Trials compare rotation length, weed numbers

Research studied one and two-year rotations, with and without fungicide treatments, and assessed weed populations

LACOMBE, Alta. — Current prairie crop rotations are too short to beat back weeds and disease.

“If you look at our cropping systems here in Western Canada, we are largely a cereal-oilseed-cereal-oilseed rotation in many areas. In many areas, pulses are an important part of the rotation,” Kelly Turkington, a plant pathologist with Agriculture Canada at Lacombe, said during a July 26 field day.

“From my perspective as a pathologist, it is not a great rotation. Unfortunately, we are not using Mother Nature to decompose that crop residue so we need to look at lengthening our crop rotations.”

The Lacombe centre is part of a five-year project assessing various combinations of rotations with and without treatments to monitor weeds, disease and other agronomic factors. Similar projects are at Melfort, Sask., and Normande, Que.

Various combinations of flax, canola and barley were used in single and two year gaps between wheat.

A single year between the two types of crops saw more weeds with more biomass. A two-year interval worked better, and plants could better outcompete weeds.

Fungicides were used at the recommended strength with some troublesome diseases. A longer rotation also gave straw residue more time to decompose to deal with overwintering diseases.

The researchers also used a spore trap to identify pathogens and assess how much pressure is being placed on a crop. Spores blowing in the wind were captured in the trap’s filters, and scientists were able to identify them and determine if the levels were high enough to spray.

Longer rotations and judicious fungicide use is needed to prevent disease resistance to these treatments, said Allan Eadie of Bayer Crop Science during a recent farm expo in Olds, Alta.

Fungicide resistance is rare in North America, but it is important to know what diseases are appearing in fields.

“You have to assess everything about the fungicide in question, the pathogen, is it a pathogen prone to developing resistance like ascochyta in lentils,” he said.

Among the most important factors is knowing the number of disease cycles per year, the abundance of spore production and its ability to spread via wind or in the soil.

A growing list of diseases is starting to show resistance to some products. For example, rust path-ogens might mutate and develop resistance to a fungicide.

Other diseases such as sclero-tinia and root rot pathogens are considered low risk for developing resistance.

“Even with low risk pathogens, the risk is more than zero,” he said.

This is a global problem. The fungicide resistance action committee is an international group working to determine how fungicides are labelled and sold.

The group also ranks low, medium and high risk pathogens and whether diseases have the ability to develop resistance.

Fungicides are broken down into chemical classes with different modes of action. Resistance development may vary within a family of chemistry.

“They are not dissimilar to herbicides where you have got them broken down into different chemical structure and mechanism of action,” he said.

“You want to use agronomic factors that use a variety of controls wherever you can.”

Agronomic risk factors include:

  • climate favouring disease
  • agronomic practices — Resistant cultivars if available, sanitation of equipment and accurate spray programs, which means getting the most of the application and not applying a sub-lethal dose. Good coverage is needed with the right sprayer speed, correct ratio of water and making sure spray nozzles are working right.
  • fungicide — The use of repetitive and sustained treatments of the same types of fungicide could encourage development of disease-resistant fungi.

The main fungicide resistance management strategies employed and recommended are:

  • Avoid repetitive and sole use of the same fungicide group.
  • Mix or alternate with an appropriate and effective fungicide.
  • Limit the number and timing of treatments.
  • Consider integrated pest management with non-chemical methods such as planting resistant plant varieties and better rotations.

For more information on fungicide resistance, visit

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