Late blight is forever

Dave Allan, writing in The Herald, a Scottish paper, almost a month ago, sang the praises of some very blight-resistant potatoes called the Sarpo group. I picked up the story because these varieties were first bred 40 years ago in Hungary by crossing local Soviet varieties with wild relatives collected by Nikolay Vavilov in South America. I stuck it on the back boiler, meaning to write something up for St Patrick’s day last Saturday.1

Meanwhile, there’s been a bit of a todo lately over field tests of GM potatoes in Ireland and England. According to The Sainsbury Laboratory’s FAQ, the potatoes have been engineered to increase their resistance to late blight, using genes from wild potato species. I think the same is true of the Irish trials, which is part of Amiga, an EU research project.2

There’ve been all sorts of responses to this news, much of it utterly predictable. People thought it “ironic” that Ireland should question the need for blight-resistant potatoes, presumably in view of the famine of the 1840s. Others questioned the need for engineering blight resistance, given that there are some extremely resistant varieties.

But few people have questioned the basic premise: that engineered blight resistance will be more durable than that achieved by conventional breeding. I’m not sure there’s evidence for this either way. In any case, I wouldn’t expect it, a priori.

The point is quite simple: overcoming resistance is what pests and diseases do. They multiply like mad, and every new individual is a new lottery ticket. Sure, the odds of a jackpot are slim. But in every case I know of, the question is not if but when.

That was my response when NPR reported last week that, according to Monsanto scientists, “considering how hard it had been to create those crops, ‘the thinking was, it would be really difficult for weeds to become tolerant’ to Roundup”. Regardless of how easy a ride reporter Dan Charles gave Monsanto, this is just daft. Natural selection has the numbers and the time to overcome anything mere researchers can come up with.

Potatoes are susceptible to late blight, and get sprayed a lot. A new variant of the blight pathogen, known as ‘superblight’ or Blue 13, destroys even the most resistant of previously resistant varieties but not Sarpo varieties. Sarpo Mira has five different resistance genes; is that enough to protect it forever from anything late blight can throw at it? No.

The Sainsbury Lab says the main reason to engineer blight resistance is because breeding is difficult; easier to insert the genes into an already desirable variety. The Savari Research Trust says Sarpo varieties are very tasty. Both laboratories, and everyone else, regardless of whether they engineer blight resistance or select for it, will have to stay in the game for as long as blight is around. Forever.

Finally, shifting back to Ireland and the famine; just how many engineered varieties (if any) are going to be deployed? Leaving aside the historical, economic and colonialist explanations for the devastation wrought by late blight in 1845, the proximate cause was the prevalence of a single potato variety, Lumpers, that was susceptible.

The danger of too little diversity remains, regardless of the crop, regardless of the source of resistance.

  1. Couldn’t manage that; Monday is probably better anyway. []
  2. Assessing and Monitoring the Impacts of Genetically modified plants (GMPs) on Agro-ecosystems, which proves, once again, that the secret to EU funding is a good acronym. []

5 Replies to “Late blight is forever”

  1. Jeremy: You argue that “the proximate cause was the prevalence of a single potato variety, Lumpers, that was susceptible.” All potato varieties in Europe seemed to have been susceptible, and that to a single clonal lineage of late blight from Mexico. (And if the disease originated in Mexico, there is no reason to think that even the vast diversity of potatoes in Peru would have any evolved resistance.). The real reason for the Irish famine was twofold. On one hand, too many people: Ireland is the only region that had a greater human population in 1845 than now. Secondly, it was not the sole variety – Lumper – that was the problem: lots of varieties would have done no better. But potato was the sole human staple food: no potatoes equals starvation.

    Large (1940) in “The Advance of the Fungi” p. 21 puts it: “… the poorest labourer in England lived on oat gruel and bread as well as potatoes. The cereal crop had not failed. In Ireland the cottiers lived almost exclusively on potatoes…If the Potato Murrain spread … millions … would starve to death.”

    Let’s hope IRRI gets it right, as rice is the present-day version of a single staple food on many millions (despite attempts to tempt populations to eat wheat noodles).

  2. When I did research on potato two years ago I noted how commercial yield had risen (in the UK) from around 30 tonnes a ha to 50 tonnes and now pushing 60 tonnes. To get this yield needs lots of water irrigation – and I came to the conclusion that blight was being facilitated by this irrigation. The answer would seem to be to apply the water in a way that does not encourage blight or to settle for less yield. If UK consumers knew how many pesticides were applied to potato the would be an outcry.

  3. Why would late blight be forever? Phytophthora infestans only parasitizes a few Solanum (and friends) species (potatoes, tomatoes, …), but not lettuce or wheat. There’s got to be a way to make potatoes a non-host like almost all other plants. More genomics please!

    Until that day, isn’t it great to have potatoes that are highly resistant, even if that doesn’t last? You call for more diversity, but I am not sure what you mean, or whether it merely is a liturgical phrase.

    Do you mean there should be more effort to breed more Sarpo’s (each with a different genetic background)? One could then try to regulate that they are grown in mixtures, or in alternating years, or regions, perhaps giving the pathogen a harder time to catch up.

  4. I don’t think it is liturgical. And absolutely I agree that it is great to have highly resistant potatoes. The Sarpo group indicates that it is possible to obtain good resistance from wild relatives. I hope others are making similar crosses, so that we have potentially different bases for genetic resistance. I also hope people are using the existing highly-resistant varieties as parents in new sets of crosses. The Sainsbury Laboratory’s rationale — let’s stick resistance into Maris Piper, because we know Maris Piper sells well (to potato growers) — is OK, but given opposition to GM (which often shares brainspace with opposition to fungicides) seems a bit short-sighted.

    True potato seed offers great opportunities to get huge numbers of plants out there, crowd-sourcing the search for plants that are both highly resistant, productive and tasty in their different ways.

    As for growing mixtures, I don’t think that’s going to work in mechanised agriculture, unless the breeders either come up with skin colour markers that permit mechanical sorting of different varieties, or get different kinds of resistance into essentially the same potato, so that cooking and taste qualities are identical.

  5. A very important factor in maintaining isolates of late blight that can overcome single or combined resistance genes in potato is that once the organism overcomes the resistance, it can over winter in potato tubers and spread infection from volunteer potato plants in the next season. Therefore, it has to evolve only once to overcome the resistance genes, and late blight has the ability during a season of severe infection to change and overcome the genes in a stepwise fashion.

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