Download 71 Denise Maurice, Manager, Technical Development – Crop Protection, AgricoreUnited, Regional... Calgary, AB, T2E 7A6 Email: New Weed Threats

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New Weed Threats
Denise Maurice, Manager, Technical Development – Crop Protection, AgricoreUnited, Regional Office,
Calgary, AB, T2E 7A6 Email: [email protected]
Biennial Wormwood (Artemisia biennis Willd.)
The growing season of 2004 presented some new weed challenges. Biennial wormwood
(Artemisia biennis Willd.), a native to North America, has colonized annual cropping systems on the
prairies. Ranked 43 rd in the recent Manitoba weed survey, this weed species would not traditionally been
viewed as a significant weed threat. However many factors such as season- long emergence,
misidentification of biennial wormwood as common ragweed (Ambrosia artemisiifolia) and an increase in
bean production, may have contributed to the increase in biennial wormwood populations in Manitoba.
Only recently was it discovered that biennial wormwood, emerges both in the fall and spring, hence this
biennial species was behaving as an annual. In fact, annual biotypes seem to be the most common due to
selection in annual cropping systems. This is consistent with reports from North Dakota where biennial
wormwood is an aggressive and prolific weed edible beans and soybean fields (Kegode et al 2003). The
invasion of biennial wormwood expanded rapidly since the majority of herbicides labeled for weed
control in these crops provided minimal control. Researchers found that significant yield losses occurred
when biennial wormwood densities were 10 plants/sq.m., competition impact is for at least 6 weeks
(Kegode et al, 2003). Biennial wormwood can grow up to 3 m tall and produce more than 400,000
seeds/plant. So this weed species can be a significant threat. Distinguishing the correct species is critical
to appropriate herbicide selection/ timing and minimizing the impact this species.
So presently how widespread are these species? Weed survey results indicated that biennial wormwood,
common ragweed and the less prevalent ragweeds, false ragweed (Iva xanthifolia) and giant ragweed
(Ambrosia trifida) are present in a low percentage of the fields surveyed (Table 1).Biennial wormwood
distribution in Manitoba has been fluctuating over the duration of the survey period (Figure 1). This
species was most frequently found in southwestern region of the province. Common ragweed populations
however showed similar distribution patterns but is more prevalent in south central Manitoba (Figure 2).
Common ragweed populations peaked in the 1990’s, with similar distribution patterns occurring in 2000
but the extent of the population was not as widespread. It is where these two weed populations converge
that it becomes problematic in distinguishing the two species. Field frequency for these species increases
significantly in local areas such as Killarney, Morden, Morris and Pilot Mound. In Killarney, for example,
biennial wormwood is found in 13.6 percent of the fields surveyed; in Pilot Mound 11.1 percent of the
fields were infested with biennial wormwood.
Table 1. Provincial summary for biennial wormwood, common ragweed, false ragweed, and giant
ragweed. Manitoba Weed Survey: Cereal and Oilseed Crops 2002, Leeson J.Y., A.G. Thomas, T.
Andrews, K.R. Brown, R.C. Van Acker
Species
Frequency
Rank
36
43
86
91
Common ragweed
Biennial wormwood
False ragweed
Giant ragweed
1.6
1.1
0.2
0.2
Field Density
Occurrence
plants/sq.m.
1.8
0.7
1.4
0.6
Field Density
Abundance
plants/sq.m.
7.2
2.2
1.4
0.6
Relative
Abundance
0.5
0.3
<0.1
<0.1
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Figure 1. Biennial wormwood populations in Western Canada, Prairie Weed Surveys from the 1970s to
the 2000s. Leeson, J.Y., A.G. Thomas, L.M.Hall, C.A. Brenzil, T. Andrews, K.R. Brown, R.C. Van
Acker
Biennial wormwood, Artemisia biennis
1970s
1980s
1990s
2000s
Frequency
Species not surveyed 0.1 to 10.0%
Absent
10.1 to 20.0%
20.1 to 50.0%
More than 50.0%
Figure 2. Common ragweed populations in Western Canada, Prairie Weed Surveys from the 1970s to the
2000s. Leeson, J.Y., A.G. Thomas, L.M.Hall, C.A. Brenzil,T. Andrews, K.R. Brown, R.C. Van Acker
C om m o n ragw eed , A m brosia artem isiifolia
1970s
1980s
1990s
F requency
2000s
Species not survey ed
A bsent
0.1 to 10.0%
10.1 to 20.0%
2 0.1 to 50.0%
M ore th an 50.0%
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Identification
Since biennial wormwood and common ragweed are frequently misidentified, it is important to
understand the morphological characteristics that separate these two species. Further distinguishing the
correct species is critical to appropriate herbicide selection and timing.
Seedling Stage
At the seedling stage, the cotyledon shape and the first true leaves can be used to differentiate the species.
Common ragweed seedlings have oval (obicular )cotyledons, 6-9mm long, more or less petiolated. The
shape of the cotyledons is generally broadest at or above the middle, apex is rounded, underside green or
with reddish –purple spots. The first true leaves are distinctly 3 – lobed, opposite, stems and leaves are
hairy (Figure 3). Biennial wormwood seedlings have oval or oblong cotyledons, very small, 2-3 mm long.
First two leaves are opposite, oval to elongated, having two small teeth on the leaf margin. Subsequent
leaves are pinnately divided and redivided, the final divisions sharply toothed. Young plants form a loose
rosette and are hairless.
