘wonderspuds’, coupled with the widening range of varieties, from first earlies to maincrops, in red, white and blue, means they’re looking increasingly like the only spuds in town worth growing.

On the trial field in early August, all of the healthy green foliage belongs to Sárpo varieties – you can clearly see blighted blocks of other varieties which succumbed earlier in the summer.

* ‘Adam Blue’ was renamed and released as ‘Blue Danube’.

Will you help support the blight-resistant Sárpo ‘wonderspuds’ today?

Gardeners everywhere can now support the future development of Sárpo potatoes by chipping into the Sárvári Research Trust’s innovative ‘crowdfunding’ project at Buzzbnk, which is live now. The Trust is raising funds to put another of its low-input, no-chemical-spray-needed, non-GM potatoes through the legal procedure required before it can be sold to gardeners and potato farmers.

Dr David Shaw and his team are already well on their way to reaching their £5,000 target, but they need more gardeners like you and me to back them to help achieve their target. This is our chance to play a small but vital part in the much bigger picture of supporting more ecologically sustainable food systems. It also means that another blight-busting Sárpo potato variety will be available for us all to grow in our gardens and allotments. With Sárpo potatoes at our side we won’t just win the fight against blight – it’ll be game over for this devastating plant disease. Backing this project is an investment in the future of food itself.

You can back the new Sárpo wonderspud as a ‘friend’ from as little as £10, and receive an invite to the Trust’s next Potato Day event at its beautiful home in North Wales. Become a ‘mover’ for £20 and you can also pick up some seed potatoes on the day to grow at home. Investing £50 in the future of safe, low-carbon food, by becoming a ‘player’, means you’ll also receive some seed potatoes of the new variety to try out yourself. If you decide to sign up as a ‘shaker’ with a £100 backing, you’ll get all of the above plus the chance to stay on for a special potato-themed meal. Join in as a ‘go-getter’, with a £500 backing, and you will receive all of the above, plus take home 25kg of Sárpo potatoes and five gift packs of Sárpo varieties.

You can find out more about how to join in on the Buzzbnk project page. I’m excited and honoured to be a part of this amazing project. Will you join me?

John Walker, November 2012

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