Glyphosate-resistant horseweed confirmed in Michigan

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Glyphosate-resistant horseweed (Conyza canadensis) has been confirmed in a Christmas tree plantation in Mason County, Michigan. This detection represents the 16th state in the United States with glyphosate-resistant horseweed (Heap 2006). Also known as marestail, horseweed is a troublesome weed in no-till field crops, fruit crops, tree plantations and nurseries throughout Michigan.

For the past several years, MSU Diagnostic Services has been screening horseweed populations for glyphosate, ALS, triazine and PPO resistance utilizing greenhouse whole plant assays. ALS-resistant horseweed has been documented in 35 field crop locations and seven Christmas tree plantations spanning 14 Michigan counties. Triazine-resistant horseweed has been documented in two field crop locations, 12 Christmas tree plantations and four blueberry plantations spanning five Michigan counties. While glyphosate resistance was suspected by the growers in some of these populations, herbicide resistance proved not to be the case until now.

Christmas tree growers commonly use a soil-applied, residual herbicide in early spring followed by one to several applications of glyphosate during the season. The initial greenhouse screen of this Mason County horseweed population, yielded survivors of glyphosate at 1X and 4X the labeled rate (22 and 88 oz/A Roundup WeatherMAX 5.5L, respectively, plus AMS at 17 lbs/100 gallons). These plants were stunted and delayed in development, but were able to bolt and produce seed. Seed was collected from these survivors, and subsequent dose response experiments were established with the original field population and greenhouse population.

Horseweed plants were treated with glyphosate at 0.01X, 0.1X, 0.25X, 1X, 4X, 10X and 100X the labeled rate (0.22, 2.2, 5.5, 22, 88, 220, and 2200 oz/A Roundup WeatherMAX 5.5L, respectively, plus AMS at 17 lbs/100 gallons) when rosettes were two to three inches in diameter. Visual weed control ratings and dry weights were taken 21 days after treatment.

While GR50 values have not yet been tabulated, these experiments consistently show the ability for this population to survive glyphosate at 1X, 4X and occasionally 10X the labeled rate.

Testing for herbicide resistance

If herbicide resistance is suspected in any weed species, samples may be submitted to MSU Diagnostic Services for a resistance screen. In most circumstances, a whole plant pot assay established from seed will be the standard test for herbicide resistance confirmation. Mature, high quality seed or seedheads should be collected from suspicious plants in late summer or fall, and submitted in a paper bag or envelope. Do not seal plants or seed in plastic!

Generally, fees associated with herbicide-resistant weed testing are $50 per sample per herbicide site of action (i.e. ACCase inhibitors, ALS inhibitors, Photosynthesis inhibitors). Each additional site of action is $20 per sample. However, costs associated with horseweed samples submitted by Michigan soybean producers are covered by check-off dollars through the Michigan Soybean Promotion Committee. Samples submitted from out-of-state producers are $75 per site of action and $30 for each additional site of action.

Please contact
Steven Gower (517-432-9693, .(JavaScript must be enabled to view this email address)) with any questions regarding resistance confirmation or sample collection. Samples can be mailed to:
Michigan State University
Diagnostic Services
101 Center for Integrated Plant Systems
East Lansing, MI 48824-1311
Attn: Steven Gower

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