Some parasites warrant international attention. Earlier this month, a report from the UN Food and Agriculture Organization announced that stem rust fungus had been identified in fields in Iran. Why the big deal about some smutty plants? Because they’re a strain of wheat that was supposed to be resistant to the fungus in the first place.
Stem rust (Puccinia graminis)
has been a pest ever since humans invented agriculture. It parasitizes cells in the stems of grass crops like wheat or oats. Different strains of the fungus specialize on different host plants – a wheat stem rust won’t be caught dead on an oat stem, for example – but their lives all follow the same general, ugly pattern: fungal spore lands on plant, invades the cells in its stem, proliferates, and builds spores that eventually erupt outward in large red masses on the surface of the plant. Each mass releases more spores, which are carried off by the wind to find new plants to infect. The fungus is devastating enough to an individual plant. Now imagine what can happen when millions of identical plants are planted side by side.
Historically, farmers feared rust because it could turn a promising crop into a field of broken and dying plants. Then came the Green Revolution of the 1950s, and new wheat strains bred (by Norman Borlaug himself!) to resist rust infections were planted around the world. Crop yields soared, and starvation was held at bay for millions and millions of people – a good thing, without doubt. But given enough time, natural selection has a way of messing these things up.
In this case, it’s probably the same old arms race we’ve heard about so often. If you repeatedly expose a population of bacteria to an antibiotic, eventually the only bacteria that survive are the tough bastards with some natural resistance to that antibiotic and their descendants. If you grow nothing but rust-resistant plants, the only fungus that survives is the one with ways to get around the plants’ resistant properties. And that’s what scientists found growing on a resistant strain of wheat in Uganda in 1999. Its discoverers called it Ug99. And thanks to wind-borne spores, it spread to Kenya in 2001, Ethiopia in 2003, and has now jumped the Red Sea and appeared in Iran. Scientists are concerned that the spores will continue to spread unnoticed on the wind and affect crops in Afghanistan, India, and Pakistan, and eventually in countries even further east. The fear is that the new strain of rust will restart the whole cycle of crop death and famine that Borlaug helped stop sixty years ago.
But unlike a giant rock from space, we can do something about this threat. Borlaug bred the first strains of rust-resistant grain with nothing more than the persistent repetitive crossing of plants. Nowadays we have the ability to analyze and manipulate genes. All we need is the will.
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