The weed spreads by the distribution of pollen and seeds but also by the spontaneous emergence of mutations
Waterhemp has crept its way along rapidly in recent years and is now considered one of the most problematic agricultural weeds in North America.
It is native to the northeastern United States and first appeared in Canada in Ontario’s Lambton County in 2002. It has since spread to seven counties in the province and was detected in Manitoba in 2017.
Waterhemp damages crop productivity and yield, and glyphosate resistance was detected in 2010.
At the University of Toronto, a team of biologists has now identified how the weed has spread to fields of soybeans and corn.
They found two mechanisms by which waterhemp strains spread by comparing the genomes of the weed from midwestern U.S. farms and the genomes of waterhemp in southern Ontario.
It spreads by pollen and seeds dispersed by wind, water, birds and the fur of animals. It also spreads by the spontaneous emergence of mutations that then also spread.
“Waterhemp is a wind-pollinating plant,” said Julia Kreiner, PhD candidate at the university’s department of ecology and evolutionary biology and lead author of the study.
“We’re talking about long-distance dispersals. It’s likely that they are travelling through animals as these tiny seeds get stuck in fur. They can get stuck on someone’s boot and they float on the surface of streams and can be caught up on boats. They can get into riparian areas and float in river habitats. Not only can they do long-distance invasions, but a single plant can produce up to five million seeds, which is a lot of opportunity for loads of seeds to be transported to new areas.”
Kreiner said it is like studying an evolutionary process.
“What we did was try to figure out how many times new, independent mutations are appearing and fixing,” she said. “Are they different populations or have they independently evolved? Or is it more likely that what we are seeing is the spread of resistance to achieve seed dispersal?”
She said they found waterhemp populations that are independently evolving resistance and they know that the symptoms are arising repeatedly in different populations.
As well, within a single population, researchers have found different mutations.
“It speaks to the fact that, with this plant, it’s going to be a recurring problem with populations evolving resistance repeatedly.”
She said they found that the genomes of some herbicide-resistant plants in Ontario were nearly identical to those in U.S. plants, suggesting a close relationship, and that plants in Ontario came from seeds transported from one field and dropped in another.
The same DNA analysis identified some resistant plants that did not genetically match any other plants, which could suggest that they appeared through the independent emergence of a genetic mutation that carried resistance.
Two regions in Ontario, Walpole Island and Essex County, have waterhemp populations that developed resistance to glyphosate independent of resistant strains that came in from the U.S.
The study has shown that weed resistance to pesticides can occur over a short time and can occur repeatedly, and that this evolutionary adaptation can use multiple mechanisms.
“What we can do is use multiple different types of herbicides that all target different genes so the likelihood of a mutation arising at each of those different sites is much lower if applying the herbicides at different times,” said Kreiner.
She said evolution can occur quickly when the same practices are relied on over and over again, which emphasizes the need for growers to rotate their crops and alter herbicides.
As well, controlling the spread of resistant waterhemp means integrated management approaches, she said, and adopting practices such as cleaning agricultural residue from equipment before moving from field to field.
Kreiner said studies continue to look for more information to better understand why waterhemp is more likely to evolve resistance so rapidly.
“We are running out of herbicides,” she said in a news release. “These plants have evolved resistance to pretty much every herbicide we’ve come up with and it doesn’t seem like there’s ever going to be a herbicide that a weed can’t eventually evolve resistance to.”