Using Genetic Tests to Confirm Herbicide Resistant Weeds in Ontario Horticulture Crops: 2020 Project Summary

Background

Since 2016, this project has created 16 genetic quick tests (5 more in progress) to assist in identifying herbicide resistance in 12 weed species and confirmed 94 new cases of herbicide resistance in horticulture crops. These tests deliver a diagnostic and a recommendation to the grower within the same growing season. Traditional resistance testing in the greenhouse can take from three months to a year to get results back to growers. Now, leaf tissue instead of seed is collected. DNA is extracted from the leaf tissue to determine if there is a change in the sequencing resulting in a mutation making the plant resistant. 

Tests have also been developed to differentiate between Brassica and Amaranthus (pigweed) species. Tests differentiating pigweed species have been instrumental in confirming new cases of waterhemp in Ontario, Manitoba and Quebec. Once confirmed, the waterhemp was tested for Groups 2, 5, 9 and 14 resistances. 

Table 1. Genetic tests currently utilized by the labs

Weed Species
Herbicide Group
Resistance & Tests
Large crabgrass
1
Metabolic: ACCase gene amplification
Common chickweed
2
Target-site (P197Q & unpublished)
Common ragweed
2
Target-site (W574L)
Eastern black nightshade
2
Target-site (A205V)
Green pigweed
2
Target-site (S653N & W574L)
Giant foxtail
2
Target-site (unpublished)
Redroot pigweed
2
Target-site (S653N & W574L)
Waterhemp
2
Target-site (S653N & W574L)
Common ragweed
5&7
Target-site (V219I)
Green pigweed
5&7
Target-site (A251V, S264G*, V219l & F274L)
Lamb’s-quarters
5
Target-site (S264G)
Redroot pigweed
5&7
Target-site (A251V, S264G*, V219l & F274L)
Waterhemp
5&7
Target-site (A251V, S264G*, V219l & F274L)
Brassica spp.
9
Presence of transgene
Canada fleabane
9
Target-site (P106S)
Waterhemp
9
Metabolic: EPSPS gene amplification
Waterhemp
14
Target-site (ΔG210 in PPX2L)
Amaranthus spp.
Species identification
Brassica spp.
Species identification
*S264G mutation only induces resistance to Group 5 herbicides, not Group 7

In 2018, the protocols for these tests were shared with the Pest Diagnostic Lab of the Quebec Ministry of Agriculture, Fisheries and Food (MAPAQ) and the weeds lab of AAFC’s Harrow Research and Development Centre as a pilot project and made available to extension personal in Ontario and Quebec to submit samples, providing the diagnostic service to growers.

In 2019, all samples were sent from Ontario to the Pest Diagnostic Lab of the Quebec Ministry of Agriculture, Fisheries and Food (MAPAQ), whom provided the testing for free. In 2020, MAPAQ could no longer accept samples form out of province.

In 2020, Harvest Genomics signed an agreement with AAFC to obtain the protocols and started to provide the service to Ontario growers for a fee. The funding obtained from the project sponsors has been utilized to pay for this service.  Due to COVID-19 there was a decrease in field sampling. Funds that were not utilized in 2020 will be carried forward to provide the same service to Ontario growers in 2021.  No funding will be invoiced or requested for 2021. For the organizations that committed to funding for 2021, we request that the project be extended to 2022.

Results

Since 2016, the most significant trend is the increase in the number of fields with multiple resistant species:

  • Common ragweed resistant to herbicide Groups 2 and 5 in pumpkins and 2, 5 and 7 in soybeans
  • Redroot and green pigweed resistant to herbicide groups 2 and 5 in tomatoes
  • Redroot and green pigweed resistant to herbicide Groups 5 and 7 in carrots and potatoes
  • Waterhemp resistant to herbicide Groups 2, 5, 9 and 14 in asparagus, peppers, soybeans and corn

Another significant trend is the increased documentation of Canada fleabane resistant to glyphosate (Group 9) in apples, grapes, carrots, onions and pumpkins.

Table 2. 2020 Results to date in Ontario (15 fields are still being analyzed)

Crop
Weed
Herbicide Group
Total Fields
Positive Tests
%
Carrots
Lamb’s-quarters
5
1
0
0
Carrots
Green pigweed
2, 5, 7**
1
1
100
Carrots
Pigweed species
5, 7
1
1
100
Corn
Pigweed species
5
1
1
100
Kidney Beans
Pigweed species
2, 5, 7
1
0
0
Onion
Green pigweed
2, 5, 7
1
0
0
Peas
Pigweed species
2*, 5, 7
6
6
100
Potatoes
Lamb’s-quarters
5
1
1
100
Potatoes
Pigweed species
2, 5, 7**
4
4
100
Seed Corn
Pigweed species
2, 5***,7
2
2
100
Soybeans
Canada fleabane
9
1
1
100
Soybeans
Common ragweed
2, 5, 7
1
1
100
Soybeans
Eastern black nightshade
2, 5, 7
1
0
0
Soybeans
Lamb’s-quarters
5
1
0
0
Soybeans
Waterhemp
2
7
6
86
Soybeans
Waterhemp
5, 7
7
0
0
Soybeans
Waterhemp
9
7
3
43
Soybeans
Waterhemp
14
7
7
100
Sweet Corn
Pigweed species
5
1
1
100
Tomato
Pigweed species
2, 5***, 7
1
1
100
Total
32
27
84
*Resistant to Group 2 only
**Resistant to Groups 5&7 only
***Resistant to Group 5 only
Note: Pigweed species includes redroot pigweed and green pigweed

This testing has been instrumental in documenting new cases of herbicide resistant weeds. 80% of submitted weed samples tested positive. Once confirmed producers were provided the resistance profile enabling a change in management to mitigate spread. Producers, agri-business and consultants that participated in the project were pleased with the timely results and welcomed the in-season management recommendations.

There are many more undocumented cases of herbicide resistant weeds in Canada. The resistance mechanism is unknown for most of them. The major concern is their distribution and economic impact for producers. Knowing where resistant biotypes are located will improve management and maintain the longevity of our crop protection tools.

Project partners include: AAFC, AAFC-PMC, Bayer CropScience Inc., FMC Corporation, FVGO, MAPAQ, OAG, OFVGA, OPVG and Syngenta Canada Inc.


Kristen Obeid
Kristen Obeid

Weed Management Specialist – Horticulture, OMAFRA