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 Palmer’s resistance to PPO-inhibiting herbicides, a group of
chemicals that disrupt chlorophyll synthesis, is especially
problematic with glyphosate out of the picture. Farmers had been
turning to PPO-inhibitors as an effective alternative, until
resistance was discovered in waterhemp in 2001 and in Palmer in
2011.
Pat Tranel from the University of Illinois has been working to
understand the mechanisms of resistance to PPO-inhibitors for
years, and was the first to discover key mutations in both weed
species. Now, in two new studies, he goes farther to explain
Palmer’s evil genius.
“We knew Palmer had the same molecular mechanism as waterhemp to
resist PPO-inhibitors, a genetic mutation known as the gly-210
deletion, and at least one more. Now we know that it evolved the
gly-210 deletion independently, rather than picking it up
through hybridization with waterhemp,” says Tranel, associate
head and professor of molecular weed science in the Department
of Crop Sciences at U of I.
This is important in two ways. It’s good news that scientists
aren’t finding evidence of hybridization between the two
superweeds, at least not so far. But the fact that Palmer
evolved the same mutation independently, and at least one more
to boot, shows just how wily the weed is.
Tranel and his team determined the evolutionary origins of the
gly-210 mutation by looking at the genetics of resistant plants
of both species that were growing together in a Kentucky field.
Being in close proximity for several years should have provided
opportunity for hybridization, if it was going to happen.
“We know from lab experiments that they are capable of
hybridizing, so the fact that it’s not happening in the field is
a good thing. The more they can and do hybridize, the more
concerns we’d have,” Tranel says.
Only about a third of the Palmer plants in the Kentucky field
had the gly-210 deletion. The rest were using a different
mutation – an arginine substitution – to ward off PPO-inhibitor
damage.
“The finding that this population of Palmer has two different
mutations is a concern because if you look forward in the
future, Palmer is well positioned to deal with future PPO
chemistries. It can use whichever is more effective against a
new PPO.
“It’s also well positioned to combine the two mutations to
create a double mutant, with both mutations on the same copy of
the chromosome. Chemistry designed to kill plants with the
gly-210 deletion won’t be able to kill double mutants,” Tranel
says. “In my opinion, it’s just a matter of time until we see
double mutants in the field.”
Tranel’s second new study explains why Palmer
amaranth took a decade longer than waterhemp to develop the
gly-210 deletion, and reveals another diabolical truth about the
species: Palmer amaranth appears to be naturally tolerant to
post-emergence PPO-inhibitor application.
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It has long been recognized that the timing of
post-emergence PPO application is especially critical for Palmer
amaranth, relative to waterhemp. If Palmer plants aren’t sprayed
before they reach about 4 inches, it’s all over.
“If you wait too long, you miss ‘em. And too long can be a matter of
a single day because Palmer grows so fast. It can go from a 4-inch
plant where you could control it to a 6-inch plant literally in a
day,” Tranel says.
For Tranel, the pattern suggests a natural tolerance to
post-emergence PPO-inhibitors. Tolerance describes the ability of a
species to handle a substance, in this case PPO herbicides.
Resistance, on the other hand, happens at the population level;
localized populations of the species evolve mutations in response to
repeated exposure to the substance. For example, corn is tolerant to
atrazine. It can handle being sprayed and doesn’t need to evolve a
mutation to handle it in a particular population.
The idea is that Palmer amaranth has a natural tolerance to PPO
inhibitors and didn’t need to develop resistance. That’s why it took
longer to evolve the gly-210 mutation. But, until now, no one had
specifically studied Palmer’s tolerance to the chemistry before.
Tranel confirmed it by growing Palmer and waterhemp plants with and
without the gly-210 mutation side-by-side and applying different
formulations of pre-emergence and post-emergence PPO-inhibitors. The
post-emergence applications were done early (smaller than 4 inches)
or late (taller than 4 inches).
“We found that ‘sensitive’ Palmer plants without the mutation
survived just as well as resistant waterhemp when sprayed
post-emergence,” Tranel says.
On the other hand, the research team found that pre-emergence
formulations effectively controlled both species.
“The difference in tolerance between Palmer and waterhemp goes away
at the pre-emergence stage,” Tranel says. “Ultimately, that’s the
take-home message here. If you’re dealing with these weeds,
especially Palmer amaranth, and you want to incorporate a
PPO-inhibitor as an alternative effective mode of action, you’ll
have much better luck if you use it in a pre-emergence application.”
The studies are published in Weed Science [DOI: 10.1017/wsc.2019.41]
and Weed Technology [DOI: 10.1017/wet.2019.84]. Kathryn J. Lillie,
Darci A. Giacomini, and Patrick J. Tranel, all from the College of
Agricultural, Consumer and Environmental Sciences at Illinois, were
authors on both papers. Jonathan D. Green of the University of
Kentucky was an additional co-author on the Weed Science paper. The
Weed Technology study was partially funded by Valent U.S.A.
[Source: Pat Tranel
News writer: Lauren Quinn] |
