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Control ratings were taken 1 DAA and 1, 2, and 4
WAA. In addition to visible control ratings, at 4 WAA
plants were identified as dead or alive. Plants exhibiting
symptoms similar to the susceptible check (necrotic grow-
ing point) were identified as being dead and plants ex-
hibiting a healthy growing point were identified as being
alive. If a single plant from a population had survived the
herbicide application 4 WAA, a population was deter-
mined to be resistant [12].
3. RESULTS AND DISCUSSION
3.1. Glyphosate Resistance
In 2011 giant ragweed seed was collected from 50
field sites in Essex (16), Kent (19), Lambton (10), Elgin
(2), Middlesex (2) and Lennox & Addington (1) counties
in Ontario. Out of the 50 populations collected, 23 were
confirmed with glyphosate resistant giant ragweed (Fig-
ures 1 and 2). Resistant populations were found in Essex
(10), Kent (7), Lambton (4), Middlesex (1) and Lennox &
Addington (1) counties (Figure 1). This study confirms
the presence glyphosate resistant giant ragweed in two ad-
ditional counties (Middlesex and Lennox & Addington)
outside of the counties surveyed by Vink et al. [10]. The
percent of glyphosate resistant giant ragweed plants in 19
populations was less than 50% (Table 1 ). There were 2 po-
pulations with 51% to 60% and 2 populations with 81 to
90% glyphosate resistant giant ragweed plants (Table 1).
In 2012 giant ragweed seed was collected from 35
sites in Kent (15), Lambton (13), Elgin (1), Middlesex
(4), Huron (1) and Brant (1) counties in Ontario. Of the
35 sites surveyed, 11 sites were confirmed with glypho-
sate resistant giant ragweed (Figure 1). Glyphosate resis-
tant giant ragweed populations were found in Kent (5),
Lambton (2), Middlesex (2), Elgin (1) and Huron (1)
counties (Figure 1). In 2012, two additional counties (El-
gin and Huron) have been confirmed with glyphosate
resistant giant ragweed in addition to the counties surve-
yed by Vink et al. [10] and those counties surveyed in
2011. The percent of glyphosate resistant giant ragweed
plants at nine sites was less than 20% (Ta b l e 1). At two
sites 31% to 40% of the giant ragweed population was
resistant to glyphosate (Table 1).
Overall, there is a low level of resistance at the sites
surveyed for glyphosate resistant giant ragweed in 2011
and 2012. Of the 34 sites confirmed with glyphosate re-
sistant giant ragweed, 30 sites had less than 50% of the
giant ragweed population resistant to glyphosate (Table
1). The percent of glyphosate resistant giant ragweed at 4
sites was greater than 50% (Table 1). It is suspected that
growers with giant ragweed populations that have a lo-
wer level of resistance may be using herbicides with
multiple modes of action as well as a diverse crop rota-
tion. Growers with giant ragweed populations that have a
higher level of resistance may have used glyphosate more
frequently in the past.
3.2. Cloransulam-Methyl
In 2009 and 2010 giant ragweed seed was collected
from 102 field sites in Essex (70), Kent (21), Lambton
(10), and Waterloo (1) counties in Ontario [10]. Of the
102 populations, 1 site was confirmed with resistance to
cloransulam-methyl (Ta bl e 2 ). This site confirmed with
cloransulam-methyl resistance was also resistant to gly-
phosate (Tab le 3). This multiple resistant site was found
in Essex County (Figure 3). The percent of cloransulam-
methyl resistant giant ragweed plants in this population
was less than 50% (Table 3).
In 2011 giant ragweed seed was collected from 50
field sites in Essex (16), Kent (19), Lambton (10), Elgin
(2), Middlesex (2), Lennox & Addington (1) counties in
Ontario. Of the 50 populations collected, 8 sites were
confirmed with resistance to cloransulam-methyl (Table
2). Resistant populations were found in Essex (2), Kent
(3), and Lambton (3) counties (Figure 2). Three of the po-
pulations with cloransulam-methyl resistance were also
resistant to glyphosate (Table 3). These multiple resistant
sites were found in Essex (1), Kent (1), and Lambton (1)
counties (Figure 3). The percent of cloransulam-methyl
resistant giant ragweed plants at all populations in 2011
was less than 30% (Table 3).
In 2012 giant ragweed seed was collected from 35
sites in Kent (15), Lambton (13), Elgin (1), Middlesex
(4), Huron (1) and Brant (1) counties in Ontario. Of the
35 sites surveyed in 2012, 2 sites were confirmed with
resistance to cloransulam-methyl (Table 2). Both resis-
tant populations were found in Kent County (Figure 2).
Of the two sites confirmed with cloransulam-resistant
giant ragweed, one site was also resistant to glyphosate
(Figure 3). The percent of cloransulam-methyl resistant
giant ragweed plants in this population was less than
10% (Tabl e 3). Overall, the percent of giant ragweed re-
sistant to cloransulam-methyl across all sites from 2009
to 2012 was less than 50% (Tabl e 2 ). The low level of
giant ragweed resistant to cloransulam-methyl may be
due to growers frequently using glyphosate for control of
giant ragweed rather than cloransulam-methyl.
4. CONCLUSION
In summary, this survey demonstrates that glyphosate
resistant giant ragweed is no longer confined to the sou-
thwestern portion of Ontario. Since the initial assessment
of its distribution in Ontario [10], the presence of gly-
phosate-resistant giant ragweed has been confirmed in
four additional counties. This is the first survey in Cana-
da to document cloransulam-methyl resistance and mul-
tiple resistance in giant ragweed in Ontario.
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