The disease affects cereals, especially wheat. It’s caused by a fungus called Puccinia graminis, which infects crops via spores that are very hardy and can travel long distances in the wind. It’s not always clear in the beginning stages whether a crop has been infected, but in the later stages, it’s obvious: red or black pustules form on the stalks and leaves, causing the plant to produce fewer seeds and, in some cases, die.
Various iterations of stem rust have been around as long as wheat has been grown, and at some points has caused disastrous problems. The last major North American outbreak was in the early 1960s, affecting both wheat and oats, and destroyed over 5 percent of the crop. But that’s nothing compared to the Ugandan outbreak in 1999, which is estimated to have decimated more than 70 percent of the crop. (That particular strain was named Ug99, in response.)
Stem rust has shown a remarkable ability to adapt to any attempts to fight it; there has essentially been an arms race for decades, with agriculture scientists attempting to stay a step ahead of the voracious fungus. A new and significant step forward from the University of Sydney was just announced in an article in the journal Science.
Many modern varieties of wheat are genetically modified to include Sr50, a gene that combats stem rust, but the fungus can sometimes overcome that gene. If that happens, the only option is to spray expensive and damaging pesticides, but even that measure is often too late, as by the time stem rust reveals itself, it may not be possible to salvage the crop.
But by sequencing the fungus’ genome, the Sydney researchers have pinpointed the genomic sign that indicates an SR50-resistant strain of stem rust. This knowledge led to the development of a DNA test that can determine what kind of treatment the crop will need far earlier than current methods allow – within the span of hours, not weeks.
It’s a major breakthrough to help reduce pesticide use and protect wheat crops, but this is, as with many developments in pest warfare, potentially temporary. As the fungus continues to evolve, scientists may have to go back to the drawing board. But for the moment, it appears that the humans are ahead.