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Weeds, disease and insects in mustard

In the final part of a 4-part series on mustard agronomy, Ross McKenzie turns his attention to pests

Weed control is generally my greatest concern when growing mustard. Weed competition can greatly reduce mustard yields by competing for available light, nutrients and moisture. Although mustard seedlings are not very competitive with weeds, there are ways that growers can reduce the early effects of weeds:

  • burndown of weeds in fall and/or early spring before seeding;
  • direct seeding as early as possible into a clean field to establish mustard as quickly as possible in the spring;
  • seed at an optimum rate with high quality seed to maximize competition with weeds;
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  • seed as shallow as possible to encourage rapid, uniform emergence; and,
  • spray as early as possible after crop establishment to remove weeds as quickly as possible to minimize competition.

Mustard should not be grown on fields with significant amounts of weeds that cannot be controlled chemically. A number of herbicides are registered for weed control in mustard. Unfortunately, there are no registered herbicides available for in-crop control of a number of weeds. Know the weeds present in your fields and check your provincial crop protection guides. Only use registered chemicals on mustard. Using unregistered products could result in legal repercussions.

Control of perennial weeds such as Canada thistle or dandelion should occur in the previous crop year. In the fall, products such as glyphosate can be used while weeds are still actively growing. Spring treatment to control perennial weeds before planting is usually less practical, but a spring burndown of winter annuals and annuals in fields that will be direct seeded is important to optimize weed control.

For in-crop control, it is important to note that the herbicide ethalfluralin (e.g. Edge) is only registered for use with yellow mustard and that ethametsulfuron methyl (e.g. Muster) is only registered for use with brown and oriental mustard.

Weed seeds that are difficult to remove from mustard seed, can cause high losses during seed cleaning and will lower market grades. For example, cow cockle, cleavers, wild mustard, wild buckwheat and volunteer canola are extremely difficult to remove from harvested mustard. Land used for mustard production should be free from these problem weeds. Ensuring optimum weed control in mustard is important to achieve the highest market grade, lowest dockage and highest net financial return.

Disease management

Using proper crop rotations and seed treatments is important in disease management. Weeds in the crucifer family can serve as hosts of mustard diseases and must be kept in control.

Seedling diseases such as damping off or root rot are caused by soil-borne fungi. Diseases such as alternaria black spot are caused by seed and soil-borne fungi. To minimize potential seedling disease problems, seed early and no deeper than one inch to assist with rapid germination and emergence. Use a registered seed treatment to minimize seedling diseases.

Mustard is susceptible to a number of diseases. The most potentially serious is sclerotinia stem rot, which is a soil-borne fungus that infects plants at flowering from air-borne spores. The incidence of this disease varies from year to year, but it is more prevalent in years with wetter than normal conditions.

If sclerotinia is a concern, fungicide application can provide effective control when the crop is at 10 to 50 per cent bloom, before symptoms are visible. Application is only economical when there is a higher level of infection risk.

The risk for sclerotinia can be assessed by monitoring environmental conditions up to the time of flowering. Warm dry weather during flowering will minimize risk while cooler wet weather will increase disease risk. Good cultural and agronomic practices are the best measures for disease control.

Some other mustard diseases that are potentially of concern:

White rust/staghead is a fungal disease; pods develop into swollen spiny stagheads that are initially green and turn brown to white. Most yellow mustard varieties have good resistance to staghead, and recent brown and oriental varieties have improved resistance.

Alternaria black spot is a fungal disease that cause abortion of florets, premature ripening, and pod splitting. Normally this disease is low in severity except in cooler wet, late summers, which are not common in mustard growing areas.

Clubroot is a soil-borne disease that causes club-shaped root galls on the roots of mustard. It is a problem with canola in the central prairies and has been found in canola in the County of Newall in the Brooks area. To date, clubroot has not been a problem in mustard growing regions in the southern prairies.

Insect Pests

Mustard growers should closely monitor fields to detect insect problems that could affect yield. The more common insect pests of mustard include flea beetle, cutworm, grasshopper, diamond back moth, armyworm and cabbage seedpod weevil.

Yellow mustard is immune to the cabbage seedpod weevil due to hairy pods and has some tolerance to flea beetles; oriental and brown mustards, are more susceptible to insect pests.

Mustard should be closely monitored for cutworm feeding at germination and emergence. Insecticide application may be necessary if significant feeding damage occurs.

Heavy feeding of flea beetles at the seedling stage can cause serious damage particularly to the Brassica mustards. Damaged plants may die or suffer reduction in vigour, particularly in warm, dry conditions. Hot weather can result in heavier feeding, while cool or wet conditions will slow flea beetle feeding. Injured plants may wilt and die during hot, dry weather, which can result in moderate to severe yield losses. The use of a registered seed treatment that contains an insecticide to control flea beetles on seedling mustard is a wise practice for oriental and brown mustard. If seedling damage by flea beetles exceeds 40 to 50 per cent at the cotyledon stage, then a foliar insecticide should be applied. Serious damage to mustard does not usually occur once the crop develops beyond the seedling stage since vigorous plants can outgrow beetle damage.

When mustard enters the early flower stage, growers should monitor oriental and brown mustard for cabbage seedpod weevils in Alberta. Saskatchewan mustard growers should be aware this pest is moving eastward into Saskatchewan. Yellow mustard is not susceptible and does not need monitoring for this pest. A nominal economic threshold of four cabbage seedpod weevils per sweep at the early flower stage can be used to make control decisions.

Diamond back moth feeding on mustard can be serious near the end of flowering and early pod formation. Control is recommended when numbers exceed an economic threshold of 30 larvae/square foot. Bertha armyworms will feed on mustard and control may be necessary when numbers exceed the economic threshold of three larvae/square foot.

As I mentioned at the start of this series of articles, I am a big fan of mustard in a crop rotation in the drier regions of the southern prairies. When well managed, mustard yield potential is good and it can be a very profitable crop to grow on your farm.

About the author

Columnist

Ross McKenzie

Ross H. McKenzie, PhD, P. Ag., is a former agronomy research scientist. He conducted soil and crop research with Alberta Agriculture for 38 years. He has also been an adjunct professor at the University of Lethbridge since 1993, teaching four-year soil management and irrigation science courses.

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