The announcement was made March 7 by the University of Guelph’s Ontario Agricultural College (OAC). Dr. Aicklen will be based at the University’s Ridgetown Campus where her research will directly benefit Ontario’s bean growers by identifying innovative and sustainable solutions for weed management, the OAC said in a release. She will also play a key role in teaching and mentoring students in Ridgetown’s diploma programs, “helping to train the next generation of agricultural professionals.”

Dr. Aicklen completed her BSc and Ph.D. at the University of Guelph, where her research focused on herbicide resistance in green pigweed populations in southwestern Ontario. The OAC said her work “provided critical insights into effective weed management strategies for Ontario growers.” She previously worked at Cargill Ltd., Syngenta Canada Inc., and Bayer CropScience, where she gained firsthand experience in agronomy, pest management, and grower engagement.

Her hiring is made possible through $2 million in funding from OBG and an anonymous donor. Funding for the professorship was announced last year to ensure weed research continued for dry beans after the retirement of Dr. Peter Sikkema, as well as help support the Huron Research Station for the next 10 years.

Dr. Aicklen’s industry experience and passion for teaching make her an excellent fit for this position, Brett Shepherd, director of U of G’s Ridgetown Campus, said in the release.

“Her expertise in weed science and passion for knowledge-sharing will be an incredible asset to our diploma students, who will gain hands-on experience in the latest weed management strategies.”
The Ontario Bean Growers Professorship in Weed Science is part of a broader effort to support Ontario’s dry bean sector and will boost U of G’s leadership in dry bean research. The recent renewal of the OBG Dry Bean Agronomy and Pest Management faculty positions at Ridgetown Campus, along with Dr. Aicklen’s appointment, ensures that research and extension efforts continue to serve Ontario’s bean industry for years to come.

“We are excited to work with Dr. Isabelle Aicklen and look forward to continuing the great work of weed management research at the Ontario Crops Research Centre sites in Huron and Ridgetown,” said Jamie Payton, chairman of the Ontario Bean Growers (OBG). “Edible dry-bean growers continue to struggle with invasive weed pressures, like waterhemp, in prime growing regions. Dr. Aicklen’s research is vital in helping farmers provide the best beans in the world by tackling these troublesome weeds resulting in increased yield, quality and profitability.” 
With Ontario producing nearly 100,000 acres of dry beans annually—most of which are exported internationally—investment in research and education remains crucial, the OAC said. Dr. Aicklen’s work at Ridgetown “will help growers optimize production practices and stay ahead of evolving weed management challenges, reinforcing U of G’s commitment to supporting a strong and sustainable agricultural sector.”

The Ontario Crops Research Centre is owned by the Government of Ontario through its agency, Agricultural Research and Innovation Ontario (ARIO), and is managed by the University of Guelph through the Ontario Agri-Food Innovation Alliance.

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Those were the words of Richard Anderson, a business representative with BASF, speaking last month at the Southwest Agricultural Conference at University of Guelph’s Ridgetown Campus. It was probably the most pertinent take-home message from a panel discussion on tackling waterhemp, held during the conference’s virtual kickoff Jan. 3.

Why it matters: Waterhemp has become a leader in resistance to herbicide modes of action. In the U.S., there are biotypes now resistant to seven different groups.

The panel included Anderson, Ryan Benjamins from Benjamins Agronomy Services, Johanna Lindeboom from Clark Agri Service and Lauren Benoit from Bayer CropScience.

Mike Cowbrough, weed specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, moderated the discussion. He led off the session with some sobering numbers on the impact of waterhemp. For starters, resistance testing in the province between 2002 and 2023 revealed that 93 per cent of plants tested were resistant. The remaining seven per cent were susceptible.

A breakdown of 195 waterhemp populations tested in Ontario offered more stunning information:

• 88 per cent showed resistance to Group 2 herbicides — Pursuit, Pinnacle and Classic.
• 72 per cent were resistant to the Group 9 herbicide glyphosate.
• 57 per cent were resistant to Group 5 herbicides — Atrazine, metribuzin.
• 52 per cent were resistant to Group 14s — Reflex, Affinity, Blazer.
• Seven per cent showed resistance to Group 27s — Infinity, Callisto.

