Apparently there's a new fungus that causes stem rust in wheat, defeated long ago but back for another round: The enemy is Ug99, a fungus that causes stem rust, a calamitous disease of wheat. Its spores alight on a wheat leaf, then work their way into the flesh of the plant and hijack its metabolism, siphoning off nutrients that would otherwise fatten the grains. The pathogen makes its presence known to humans through crimson pustules on the plant’s stems and leaves. When those pustules burst, millions of spores flare out in search of fresh hosts. The ravaged plant then withers and dies, its grains shriveled into useless pebbles. Stem rust is the polio of agriculture, a plague that was brought under control nearly half a century ago as part of the celebrated Green Revolution. After years of trial and error, scientists managed to breed wheat that contained genes capable of repelling the assaults of Puccinia graminis, the formal name of the fungus. But now it’s clear: The triumph di...