Weed of the week: Canada fleabane

Weed of the week: Canada fleabane

Canada fleabane is not considered a major weed in Western Canada.

But in Ontario, it has developed into a herbicide tolerant pest.

Fleabane is a winter or summer annual, with most of the seedlings making their appearance in a post-harvest environment from August to October.

The pest forms a dark green rosette of hairy leaves and settles in for the winter.

Fleabane, formally known as conyza canadensis, gets going again from the end of April to the end of May when the weather warms up. The plant generates its flower stalks and develops the characteristic long, narrow, pointed leaves.

Blooming begins in July, when the small white flowers emerge. Canada fleabane will continue to bloom well into September. Smaller adults, less than half a metre in height, will produce 2,000 seeds. Larger plants of up to 1.5 metres will generate as many as 250,000 seeds.

Ontario’s agriculture ministry says the weed establishes best when seeds aren’t disturbed and remain on the surface. Coverage by crop trash or burial with fall tillage suppress germination and seed survival. Nearly all seeds suffer mortal deterioration by the three-year mark.

Fall annual versions produce the most seeds, while the spring germinated plants are far less prolific.67_3col_REL_fleabane-copy-vertical

Minimal research on the economic impact has been conducted in Canada, especially in the West.

Michigan researchers found that populations of 150 plants per sq. metre reduced soybean yields by up to 85 percent.

The plant acts as a host for insects such as the tarnished plant bug and the alfalfa bug. It also suffers from aster yellows, becoming a reservoir for that disease when the aster leaf hopper is present.

Even light tillage controls the weed, so the move to reduced tillage systems has allowed it to expand its range. Control in the fall with glyphosate or glufosinate ammonium (Liberty, Good Harvest) will catch most of the rosettes.

Spring burn-off applications are also effective.

In-crop herbicides such as dicamba, clopyralid and bromoxynil with MCPA will often provide control of both new seedlings and mature plants.

Metribuzin (Sencor) is also effective.

Thifensulfuron-methyl or tribenuron-methyl with MCPA (Refine with MCPA), fluroxypyr with MCPA (Trophy), pyrasulfotole and bromoxynil (Infinity) and clopyralid (Lontrel) are effective in winter wheat, although not expressly listed for the pest in the prairie crop production guides. That research was done by the Ontario agriculture ministry and the University of Guelph.

Adding products that provide a second mode of action to glyphosate burn-offs are also effective for controlling the pest. These include florasulam (Pre-Pass, Priority) and saflufenacil (Heat). Aminopyralid with 2,4-D (Restore) can be used in pastures.

The problem with Canada fleabane comes with herbicide resistance.

Ontario reported its first glyphosate resistant version of the weed in 2011. In some counties, it also resists paraquat. Elsewhere in the world, fleabane has evolved to tolerate atrazine and Group 2 ALS inhibitors.

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