“It’s no surprise that waterhemp is resistant to some sort of herbicide,” said Cowbrough. “To make matters worse, it can be resistant to a number of different herbicide groups all at the same time.”

The majority of waterhemp, he added, is resistant to more than one mode of action: 37 per cent is resistant to at least three modes, 27 per cent to two modes, 19 per cent to four modes and six per cent to six modes.

Resistant biotypes have been confirmed in 18 counties, with most in southern Ontario. Essex and Lambton were the highest with resistance to multiple modes of action, with Chatham-Kent in a distant third position. Surprisingly, a resistant and susceptible population has been found in Rainy River in northwestern Ontario.

Managing waterhemp

Cowbrough then asked the panelists for their assessment of how to manage resistant waterhemp, noting there are two types of individuals dealing with the issue: those with first-hand experience, either on their own farm or in their area, and those who’ve heard of waterhemp but have yet to encounter it in their fields.

Anderson said finding it and familiarizing oneself with what it looks like and how it’s growing are the keys to dealing with it. It’s also vital to plot a Plan A, B and C on what to do when, not if, waterhemp is confirmed on a farm. Anderson noted later in the session that he’s working with his customers on five-year cropping plans in an effort to reduce the risk and damage. He also advised rotating chemistries as well as crops.

For Ryan Benjamins, managing waterhemp requires a good start to the growing season. He echoed Anderson’s comment about advanced planning and linked it to the use of herbicides with residual activity, which may help reduce waterhemp’s impact and help with other weed species in a field.

“Soybeans are not competitive, so having a residual to start is a huge step,” said Benjamins.

“Maybe you have a grass problem so you can add a Group 15 herbicide that might buffer you a little on waterhemp but also give you some grass residual.”

Johanna Lindeboom said soybeans is the crop where waterhemp likely appears. If it’s possible to grow IP soybeans, or grow a cover crop after winter wheat, those actions would help.

“If you don’t have a good crop rotation or you’re growing beans-on-beans, that could be an issue,” she added. “It also moves through equipment, so if you get a lot of custom work, that could be a chance for waterhemp in your area to find its way to your farm.”

Custom operators often have a tough time doing a thorough cleanout, given the number of acres they’re servicing. There is also a need for awareness when purchasing used equipment and whether that unit is coming from an area where waterhemp is a problem.

Asked if there were other “risky behaviours,” Laura Benoit noted that waterhemp seeds can float and animals can move seed onto a farm. Those are hard to control, but scouting and knowing what to look for are the two most important factors after equipment-spread seed.

In the absence of carrying a pocket magnifying glass for identification, Benoit suggested using a smartphone camera with a zoom function to get a clear glimpse of waterhemp’s key features, including hairless leaves and stems, a shiny or waxy texture to the leaves and leaves narrower than other pigweed species.

Above all, if you’re not sure what you’re looking at, get someone who is to have a look.

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Even if the two species are not an immediate concern, adopting a longer-term, more multi-faceted approach to weed management could help keep these challenging pests at bay.

Why it matters: Multiple herbicide group resistance in problem weeds is an accelerating problem. Proactive herbicide use and planning are necessary.

Sikkema, a University of Guelph professor at the Ridgetown campus, emphasized this position during the 2023 Ontario Agricultural Conference. Speaking Jan. 4, he stressed the tenuous position of Ontario growers regarding waterhemp and Canada fleabane, two highly competitive weeds that have developed, and continue to develop, resistance to an array of herbicide groups.

Herbicides are not the only methods growers can use to kill problem weeds or reduce weed seed populations. Cover crops, tillage, good rotation and even combine-mounted seed crushers can play a role. Because herbicides remain the critical tool, however, Sikkema highlighted approaches and product combinations that Ontario growers can use to hedge their bets.

