Return of the Potato Blight


Irish infestationGlobal blightA resistant spud?The poor farmers' stake


Fries are the essential fast food. Will the new blight kill our nicest vice?











The late blight pathogen can reproduce either sexually or asexually, depending on the genetics of nearby blight organisms.
© 1998 The American Phytopathological Society, courtesy Gail Schumann.
























Grayish-white blight appears on top of two green tomatoes.
Late blight also affects tomatoes, and has forced some farmers to abandon these toothsome fruits.
Copyright and courtesy Ohioline

    Target destroyed
The disease that destroyed Irish potatoes is now killing spuds in many potato-growing regions around the globe. Late blight, is more dangerous than ever, partly because many strains resist chemicals that once controlled it, says A batch of yellow-brown friesWilliam Fry, a Cornell University plant pathologist who concentrates on the organism.

In places that are heavily reliant on potatoes, a second coming of the Irish potato famine could be on the menu... (Late blight can strike at any time in the season but is mainly troublesome in wet weather. Early blight is a different, less ferocious disease.)

The late blight pest, Phytophthora infestans, has a complex life cycle and can attack potatoes (and tomatoes) in several ways. The assault usually begins when airborne spores land on the foliage and either germinate directly or release swimming zoospores. In either case, the pathogen penetrates the leaf or stem and grows into a dark lesion that releases thousands of new spores. Carried by wind or rain-splash, the spores can infect the same or other plants, sometimes miles away.

Zoospores can be washed from the foliage to the potato tubers in the soil, causing them to rot immediately or in storage. Infected seed tubers can start a new cycle of infection in a following season.

A complicated life cycle, showing sporangia and their interactions with plant tissues

A sexual predator
Diseases don't just happen, so it's fair to ask why late blight is making a comeback. Major credit goes to a strange sexual practice in Mexico, where two strains of the P infestans pathogen have joined to create super-microbes that are destroying potato fields and threatening the souls of french-fry freaks from France to Philly.

The late blight pathogen is bisexual, meaning that it can make male or female reproductive organs.

If both mating types, imaginatively labeled A1 and A2, are present, sex can happen. Otherwise, P. infestans makes virtually identical offspring without bothering (is "bothering" the right word?) with sex.

Say you've got some type A1 blight. Any nearby A2 mating strain will release pheromones -- chemical signals -- that direct the A1 to form sexual structures, causing sexual reproduction and mixing genes between two plants. As you'd expect, the opposite attraction also holds -- A1 can trigger A2 to get sexual.

Now, instead of identical offspring, you have a wealth of blight strains with genetic differences. From there, evolution through natural selection allows the organism to change and master new conditions.

That's a big improvement -- unless you grow spuds or like baked potatoes...

Theoretical sexuality
These sexual politics were irrelevant to the Irish famine, which was caused by asexually reproducing A1 pathogens. It wasn't just that (shall we say) sex was just a gleam in the blight's eye, but nobody knew mating type A2 even existed.

Then, in the 1940s or 1950s, A2 was discovered west of Mexico City, where the pathogen originated. The orgy began. "It was very clear this was a sexual population," says Fry. By the 1970s or 1980s, strains of A2 had escaped and were spreading through the Americas, Europe, Asia, and North Africa.

A second potato blight was on the move.

Now for the bad news
The increased diversity lead to a major genetic "improvement:" Many new strains of late blight resist metalaxyl, a fungicide that used to control the disease rather well.

Given the lack of good alternative chemical controls, that would be a scary enough. But the sexual stage has another trick. It can two produce oospores that can survive even without spuds (something we Why Filers would hate to attempt). The earlier, non-sexual versions are "obligate parasites" -- to survive, they need a living host plant.

The plant-path jargon conceals a disaster. Nowadays, oospores of the new, "improved" blight can contaminate soil, which alters the whole course of the disease. Late blight outbreaks once required both newly arrived blight spores and damp weather. Now, fields can remain infected for years, waiting for the damp weather that triggers outbreaks. "That changes the way we manage disease," says Fry. "It used to be spectacularly erratic and sporadic, now it's predictable, more persistent and troublesome."

If the disease is present in the soil, farmers who can afford it may be able to dollop on more fungicide and hope for the best. (Crop rotation, which deters some diseases, is less helpful since oospores can survive two or more years in the dirt.)

Often, faced with the extra expense and risk of late blight, "Growers quit growing potatoes, get out of the business," say Fry.

An apparently healthy-looking potato field, thickly leaved, purple flowers

Blight strikes quickly. Six days after infection (above), these Yungay potatoes look great, but are living dead. After 18 days (below), they are kaput.
Courtesy Rebecca Nelson, International Potato Center.

lots of dirt, a few scraggly, wilted plants are left

Not a fungus? Rot, I say!
It's a bit peculiar that any fungicide would control Phytophthora infestans, because the "late blight fungus" is not a fungus at all. Rather, it belongs to a group called the Oomycetes, which includes diatoms, brown algae, and kelp.

(Help! said the taxonomist with a yelp. The little whelp is not a rot, it's a kelp -- or at least it's brown algae!!)

The reclassification, which has been discussed for 50 years, is achieving "gradual acceptance" among biologists, says Fry. Sequence data on genes and structural data from proteins both show that Oomycetes are more closely related to plants and fungi are closer to animals.

The reclassification could matter in the real world. Despite the success of the fungicide metalaxyl, it makes little sense to test other fungicides if you're interested in killing things that are related to kelp. Instead, chemicals that are active against algae would be more appropriate.

Teaching spuds to quell kelp
For cost and environmental reasons, Fry says farmers would prefer to plant potatoes that mount their own defense against late blight. "Resistant potatoes are mandatory, we're moving very slowly to get there," he says. One major obstacle is that the "market is so demanding, it's controlled by big buyers, Frito-Lay and McDonald's, and is very traditional."

You can't make long fries, obviously, without long spuds. So you don't need to simply breed resistant potatoes -- you need to make sure they have all of the other characteristics that make big buyers spend bucks.

Fry notes that a single variety, Russet Burbank, accounts for 30 percent to 40 percent of United States production. "That variety is 100 years old, but everybody wants the same thing; they know how to work with it."

Since resistant spuds are only likely to have much impact, at least domestically, if they meet the cosmetic needs of large processors, Fry suggests using genetic engineering to move genes into potatoes that already make big processors want to heat up the fryer.

Want more on spuds that resist late blight?



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