Early detection, monitoring and management of herbicide resistant weeds in field vegetables
Project Code PRR16-010
Marie-Josée Simard and Robert Nurse - Agriculture and Agri-Food Canada
To develop and communicate to growers tools and approaches for early identification and risk mitigation of herbicide resistance in vegetable crops
Weeds are an important challenge for viable vegetable production that contribute significantly to yield losses. Typical weed management in vegetable crops relies on regular application of herbicides. However, because only a few herbicides are registered and available to growers, weed resistance to herbicide groups used in these crops has become a major problem. The rapid proliferation and spread of herbicide resistant weeds poses a constant threat to the sustainability of vegetable crop production. To effectively manage the risk of resistance development it is essential that resistant weeds are detected early, monitored regularly, and properly controlled using integrated management practices.
In 2015, a Pest Management Centre (PMC)-led stakeholder working group identified herbicide resistance and early detection of resistant weeds as a priority issue to be addressed by the PMC’s pesticide risk reduction strategy for Integrated Weed Management in Field Vegetables. The traditional resistance detection methods (dose response assays using living plants grown in the greenhouse) take up to a year to get results back to growers. This 3-year project set out to survey and document new herbicide resistant weed biotypes in vegetable crops in Ontario (ON) and Quebec (QC) and develop rapid, molecular-based herbicide resistance diagnostic tests to generate results in less than a week.
The project was conducted in the labs, greenhouses and experimental fields of Agriculture and Agri-Food Canada’s (AAFC) Saint-Jean-sur-Richelieu (QC) and Harrow (ON) Research and Development Centres. Suspected herbicide resistant weed populations were sampled in commercial vegetable farms by Ontario Ministry of Agriculture, Food and Rural Affair (OMAFRA) and Quebec Ministry of Agriculture, Fisheries and Food (MAPAQ) specialists, agri-business, agronomists and growers and the resulting leaf materials were submitted for analyses to AAFC, OMAFRA or MAPAQ labs. Molecular markers were developed in the lab as rapid diagnostic tools for each suspected weed x herbicide combination received. This was achieved by extracting DNA from leaf samples of resistant plants and comparing it to the DNA of susceptible (non-resistant) populations in order to find mutations that confer herbicide resistance and then develop markers that would detect these mutations. For some combinations (for example, pigweeds resistant to linuron), mutations had already been discovered and published, thus only marker development was necessary. Other cases required the discovery of new unpublished mutations. Samples were also subjected to traditional diagnostic method to validate molecular results.
Between 2016-2018, a total of 38 cases of weed species x herbicide group resistance combinations have been reported in horticulture crops in ON and QC, and at least 12 quick genetic tests were successfully developed for 10 weed species during this project. Two of these testing methods are new discoveries and one of these unveiled an unexpected mechanism of resistance (for example, crabgrass resistant to AACase herbicides). Since 2018, the protocols for these tests have been shared with the MAPAQ’s Pest Diagnostic Lab (available in French only) as a pilot project so that this Lab offers the testing as a free service to commercial growers. About 1,000 samples (several samples are tested for each case) of suspected weeds (for example Canada fleabane, large crabgrass, lamb's-quarters, pigweed species, common ragweed, Eastern black nightshade, redroot pigweed) collected from farmers’ fields in ON and QC have been processed by this lab to date. At least 52% of submitted weed samples have tested positive for herbicide resistance and all were validated. Based on test results coming out of the diagnostic lab, recommendations on type of herbicides to avoid were delivered regularly to growers through the provincial specialists collaborating in the project within 10 days from sampling.
Survey data from Ontario suggested that the presence of linuron resistant pigweeds in a field was not related to the number of herbicide applications in the given field but to the presence of resistant biotypes on the farm. Once resistance is present on the farm, it can be observed in any field regardless of its history of herbicide applications. The project also demonstrated that both target-site (mutations in a single gene related to the herbicide mode of action) and non-target site (resistance is more complex and conferred by mutations that are not located where we would expect them) linuron resistant common ragweed biotypes were present in carrot fields in Quebec. The presence of these different mechanisms have implications for weed management and resistance spread.
The project results were regularly communicated to scientific communities and growers emphasizing the importance of early detection of resistant weeds present in a field and making informed field-specific decisions on the selection of appropriate herbicide to be used. These tests were also beneficial in detecting cases of multiple resistance in some weed species.
Rapid diagnosis can lower the use of ineffective herbicides in the field where the resistant population is found and can help to maintain the effectiveness of current tools, while proper management of resistant population can reduce its spread. Improved understanding of weed resistance dynamics and early detection of resistant weeds enables growers to make better choices for effective within-season weed management and reduce the use of unnecessary pesticides, thus preventing further resistance proliferation.
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