Molecular herbicide resistance testing is bad news for weeds

By Carol Brown, Soybean Research Information Network

The main method for getting rid of weeds in a crop field is fairly simple: spray them with a herbicide. But some weeds are resistant to herbicides. Knowing which ones are resistant ahead of time could save farmers time and money by not applying a herbicide that isn’t going to work.

Michigan State University weed geneticist Eric Patterson is working to identify herbicide-resistant weeds faster. He led a research project with Michigan soybean checkoff support to detect resistance earlier through molecular diagnosis.

“The current process for farmers to find out if weeds are herbicide resistant is to send seeds from weeds that survived herbicide applications to a weed diagnostics clinic. At MSU, Erin Hill is one of the few dedicated weed diagnosticians in the country,” Patterson explains. “When she receives weed seeds in the fall, she grows the plants and conducts a ‘dose response’ assay. Hill sprays the weeds at increasing herbicide doses to see at what level the plant dies. Simply speaking, if the amount is higher than the dosage allowed to spray in the field, the weed is considered resistant. It could take up to a year to go through this process.”

Patterson’s research project explores whether finding the resistance genetically could predict if a herbicide will be effective or not.

“We have a series of lab techniques where we look at a weed’s extracted DNA code. If a particular DNA pattern changed, or mutated, we can say whether it is resistant or not, without the need for growing seeds or spraying anything,” Patterson says. “This process takes about three or four days between getting the plant tissue and finding the results.”

In this research project, Patterson and graduate student Juliano Sulzback focused on five prominent Michigan weed species: waterhemp, Palmer amaranth, marestail, and common and giant ragweed, and their reactions to ALS-inhibiting herbicides. These herbicides inhibit the acetolactate synthase, or ALS, enzyme in the weed, preventing the plant from making essential amino acids, which interrupts plant growth. Resistance to this type of herbicide is nearly always caused by a gene mutation, Patterson says, which are well-characterized in most weed species, making it an easy trait to work with. They also worked with glyphosate resistance mutations in pigweeds and developed rapid molecular diagnostics for them as well.

Sulzback sequenced nearly 200 plant samples to identify if they had the gene mutation and to document its frequency in Michigan soybean fields. He was able to reach greater than 90% accuracy in marestail, common ragweed, waterhemp and Palmer amaranth through this molecular testing. In other words, they could correctly identify if there was an ALS resistance mutation or not in 90% of individuals of these four weed species.

Patterson and his team began this work in the middle of the COVID-19 pandemic, which created unforeseen obstacles.

“When we began this project, we asked farmers across the state to send us weed samples in the mail. We didn’t anticipate the slower mail service during this time,” Patterson comments. “We received plants that had rotted and turned to mush. We tried the tests with the degraded material, and we were still able to diagnose about 50 to 60% of them. But with fresh tissue, our results are in the 90% range.”

The team conducted the work through crop year 2022 with funding from the Michigan Soybean Committee, Michigan Corn Marketing Program, and MSU Extension’s Project GREEEN. Patterson hopes to expand this work to look at resistance to different herbicides and in other weed species across the north central region.

“As ALS resistance is known, we are now concerned with rarer things such as 2,4-D, dicamba, or PPO resistance, as there are new soybean traits for these chemistries,” Patterson says. “Those are harder because we don’t necessarily know the molecular mechanisms, but when we do, we can then develop diagnostics and employ them.”

If Patterson’s research studies are successful, identifying herbicide-resistant weeds could become much faster and someday possibly be done in the field – which could mean big trouble for weeds in the future.

This research is part of the Soybean Research Information Network and was funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program.

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