2022 status of herbicide-resistant weeds in Michigan
Each year new herbicide resistance cases are identified by MSU Plant & Pest Diagnostics
In 2021, Michigan State University Plant & Pest Diagnostics (PPD) received 75 samples for herbicide resistance screening. The majority of these samples (61) were submitted during the summer months as part of a molecular testing pilot program sponsored by the Michigan Soybean Committee (MSC) under the direction of Eric Patterson and Erin Hill, as well as M.S. student Juliano Sulzback. The goal of this program is to develop new molecular assays for detecting resistance to glyphosate and/or acetolactate synthase (ALS) inhibiting herbicides on some of Michigan’s most economically important weeds so that it may become a service offered by PPD. Thanks to the submissions from a few perennial PPD clients, the following weeds were investigated: Palmer amaranth, waterhemp, common ragweed, giant ragweed, and marestail/horseweed.
The processes of sample handling, DNA extraction, and DNA amplification presented some learning curves; 39 of the samples were successfully screened and we identified a novel mutation, not yet reported in marestail/horseweed for ALS herbicide resistance. The other 22 tissue samples had insufficient DNA as a result of degradation. These issues are being resolved in 2022 by expediting delivery or drop off of samples. We worked diligently throughout the fall and into the spring and summer months of 2022 to redesign some primers needed to amplify DNA regions of interest. Previously published primers did not always work for Michigan biotypes of particular weed species. We are continuing to streamline this process with the expectation that this service will be offered publicly in the future.
The remaining 14 samples submitted in 2021 were screened for herbicide resistance using traditional bioassays in the greenhouse. Traditional bioassays involve growing weeds from seed and spraying them under controlled conditions. This process is time consuming, however traditional bioassays offer comprehensive results and are not limited to what is currently understood regarding the mechanisms of resistance. Eight of these 14 samples were from Michigan and five from out of state. All but two of the Michigan samples were sponsored by the MSC or the Michigan Vegetable Council (MVC) in partnership with Sushila Chaudhari. The MSC covers screening for select weed species growing in soybean rotations (i.e., pigweeds, ragweeds, marestail/horseweed, and common lambsquarters) and the MVC covers any suspected resistant weed found in vegetable rotations.
Of the seven Michigan samples, six were confirmed to be resistant to various herbicides. Three of these cases had not previously been confirmed in the counties in which they were sampled. ALS inhibitor resistance (G#2) in redroot pigweed was documented for the first time in Gratiot County and resistance to glyphosate + ALS inhibiting herbicides (G# 9 + 2) in waterhemp was confirmed in Allegan County. Finally, in large crabgrass, acetyl coA carboxylase (ACCase, i.e. graminicide, G#1) resistance was confirmed for the first time in Oceana County, making it the second Michigan county to have a confirmed case.
Two new species were screened in 2021 and found to be resistant. The native species six-weeks fescue (Festuca octoflora) is not a common weed in field crops; however, struggles with control were noted in a rotation in Hillsdale county. This population was confirmed to be resistant to glyphosate. The second novel case noted in 2021 was discovered by accident. Seed from a St. Joseph county population of johnsongrass (Sorghum halapense) was collected, with the help of MSU Extension educator Lyndon Kelley, for comparison to two out of state samples. The expectation was that this Michigan sample would be susceptible to the modes of action being screened, however ALS resistance was confirmed. Fortunately, one of the out of state sample proved to be susceptible to the modes of action screened.
Herbicide resistance testing via bioassays will again be available in the fall of 2022. Fully mature seeds are needed for these screens, with most annual species expected to have mature seeds starting in mid-September through frost. All submissions are due by mid-November 2022. Information on these supported programs, fees for other types of samples, and the submission process can be found on the MSU PPD website or in the back of the 2022 Weed Control Guide for Field Crops (E-434). Information and photos showing how to collect seeds for the bioassays is available in “Tips for collecting weed seeds: Ensure your resistance sample gets tested.” If you have any questions regarding the 2021 results or future sample submissions, please contact Erin Hill (email@example.com) for more information.
Thank you to Christy Sprague for reviewing this article.
New county locations and/or incidents of resistance confirmed from 2021
*G# refers to the herbicide site of action group
- ALS-resistant (G#2): St. Joseph
- ACCase resistance (G#1): Oceana
- ALS-resistant (G#2): Gratiot
Six weeks fescue
- Glyphosate-resistant (G#9): Hillsdale
- Multiple resistant (ALS-G#2 + glyphosate-G#9): Allegan