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Weed Resistance Testing in Ontario

Mike Cowbrough and Cesar Cappa – OMAFA (Guelph)

Weed resistance can be tested using two methods: molecular testing and plant bioassay. Each has its advantages and limitations.

1. Molecular Testing

Conducted by: TurnKey Genomics
Submission form: https://turnkeygenomics.ca/sample-submission-forms/

Strengths:

• Easy sample collection (leaf tissue).
• Fast turnaround time.

Limitations:

• May produce false negatives if resistance is due to non-target site mechanisms, which molecular tests cannot detect.
• Limited to specific weed species and herbicide groups.
• A positive result indicates resistance to an entire herbicide group, even though some herbicides within that group may still be effective (e.g., resistance to Group 5 may be based on atrazine, but metribuzin could still work).

2. Plant Bioassay Testing

Conducted by: Tardif Lab, University of Guelph
Submission form: https://www.plant.uoguelph.ca/resistant-weeds

Strengths:

• Can test any weed species and herbicide group.
• No false negatives.

Limitations:

• Requires mature, viable seed in sufficient quantity.
• Testing is conducted over winter, so results take several months.

Example: False Negatives in Molecular Testing

In a recent molecular test of waterhemp for resistance to Groups 2, 5, 9, and 14 herbicides, all samples were reported as susceptible to Groups 5 and 9, and resistant to Groups 2 and 14. However, historical bioassay data shows high resistance to glyphosate (Group 9) and atrazine (Group 5) in waterhemp, suggesting the presence of non-target site resistance that molecular testing missed.

Test Summary:

• Samples submitted: 10 waterhemp leaf samples.
• Method: Detection of genetic mutations linked to target-site resistance

Best Practice Recommendation

Use molecular testing to quickly identify target site resistance, any susceptible results from molecular testing should be followed up with a plant bioassay test.

Using the above example, the molecular test quickly identified group 2 and group 14 resistance in waterhemp. The submitter should follow-up by collecting seed and submitting it to the University of Guelph to test for group 5 and group 9 resistance.

Plant bioassay remains the gold standard for herbicide resistance testing. While molecular testing offers speed and convenience, its inability to detect non-target site resistance means “susceptible” results should be interpreted with caution.