[RELATED] How to keep soybeans weed-free

Managing Canada fleabane, Group 2 and 9 resistant

Herbicide resistant fleabane is considered the top weed issue in Ontario. Populations resistant to herbicide Groups 2 and 9 have established from Essex County to the Quebec border and likely beyond.

On the positive side, a number of herbicides control fleabane in corn. Pre-emergence combinations of Acuron and Flexi, for example, provide 91 per cent control.

Others like Calisto, plus Atrazine and Integrity, reach 96 and 99 per cent control, respectively. Older herbicides like Pardner bring 90 per cent control in post-emergent applications. Sikkema said there are many options to achieve “commercially acceptable control” of fleabane.

Soybeans, on the other hand, present a greater challenge.

“The first rule of thumb in managing fleabane in soybeans – if you’re growing (identity preserved or Roundup Ready) soybeans, the bottom line is to avoid trying to control it with post-emergence herbicides since they largely do not work. You have to have it controlled before soybeans come out of the ground,” said Sikkema.

Eragon plus glyphosate in a pre-emergence application is, in his experience, the best starting point for achieving good control, though it’s not a magic bullet. Results can vary even under tightly controlled experimental conditions.

“We did 90 trials where we had Roundup plus Eragon… In 58 per cent of trials, the control was 90 to 100 per cent… What’s really disappointing is, in nine per cent, the control was between 80 to 89,” he said. The lowest control rate was six per cent.

Sikkema said there are many reasons for variability — time of day for application, size of the fleabane plants, and variability in biotypes within the weed species. He recommends growers add a third component.

“I think the best one to add to Eragon is metribuzin… You can buy it in all sorts of different trade names.”

For Xtend soybeans, Sikkema said Dicamba works well if mixed with Eragon or Integrity. Metribuzin should not be part of the formulation because it can reduce the control from Dicamba. As well, the potential for off-target movement of the herbicide should be taken seriously.

For Enlist soybeans, Group 4 herbicides such as Enlist Duo and Elevore brought good results. Chemical control of fleabane in winter wheat is another option, and with many near equivalent options available, Sikkema suggests using the cheapest one, provided the same product is not used repeatedly.

“Aggressive tillage can be effective at fleabane control … but just like herbicides, tillage can be variable. We’ve done trials where we’ve done three passes with the cultivator, and fleabane will feather its way through the cultivator,” he said.

Cover crops have also proven valuable for early season fleabane suppression, he added.

Wrangling waterhemp

The first glyphosate-resistant waterhemp populations in Ontario were found in 2014. Just eight seasons later, plants with multiple resistance grow from Essex County to Stormont, Dundas and Glengarry. In some areas, the species has developed resistance to herbicides in Groups 2, 5, 9, 14, and 27, eliminating 80 per cent of herbicide options.

Herbicide-induced selection aside, waterhemp’s province-wide conquest can be attributed to its competitiveness, full-season emergence and ability to thrive in reduced-tillage fields.

“This plant starts coming up in May and comes up all summer long. Therefore you need a herbicide program that provides long control,” said Sikkema.

Pre-emergence applications of Acuron now provide the best control. A related product, Acuron Flexi, shows less control at 76 per cent because it does not contain Atrazine.

Post-emergence application is challenging. Sikkema said all post-emergence products are Group 27, so plants resistant to that group are a problem, but Laudis plus Atrazine still works well.

In soybeans, research data suggest Fierce is the most effective waterhemp herbicide with 90 per cent control. However, it’s also hardest on the crop.

“You as the grower have to choose whether weed control is more important or crop safety,” Sikkema said. “I think you have to have a two-pass program… The disappointing thing is we now have Group 14 resistance.

“I think every farmer in the winter, if you have this weed, you should plan to use a two-pass program. Use your best soil-applied herbicide, and plan to come back post-emergence if needed.

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An international team led by researchers at the University of British Columbia compared 187 waterhemp samples from modern farms and neighbouring wetlands with more than 100 historical samples dating as far back as 1820 that had been stored in museums across North America. 