Co
Figure 3. Common ragweed and biennial wormwood showing the distinct 3- lobed leaf of the common
ragweed
Figure 4. Common ragweed and biennial wormwood juvenile plants, common ragweed has hairs on
leaves and stem, biennial wormwood leaves hairless with distinct carrot/sage-like odour.
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Mature Plant
As the plant matures, it is more challenging to separate the species. (Figure 4). Since biennial wormwood
can be significantly more difficult to control at the later stages of development, early identification is very
important. With few registered herbicides, it is critical to identify this weed problem early in the crop
rotation so appropriate control methods can be taken. Mature plants of biennial wormwood are erect,
hairless, with slender stems, with little or no branching. Leaves appear to form a whorl around the stem.
Upper leaves, stems and flowers release a carrot or sage-like odour when crushed. Common ragweed
mature plants are erect, much-branched, somewhat hairy, hairs short and appressed, lower leaves opposite
but becoming alternate towards the apex of the plant. Plants become greenish gray as they mature. Leaves
compound and finely divided, the final divisions usually coarsely toothed. Flowers of both species are
inconspicuous.
Management and Control
Biennial wormwood seedlings begin emerging relatively early and emergence continues throughout the
growing season as long as moisture and light are available. From a management perspective, control with
a preseed application of glyphosate will control the overwintering rosettes and newly emerged seedlings
but in-crop alternative need to be considered to control emergence of seedling later in the season.
Seedlings grow slowly at first and remain in rosettes for a relatively long period ( Kegode et al 2003),
once it reaches this stage of development biennial wormwood is more difficult to control.
Biennial wormwood is tolerant to most PPI and PRE herbicides including, trifluralin, and ethalfluralin,.
This is where the preseed applications of glyphosate have a good fit. Biennial wormwood is tolerant to
many POST herbicides used in row crops including Basagran, Pinnacle, Linuron, Reflex, and Pursuit.
Biennial wormwood is susceptible to phenoxy or growth regulator herbicides, such as 2,4-D, Banvel, and
clopyralid based products such as Curtail M. Unfortunately only 2,4-D and MCPA are labeled for control
of this weed and rates refer control of the winter annual form. Research from North Dakota indicates that
herbicides with other modes of action such as Liberty, and glyphosate effectively control seedling
biennial wormwood.
Research has shown some options in control, in corn including, Banvel II, all premixes that contain
dicamba or 2,4-D such as Distinct, Marksman PeakPlus, Shotgun also effectively control wormwood
seedlings. Basagran on wormwood less than 3 inches tall, followed by a second application has shown
some promise in research trials in North Dakota.
Biennial wormwood is easily misidentified with common ragweed. Many herbicides that control ragweed,
such as Basagran, and Pursuit do not control wormwood. Researcher are investigating herbicides that
were thought not to be effective on biennial wormwood may in fact be stage dependent so small
wormwood, 1 to 3 inches tall, may be controlled with some herbicides now thought are be ineffective.
Wormwood quickly becomes tolerant to herbicides as size increases, so rescue treatments can show
minimal effect.
Japanese Brome (Bromus japonicus Thunb. ex Murr.)
Another species found in southwest Manitoba in the Treherne area this growing season, is Japanese
brome (Bromus japonicus Thun.ex Murr). Although Japanese brome has been detected in southern
Saskatchewan several years ago, this is the first appearance of this winter annual grass species in
Manitoba. Just recently, Japanese brome has also been detected in southern Alberta. In addition an
infestation of downy brome (Bromus tectorum L.) has been identified in the same area as the Japanese
brome infestation. The weedy brome species such as downy brome and Japanese brome are usually
associated with winter wheat production. The different brome species are distinctive at heading, but are
easily confused at earlier stages of growth. The most reliable method for identification in the first month
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of growth is to check the seed.(Table 2). These seedling grasses still have seeds attached to their roots, if
gentle removed from the soil the seed can be examined for distinguishing characteristics, such as awns,
hairs, length, and width.
Table 2. Key distinguishing characteristics of the brome seedlings
Feature
Leaf Sheath
Ligule
Leaf Blade
Seed
Japanese bromeligule
Downy brome
margins fused, hairs 0.6 mm
3.0 mm long, ragged
edges smooth, hairs 0.6 mm
Seed unit long and narrow and
includes lemma and palea, lemma
pubescent and awned (1/2 to 3/4
inch awn).
Japanese brome
margins fused, 0.6 –0.8 mm long
1.5 mm long, hairy erose-ciliate
edges smooth,
Seed unit with lemma and palea
attached, awn on lemma about 1½ -2
times length of seed.
Japanese brome- head
References:
Guide to Crop Protection 2004, Manitoba Agriculture, Food and Rural Initiatives in co-operation with
Saskatchewan Agriculture, Food and Rural Revitalization.
Guide to Weed Control 2004- 2005. Publication 75. Ontario Ministry of Agriculture and Food.
Kegode,G.O. and M.J. Christoffers.2003. Weed Technology. Volume 17:17646-649.
Leeson,J.Y., A.G. Thomas, L.M.Hall, C.A. Brenzil, T. Andrews, K.R.Brown and R. C. Van Acker. 2005.
Prairie weed surveys of cereal, oilseed and pulse crops from 1970s to 2000s. Weed Survey Series
Publication 05-1CD. Agriculture and Agri-Food Canada, Saskatoon, Saskatchewan.
North Dakota Weed Control Guide 2004, Circular W-253. NSDU Extension Service. North Dakota State
University, Fargo, ND.
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The Weedy Bromes: recognizing them at an early stage. L-814.1990. Kansas State University
Agricultural Experiment Station and Cooperative Extension Service.