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New Weed Threats Denise Maurice, Manager, Technical Development – Crop Protection, AgricoreUnited, Regional Office,

Calgary, AB, T2E 7A6 Email: dmaurice@agricoreunited.com 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 Rank

Species Frequency Field Density Occurrence plants/sq.m.

Field Density Abundance plants/sq.m.

Relative Abundance

36 Common ragweed 1.6 1.8 7.2 0.5 43 Biennial wormwood 1.1 0.7 2.2 0.3 86 False ragweed 0.2 1.4 1.4 <0.1 91 Giant ragweed 0.2 0.6 0.6 <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

Species not surveyed 0.1 to 10.0% 20.1 to 50.0%Absent 10.1 to 20.0% More than 50.0%

Frequency

2000s1990s

1980s1970sBiennial wormwood, Artemisia biennis

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

Species not surveyed 0.1 to 10.0% 20.1 to 50.0%A bsent 10.1 to 20.0% M ore than 50.0%

Frequency

2000s1990s

1980s1970sC om m on ragw eed, A m brosia artem isiifolia

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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 Downy brome Japanese brome Leaf Sheath margins fused, hairs 0.6 mm margins fused, 0.6 –0.8 mm long Ligule 3.0 mm long, ragged 1.5 mm long, hairy erose-ciliate Leaf Blade edges smooth, hairs 0.6 mm edges smooth, Seed Seed unit long and narrow and

includes lemma and palea, lemma pubescent and awned (1/2 to 3/4 inch awn).

Seed unit with lemma and palea attached, awn on lemma about 1½ -2 times length of seed.

Japanese brome- ligule

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.