Much like the sequencing of ancient human and Neanderthal remains has resolved key mysteries about human history, studying the plant’s genetic makeup over the last two centuries allowed researchers to watch evolution in action across changing environments.  

“The genetic variants that help the plant do well in modern agricultural settings have risen to high frequencies remarkably quickly since agricultural intensification in the 1960s,” said first author Julia Kreiner, a postdoctoral researcher in UBC’s botany department. 

The researchers discovered hundreds of genes across the weed’s genome that aid its success on farms, with mutations in genes related to drought tolerance, rapid growth and resistance to herbicides appearing frequently. 

“The types of changes we’re imposing in agricultural environments are so strong that they have consequences in neighbouring habitats that we’d usually think were natural,” said Kreiner.

The findings could inform conservation efforts to preserve natural areas in landscapes dominated by agriculture. Reducing gene flow out of agricultural sites and choosing more isolated natural populations for protection could help limit the evolutionary influence of farms.

Common waterhemp is native to North America and was not always a problematic plant. Yet in recent years, the weed has become nearly impossible to eradicate from farms thanks to genetic adaptations including herbicide resistance. 

“While waterhemp typically grows near lakes and streams, the genetic shifts that we’re seeing allow the plant to survive on drier land and to grow quickly to outcompete crops,” said co-author Sarah Otto, Killam University professor at UBC. 

“Waterhemp has basically evolved to become more of a weed given how strongly it’s been selected to thrive alongside human agricultural activities.”

Notably, five out of seven herbicide-resistant mutations found in current samples were absent from the historical samples. 

“Modern farms impose a strong filter determining which plant species and mutations can persist through time,” said Kreiner. “Sequencing the plant’s genes, herbicides stood out as one of the strongest agricultural filters determining which plants survive and which die.” 

Waterhemp carrying any of the seven herbicide resistant mutations have produced an average of 1.2 times as many surviving offspring per year since 1960 compared to plants that don’t have the mutations.

Herbicide resistant mutations were also discovered in natural habitats, albeit at a lower frequency, which raises questions about the costs of these adaptations for plant life in non-agricultural settings. 

“In the absence of herbicide applications, being resistant can actually be costly to a plant, so the changes happening on the farms are impacting the fitness of the plant in the wild,” said Kreiner.

Agricultural practices have also reshaped where particular genetic variants are found across the landscape. Over the last 60 years, a weedy southwestern variety has progressed eastward across North America, spreading its genes into local populations as a result of its competitive edge in agricultural contexts.

 “These results highlight the enormous potential of studying historical genomes to understand plant adaptation on short time scales,” says Stephen Wright, co-author and professor in ecology and evolutionary biology at the University of Toronto. 

“Expanding this research across scales and species will broaden our understanding of how farming and climate change are driving rapid plant evolution.”

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First detected in Lambton County in 2002, the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) describes common waterhemp as “extremely difficult” to identify. Grain Farmers of Ontario (GFO) also describes it as a “highly adaptive species” which, combined with a high seed production rate, consistently spreads to new counties. 

Yield losses from the prolific pest can be significant. But although resistance to herbicides is an acute problem, control is not impossible.

Why it matters: Waterhemp can have significant impact on yield and continues to quickly adapt to a variety of herbicide groups.

Common waterhemp is an annual weed that produces male and female plants. Female plants produce seed for reproduction. As a member of the pigweed family, it can easily be mistaken for close relatives such as redroot pigweed. This is particularly true in early stages of growth. 

Characteristics of common waterhemp include: 

• Narrow leaves with a waxy surface
• Narrow leaf margins
• Green to purple and red stems 
• Completely or nearly hairless stalks
• Very long seed heads (when plants are large)

Mature plants are also taller than other pigweed species, growing from 1.5 to 2.5 metres (4.5 to 7.5 feet) in height. Stem colouration ranges from green to purple and red. A spring 2021 Agronomy Alert from GFO adds male and female plants look like completely different weed species when mature. 

Mike Cowbrough, field crop weed management specialist with OMAFRA, details best herbicide application practices for corn, soybeans and cereals in a May 3, 2022, Field Crop News update. 

