Beating the blight: how to grow healthy potatoes

 

Vigilance is vital to stop disease spreading to nearby gardens

The Penguin Reference Dictionary

blight/ bliet/ n 1 (an organism that causes) a disease or injury of plants resulting in withering, cessation of growth, and death of parts without rotting 2 sthg that impairs, frustrates or destroys 3 a condition of disorder or decay

IT'S NOW 165 years since the late-blight pathogen, Phytopthora infestans, first crossed the Atlantic Ocean to devastate crops like the widely planted "Lumper" potato and forever change the course of Irish history. Forty years later, scientists discovered that spraying a still-healthy potato crop with copper sulphate would effectively protect the leaves and tubers against possible infection, but that was too late for many. By then, Ireland had suffered through the Famine or An Gorta Mór, causing approximately a million deaths and twice that number in emigration. As president Mary Robinson once said: "It was an event which more than any other shaped us as a people."

The exact same genetic strain of blight that caused the Famine predominated for the next 100 years or more throughout Europe, the US and Russia, even as modern fungicides gradually took their place alongside copper-based sprays in providing both commercial growers and private gardeners with effective treatments. But the fungus Phytopthora infestansis genetically complex, ever-adaptable, and ominously capable of mutating to produce more virulent forms.

By 1976, many new strains now able to interact sexually had made their way to Holland, probably from Mexico (the most likely birthplace of blight). Growers now faced the possibility of new, more potent and more persistent hybrid strains being produced. Blight researchers began to track the disease, genetically fingerprinting different infections to follow its development. In 2005, they found a new, more aggressive form of A2 blight was evolving, and it was eventually given the name Blue 13. This form has since become the predominant strain of blight in the UK and has quickly been gaining ground in Ireland.

According to David Shaw of the Sárvári Research Trust (a blight-researcher for 40 years and the man who discovered and named Blue 13 in 2005) this new strain is more destructive, faster-acting and capable of infection at lower temperatures than older forms of the disease (making it active earlier in the growing season). Not only that, he says, but it's also proving both resistant to a widely-used synthetic fungicide (phenylamide), and damaging to many so-called blight-resistant varieties of potato such as Orla, Santé and Setanta. "Once great blight-beaters, these varieties are now becoming as susceptible as Maris Piper or British Queen, a fact that's not generally known amongst the potato growing fraternity," comments David.

And if all that weren't enough to strike fear into the heart of every urban farmer, he explains that there's also the (as-of-yet-unlikely) possibility that if two sexually active strains were present together in the same crop, they could produce the overwintering form of the disease (known as oospores). These could lie dormant in the soil before re-infecting any newly-planted seed potatoes the following spring. Worse again, the disease can also cross-infect other related crops, such as tomatoes and aubergines as well as ornamentals like daturas. It's a chilling prospect for all us urban farmers as the potato-planting season begins.

Teagasc advisor Stephen Kildea agrees that Blue 13 will change the way in which potato crops are managed, whether for the private gardener or the commercial grower. "David Shaw is correct in saying that Blue 13 does appear to affect some of the varieties that previously showed good blight resistance, Orla and Santé included . . . That said, there is no reason why good management can't yield good potatoes of any variety. Regardless of which strain of blight is present, some basic disease practices should always be observed. Good hygiene is essential."

Stephen advises always using good quality, certified seed potatoes, planting as early as possible (ideally chit and preferably use early varieties) and using a fungicide like Dithane regularly from early in the season, as prevention is far better than cure. "What is of the upmost importance for the allotment grower is to get a fungicide that will prevent disease from getting into the crop. If you have a good crop and are weary of the disease, don't save the fungicide, use it! The tubers still appear to have good levels of resistance - unlike the leaves - so small amounts of fungicide can help immensely; an example is Setanta."

Both he and David Shaw warn against short crop rotations - so don't plant potatoes this spring where they grew last year. Be very aware that both old potato "dumps" left over from last year, or "volunteer" potatoes accidentally left in the ground to sprout can act as overwintering sources of infection - dispose of them now carefully by either burning or bagging them but don't leave on site. Where blight does strike, the infected plant should be lifted straight away and similarly disposed of. This will slow down but not prevent the spread of the disease. Both men also stress the importance of allotment holders/adjacent gardeners being vigilant and seeing their individual potato patches as a shared crop. "In the allotment or suburban plot situation, it is vital that people are acting as if all potato crops were one single crop from the start of the season, as blight will attack all stages of potato growth irrespective of who owns them."

But Blue 13 leaves organic gardeners, in particular, facing a bleak dilemma, as the use of copper sprays as an organically-approved form of blight protection is in the process of being phased-out (there are well-founded concerns about the build-up of copper in the soil). This leaves organic growers with no organically-approved alternative just at the time when they need it most. "We've now had three summers in a row where blight weather has predominated. If we have another one this year, many popular varieties will be 'dead in the water' without chemical protection," says David.

Which brings us back to the Sárvári Research Trust (whom David works for), a not-for-profit company established in Wales in 2002, that's working to develop new potato varieties with high levels of blight resistance. Most promising are the Sárpo (pronounced Sharpo) range, the result of a 40-year breeding programme first begun by the Sárvári family from the Lake Balaton region of Hungary as part of an initiative by the then Soviet empire. Using South American and Mexican wild potato genes, their aim was to produce "a hardy strain of potatoes for growing across the USSR which would survive the ravages of climate and disease".

The resulting potatoes have been proven to have a unique combination of high resistance to late blight, including Blue 13, and most major viruses. They're typically high-yielding, vigorous, weed-smothering and drought-resistant, giving them a light carbon footprint. In short, the Sárpo is the perfect communist potato. The range presently includes Sárpo Mira and Sárpo Axona (red-skinned maincroppers that are "good, old-fashioned and floury" according to David), with Sárpo Una, Kifli (earlier, waxy), Sarpo Shona (white-skinned) and Blue Danube (blue-flowering, blue-skinned) soon to follow. Their uniquely high resistance to Blue 13 completely does away with the need for spraying, says David.

But what about the possibility that blight-resistant potatoes can still act as an asymptomatic host to the disease, a point that Stephen Kildea makes in recommending spraying even Sárpo varieties? By the time that Sárpo varieties have a few infections, all other varieties will be dead, argues David. And their tuber resistance is such that even the volunteers would be very unlikely to transmit blight the following year. "If we're going to spray resistant varieties, then why bother breeding or growing them? Of course the answer is that if everyone grew only Sárpo, there would not be any blight around!"

Whatever potato the urban farmer decides to grow this year (and David agrees that a handful of other favourites such as Cara and Charlotte still show good tuber resistance), Blue 13 will be of huge concern to Irish gardeners. As Stephen Kildea puts it: "I suppose, summing up, the best hope we have of controlling blight this year is to pray for a nice dry summer."

Further information can be found at sarvari-trust.org as well as eucablight.org and euroblight.net. Sárpo potatoes are available from Fruit Hill Farm (fruithillfarm.com, Mr Middleton (mrmiddleton.com) and the Secret Garden (thesecretgardener.com).

  • The OPW's Victorian walled kitchen garden is in the grounds of the Phoenix Park Visitor Centre, beside the Phoenix Park Café and Ashtown Castle. The gardens are open daily from 10am to 4.30pm
  • Next week's Urban Farmeris about planting pear and plum trees in the walled garden
  • Fionnuala Fallon is a garden designer and writer