Speaking at the Indiana Certified Crop Advisers Conference in December, 2014, Bill Johnson signaled that Palmer amaranth might not become the problem in Indiana that it is in the South. Johnson is the Purdue University Extension weed control specialist.
Related: Weed resistance is tough to predict
He wasn't implying that if you have it, it isn't a problem, because it certainly is, he says.
"What we're saying is that its advance across Indiana appears to have slowed down. That may be partly because people got the message and got ahead of it."
It's also partly due to nature itself, he adds. Palmer amaranth prefers a slightly drier environment than what Indiana offers on average, and certainly drier environment than in 2015.
What Johnson is seeing is an increase in a phenotype of giant ragweed resistance that is different from the typical response from glyphosate resistant giant ragweeds.
"The type of resistance we had in giant ragweed until recently allowed you to respray and burn the weed down for most of the season, " he says. "This new biotype of resistant giant ragweed looks like it is dying faster, but then it actually looks more normal again, and isn't knocked back much. You won't knock it down with respray."
The new biotype features yellowing and some necrosis on giant ragweed leaves, he says. It typically shows up fairly quickly after application, and is called "fast-acting necrosis."
It's possible to have both biotypes of giant ragweed in the same field, he adds. Currently, glyphosate-resistant giant ragweed has been documented in 30 Indiana counties, Johnson says. He expects it's actually in a greater number of counties than just 30.
Of the 30 counties where glyphosate-resistant giant ragweed has been documented, about half have the original phenotype, and half have the new, fast-acting necrosis version. As mentioned earlier some of these fields will have both types.
In the coffee shop, it is known as Palmer pigweed. In university circles, it is referred to as Palmer amaranth. Whatever you want to call it, this weed is the No. 1 weed to watch. Stay on top of your control plan with our new free report, Palmer Amaranth: Understanding the Profit Siphon in your Field.