He says for corn and soybeans, a two-pass herbicide program achieves best control, with the first pass including a pre-emergence soil applied herbicide and the second a post-emergent herbicide. 

Consistency of waterhemp control with soil applied herbicides varies. Cowbrough says growers should not expect perfect control for the entire season. However, a soil applied herbicide will eliminate most early seedlings, reducing the chances for larger and less susceptible seedling plants to be present during a second herbicide application. 

Overall, Ontario data indicates winter cereals are a good rotational crop for reducing waterhemp populations. Cowbrough says a limited amount of cereal-specific research has been completed in the province, primarily because waterhemp has not been a problem in winter wheat to date. 

This could be, in part, because fall seeded winter wheat tends to out-compete spring emerging summer annuals like waterhemp and other pigweed species.

Cover crops and tillage can be effective tools, particularly given the presence of populations with multiple herbicide resistances. 

Data from Illinois shows fall seeded cereal rye, when crimped prior to planting soybeans, consistently reduced biomass of waterhemp over a three-year period. However, the thick residue left by the cereal rye can pose problems for soybean stand and establishment, making residue management at planting important (e.g. through row cleaning or strip tillage). 

GFO adds oats, or a blend of oats and radish, are also potential alternatives. Depending on the cover crop used and level of growth, mowing or clipping may be required. 

Waterhemp emergence is greater in no-till cropping systems. Research from Iowa has demonstrated the duration of waterhemp emergence was 26 days longer in a no-till cropping system compared to one employing a chisel-plough. 

Isolating waterhemp-contaminated fields can also reduce the weed’s overall impact. This can involve restricting equipment movement, thoroughly cleaning equipment and actively testing for herbicide resistance when resistance is suspected.

Where is resistant waterhemp in Ontario?

Essex, Bruce, Glengarry: these counties mark the known boundaries of herbicide-resistant common waterhemp. Severity of herbicide resistance varies by region (see above). The following are confirmed resistances as of mid-May, 2022: 

Group 2, 5, 9, 14, 27

• Essex, Chatham-Kent, Lambton, Middlesex

Group 2, 9, 14, 27

• Elgin, Northumberland, Stormont, Dundas and Glengarry

Group 2, 5, 9, 14

• Huron, Bruce, Haldiman, Leeds and Grenville

Group 2, 9, 14

• Wellington, Norfolk

Group 2, 5, 9

• Hamilton

Group 2, 9

• Brant

Herbicide resistance identification programs are ongoing. Sample kits and directions are available through OMAFRA. The University of Guelph also offers a free tissue sample service.

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In this edition of Field Talk, Farmtario contributor Matt McIntosh talks with Chris Snip, crop sales specialist with Agris Co-operative, about waterhemp in Essex County, wider herbicide management concerns – and the importance of dialogue with your farm neighbours.

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Quebec’s Reseau d’avertissement phytosanitaire (RAP) last Friday reported a patch of waterhemp with resistance to mesotrione herbicide in the Haut-Richelieu municipality, in the province’s Monteregie.

A Group 27 HPPD enzyme inhibitor, mesotrione is the active ingredient in Syngenta’s Callisto herbicide and one of several in products such as Halex and Acuron, all used mainly for broadleaf weed control in corn crops.

Tests by the Centre de recherche sur les grains (CEROM) also confirmed resistance to Groups 2, 5 and 9 (atrazine, metribuzine and glyphosate respectively) in the same waterhemp patch, RAP reported.

Established in Ontario since 2002, Manitoba since 2016 and Quebec since 2017, having arrived via the central and eastern U.S., waterhemp is able to germinate all throughout a growing season.

The weed is also known to be highly prolific — a single plant can produce up to 300,000 seeds — and if left unchecked, has been known to cause yield losses of up to 73 per cent in infested corn and soy crops, RAP said.

The plant also grows rapidly, at a rate of up to 2.5 to three centimetres per day, and is tough to distinguish visually from relatives such as redroot and green pigweed.

