Resistant Wild Mustard Increased In 2000

Conditions for herbicide applications were not ideal for many growers in 2000. Cool, moist conditions during the spray window meant that many products suffered from either unsatisfactory crop injury, poor weed control or both. This follows on from a case in 1999 when a number of growers in the Landmark area reported wild mustard escapes after 2 Muster applications. Subsequent testing showed that all plants were in fact susceptible. A check of field records identified the problem as a localized frost which reduced efficacy of Muster at the first application and that surviving plants had outgrown the staging window by the time of the follow up application.

A number of growers in the Red River valley noticed persistent wild mustard infestations in 2000 after 1, and in some cases 2, Muster applications. Even though environmental conditions were variable, herbicide resistance was considered possible. As a result, DuPont Canada collected seed samples from suspicious wild mustard patches in 23 fields. Testing by the University of Manitoba has shown that 17 of these samples have some level of resistance to ethametsulfuron, the active ingredient in Muster.

Most resistant samples were collected in an area between Morris and Roland although resistance was also confirmed around High Bluff (near Portage), Crystal City and Altamont. The level of resistance to Muster was variable, with preliminary tests confirming that some samples were more resistant when the herbicide was applied at a later leaf stage. So far, samples have only been tested in the lab for cross resistance to thifensulfuron- methyl ("Refine", "Plus" or Pinnacle), with just 1 sample being cross resistant.

Although little else is known about these Manitoba populations so far, Alberta researchers reported wild mustard resistance to ethametsulfuron in 1993. Like the majority of Manitoba samples, the sample from Alberta was only resistant to Muster. In the Alberta case, resistance was shown to be metabolism based, rather than a target site mutation. Metabolic resistance generally confers lower and more variable levels of resistance than target site mutations. (Group 1 resistant wild oat, Group 2 resistant kochia and Group 3 resistant green foxtail are all based on target site resistance). It is also important to note that, unlike green foxtail and wild oat, wild mustard outcrosses readily. As a result, there is potential for rapid spread of resistance via pollen as well through seed movement.

The fact that numerous wild mustard samples have been confirmed resistant at this time is not necessarily surprising. Mathematical models that have assumed the likelihood of naturally occurring resistant plants, outcrossing rates and other variables have been used to predict outbreaks of noticeable resistant weed patches in a field. These models have showed that a long period of low level build up is followed by a rapid increase in the proportion of resistant plants. Such a pattern was evident in the evolution of trifluralin resistant green foxtail that covered large areas of southwest Manitoba in a fairly short time.

It is possible that there are more resistant wild mustard populations in Manitoba than those reported here. Any grower who noticed wild mustard that was difficult to control with Muster in 2000 should consider the effect of local environmental conditions as well as the possibility that these populations are resistant. This is especially true for growers who noticed variable control of wild mustard but good control of other broadleaf weeds listed on the Muster label.

The confirmation of Muster resistant wild mustard is an important development that deserves the consideration of growers and extension specialists. Even though around 70% of canola acres were herbicide tolerant varieties in 2000, this meant that 30% were still conventional varieties, and many of these acres would have been sprayed with Muster. Considering that canola samples with greater than 5% wild mustard contamination is downgraded to sample, at least three fields inspected in 2000 would have been in this category.

Growers who have Muster resistant wild mustard confirmed in a particular field could consider replacing conventional canola with a HTC variety in future (Liberty and glyphosate should be effective while CLEARFIELD herbicides such as Pursuit and Odyssey are also thought to control Muster resistant populations). In cereals and other rotational crops, Muster resistant wild mustard can be controlled by tankmixing group 2 herbicides with other commonly used herbicides from Group 4 (2,4-D, MCPA, etc), or by using herbicides from Group 5 (Sencor) or Group 6 (Basagran, Buctril M). The use of good crop and herbicide rotations, recommended sanitation practices and other integrated management techniques will help ensure that Muster will continue to be an important and effective weed control tool in conventional canola systems.

Summary

• Wild mustard samples were collected from 23 conventional canola fields in 2000. Of these, 17 have so far tested resistant to Muster (ethametsulfuron).
• Most resistant samples were collected in the Red River Valley (see map).
• The level of Muster resistance between samples was variable. Some samples showed low levels of resistance that may not guarantee survival if treated under ideal conditions or at an early leaf stage.
• Except for 1, all samples were controlled by thifensulfuron-methyl ("Refine", "Plus" or Pinnacle).
• Limited field testing in 2000 has shown that the herbicides Refine Extra, Express, and Odyssey appear to be effective against these Muster resistant wild mustard samples.
• Wild mustard outcrosses readily so it is likely that resistance could spread long distances via pollen movement. Growers with affected fields could consider growing HTC varieties on those and surrounding fields in the future.