Waterhemp patches in Ontario have previously shown resistance to groups 2, 5 and 9 and, more recently, Group 14. That group includes PPO inhibitors such as Syngenta’s Reflex and Corteva’s Goal, also used mainly for broadleaf weed control.

The Quebec agriculture ministry previously set up a support program to help corn and soy growers deal with waterhemp infestations. RAP, in its notice last Friday, urged affected producers to sign up for the program, to help limit the weed’s further spread in the province. — Glacier FarmMedia Network

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The weed — a non-native member of the pigweed family first discovered in Lambton County in 2002 – is an annual plant producing tens of thousands of seeds every year. It features narrow leaves, wavy leaf margins, and (almost completely) hairless stems which OMAFRA resources describe as “smooth to the touch.”

Waterhemp (Amaranthus Rudis L.) can be easily confused with redroot or green pigweed during the early stages of growth, which also happens to be the best time to control the plant. However, the lack of hairs is a notable indicator.

Mature plants are taller than other pigweed species, growing from 1.5 to 2.5 (4.5 to 7.5 feet) in height. Stem colouration ranges from green to purple and red.

As described by OMAFRA, the first line of defense against waterhemp is proper identification This also applies to palmer amaranth, a related species now identified as the worst weed in United States agriculture. Difficult early identification of waterhemp means early control is also a challenge.

Concerning instances of resistance

So far, waterhemp has been found in Bruce, Chatham-Kent, Essex, Lambton, Haldimand and Middlesex counties. Varieties with cross-resistance have been found for a variety of herbicide groups:

• Group 2 (ALS inhibitors like Pinnacle, Prism, and Pursuit),
• Group 5 (Photosystem II inhibitors like Gesagard, Sencor, Simazine, Sinbar)
• Group 9 (EPSP synthase inhibitors, namely glyphosate)
• Group 14 (PPO-inhibitors such as Authority, Aim, Chateau, Goal).

As of August 2018, waterhemp remained the first weed in Ontario with Group 14 resistance (though all samples were found within 25 kilometres of each other in Essex County). OMAFRA resources reiterate samples also have cross-resistance to Groups 2, 5 and 9 herbicides, and that waterhemp – like redroot pigweed – is a cross-pollinating species. This makes it easier for resistance to spread.

Controlling waterhemp

Waterhemp emerges over a long period of time in the spring, though germination occurs whenever soil temperatures are between eight and 30C. But the tendency for early germination also allows for effective early-season control with soil-applied herbicides.

In corn, OMAFRA resources say Converge and Primextra II Magnum pre-emergence herbicides (a two and three-blend mix respectively) have been shown to provide 99 per cent control. A high rate of Boundary (another three-ingredient mix) applied pre-emergence has also been shown to bring 97 per cent control in soybeans. If an additional post-emergent herbicide application is required, Callisto mixed with atrazine, as well as a dicamba-atrazine tank mix (under the trade name Marksman), are effective solutions in herbicide some resistant crops.

OMAFRA resources add the possibility of resistance to these and other chemistries is an important consideration in any control strategy.

A minimum of two modes of action based on Group 14 or 15 products should be used.

Herbicide programs identified by the University of Guelph Ridgetown campus as effective on waterhemp are equally effective on pigweed and other species. This means addressing pigweeds in general inevitably provides preventive control of waterhemp, should it be present.

When practical, washing equipment after harvest and donning clean apparel after entering fields with significant waterhemp populations helps reduce the spread of its minutely-sized seeds. Significant canopy growth from other crops – namely wheat – also prevents waterhemp growth and proliferation season-to-season.

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Obeid is part of a team that has developed a process for leaf sample analysis of weeds which potentially have resistance to herbicides commonly used in vegetable crops. She spoke at the Ontario Processing Vegetable Industry conference held recently in London.

“As soon as you have a miss on your field, we can take a quarter-size piece of the leaf, and we can get a DNA analysis,” she reported. “We’re getting results back in one or two days” a huge improvement over the previously-available turn-around of two weeks or more.

A benefit, Obeid says, is growers might have the opportunity to try a different control measure than they used in the previous year, rather than simply finding out after they’ve already applied herbicide that there is resistance.

And, despite concluding her talk by stating, “sometimes, I do have good news” — that a high percentage of the tests undertaken through the initiative in 2019 showed no resistance — Obeid cautioned that her team is watching a few potentially troublesome weeds very closely.

For vegetable growers, the most significant among the 21 weeds so far identified as possessing resistance in Canada, are in the amaranth/pigweed group. In 2019, herbicide-resistant waterhemp was confirmed in two vegetable fields in Ontario: one in Norfolk County and one in Elgin County. Resistance was detected to Group 2 and Group 9, as well as Group 14 in the Norfolk instance. Group 14 is concerning, Obeid suggested, because this is one of the newer active ingredients that veggie growers have available.

With waterhemp, however, don’t just assume it’s resistant. Get it tested. She even had an instance of a plant she believed was waterhemp actually turning out to be a somewhat mutated version of pigweed. “It can be confusing.”

Others to watch include large crabgrass in peas, onions (Obeid shared a photo of a crop that had been sprayed 10 times by the grower, and the onions were still difficult to discern) and other vegetable crops, redroot pigweed in celery, and fleabane in carrots. She has also tested for resistance in velvetleaf in red beets and tumble pigweed in carrots, but so far there’s no DNA confirmation.

A big threat with resistance, she explained, is that a seed bank has been established. So next time you grow that crop in that field, it will resurface. “If we do have (resistance), what are we going to do next year? We’re going to rotate to a different crop that will hopefully get that under control.” But rotate wisely. “When we rotate crops, we don’t necessarily rotate the groups of herbicides in use.”

Either way, once you get herbicide-resistant weeds, Obeid said, “you’re always going to be managing them so you don’t have an outbreak.”

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They found that the resistance, which was first detected in Ontario in 2010, has spread thanks to two mechanisms:

• Pollen and seeds of resistant plants are dispersed by wind, water and other means.
• Resistance has appeared through the spontaneous emergence of resistance mutations that then spread.

Researchers found evidence of both mechanisms by comparing the genomes of herbicide-resistant waterhemp plants from midwestern farms in the United States with the genomes of plants from southern Ontario.

“We used modern methods of genome analysis to look at the genetic similarity of different populations of these plants,” explains Julia Kreiner, a PhD candidate in the Department of Ecology & Evolutionary Biology (EEB) in U of T’s Faculty of Arts and Science and lead author of a study published in October in Proceedings of the National Academy of Sciences.

“To our surprise, we found that the genomes of some resistant plants in Ontario were nearly identical to those in very distant U.S. plants. This was evidence that the Ontario plants were very closely related to the U.S. plants and suggests that the former came from seeds that were just picked up from one field and dropped in another.”

While Kreiner and her collaborators did not determine exactly how the seeds were physically transported, this propagation, known as gene flow, is typically accomplished in different ways. Seeds can be carried by water, or in the digestive tracts of animals, or from field to field by way of farm equipment. As well, with a wind-pollinated plant like common waterhemp, genes can also be spread via wind-borne pollen.

The same DNA analysis identified some resistant plants that did not genetically match any other plants suggesting they appeared through the independent emergence of a genetic mutation conveying resistance.

Researchers were surprised to discover both mechanisms at play.

“We have two regions, Walpole Island and Essex County in southwestern Ontario, where waterhemp populations evolved resistance,” says Stephen Wright, a professor in ecology and evolutionary biology at University of Toronto and a co-author of the study.

“Because of their proximity, our expectation was that they would have shared the same origin of resistance. But our results suggest different origins — from the movement of seed from a source population in the U.S., as well as independent evolution of resistance in a local population.”

In addition to the University of Toronto cohort, co-authors included weed scientists from the University of Illinois and the University of Guelph Ridgetown Campus; and genome and developmental geneticists at the Max Planck Institute for Developmental Biology in Germany.

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