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Crop advisor casebook: Irregular canola pod shatter a mystery

A Crop Advisor's Solution from the August 27, 2019 issue of Grainews

Jaclyn Hunter.
photo: Supplied

Dave, who farms 6,000 acres of wheat, barley, peas and canola in Central Alberta, was trying out a new canola seed variety, which was in its first market year. The variety had clubroot resistance and had also been selected for some pod shatter tolerance.

Throughout the growing season, the canola crop had developed normally, and it was a generally healthy crop. Dave hadn’t noticed anything unusual about the crop and I had been out to the field a few times to scout throughout the season. Nothing seemed out of the ordinary.

The crop was planted on a quarter section new to Dave — he’d only been farming it for a few years. It was highly productive land and had been fertilized with a premium blend. A fungicide application had been made on the canola crop at 30 per cent flower for protection against sclerotinia as in-season rainfall and a dense crop canopy were conducive to disease development.

In early September, when Dave was swathing the crop, he noticed the pods were shelling out more than he expected. Dave thought that perhaps there was something wrong with the new variety.

“Something is wrong with this variety,” he said. “It is shelling out at swathing time. I have never had a variety shell like this while swathing before!”

He said the field’s hilltops, where the soils were drier and lighter, were most affected by the shelling. He hadn’t noticed any shelling prior to swathing. The day he swathed had been hot, Dave added.

Eager to get to work on this problem, I met Dave at the field, parking by the roadside and donning my booties for walking in the field. Because the field was completely swathed, it was easier to walk through; however, I was worried Dave’s call had come too late for a diagnosis.

Dave took me directly to the spot where he first noticed the shelling and then we moved on to some other locations. The field was rolling with a few high and low spots.

While we were walking through the field, possible sources of the shelling issue were running through my mind. For example, a nutrient deficiency can cause some areas to stage or ripen differently than the rest of the field. If the plants ripen off too quickly, it can cause the pods to shell out.

However, annual soil tests have been conducted on this field and fertilizer is applied accordingly. This field had been managed with a balanced fertility program, so there was no reason for the plants to run out of nutrients and ripen prematurely.

Excess moisture or drought conditions can also encourage plants to prematurely ripen. Yet, precipitation levels had been average during that growing season, so soil moisture was adequate for the crop to develop normally.

Another possible answer could have been the conditions at which the crop had been swathed, as hot, dry conditions in the afternoon can cause excessive pod shelling.

When I asked Dave about the conditions at swathing, he said most of the field was swathed in the early morning or late evening, during the dewy or cool hours of the day so I knew environmental conditions at swath-timing shouldn’t have contributed to more pod shatter.

Blackleg disease incidence in Dave’s crop was medium to medium-high, which may have contributed to the advanced maturity and high levels of pod shatter in this field. However, I thought it was likely that blackleg was compounding another larger issue in the crop.

As Dave and I were examining plants by pulling them up out of the soil, we noticed some of the roots looked funny — almost like there was an abundance of organic matter or extra loamy soil collecting at the roots.

Here was my first bit of concrete evidence for the cause of the pod shelling.

Crop Advisor’s Solution: Clubroot didn’t get the best of this producer. Not this time, anyway

Clubroot, which is caused by a fungus-like protist, causes galls to form on the plants’ roots. Once a field starts to ripen off, especially after swathing, the galls start to break down and release spores, causing the odd appearance of the plants’ roots.

This evidence led me to believe there was a good chance of a clubroot breakthrough in this field: the pathogen had overcome the genetic resistance in the variety. Dave and I spent a good amount of time in the field pulling plants and examining roots. Eventually, we found some galls that hadn’t broken down as much as the others. We collected these samples to send to the University of Alberta’s Plant Pathology Laboratory for testing.

The test results confirmed clubroot had overcome the variety’s genetic resistance because of the high spore load and the strain of clubroot present in the soil. The crop had been planted in a quarter section Dave had been farming for only a few years.

It was possible the affected plants could have been volunteers, which would not have clubroot resistance. However, this was unlikely because the field was very clean and the plants were pulled from the seed row.

The spores causing clubroot can linger in the soil for up to 20 years. There is no cure for clubroot and no fungicides are available to mitigate the disease’s effect on canola plants.

When scouting, identify the cause of symptoms in a crop, such as wilting, stunting, yellowing, and premature ripening (as in this case). Do not make assumptions that the symptoms are a result of environmental conditions, such as drought or excess moisture, new variety issues, or other diseases that may not affect the crop as greatly, such as blackleg.

There are some measures growers can take to prevent or mitigate clubroot symptoms, such as implementing longer crop rotations, choosing clubroot-resistant cultivars, and controlling weeds that are hosts to clubroot. Weeds that can also be infected by the disease include stinkweed, wild mustard, flixweed, shepherd’s-purse, and perennial ryegrass.

Resting clubroot spores can be readily moved in soil and transported in many different ways including, but not limited to, wind, water, erosion, animals, implements, and vehicles, to name a few. Direct seeding is another cultural practice that may reduce erosion and spore movement. You can also seed clubroot-infected soils last and clean implements thoroughly after you are finished in that field.

Growers should sanitize equipment, vehicles, tools, hands, clothes and shoes that enter canola-infected fields with a solution of at least two per cent hypochlorite bleach to prevent clubroot spread. All soil should be removed from vehicles and implements, especially if it is suspected that the soil on the equipment has clubroot spores in it.

Growers and all visitors should wear washable boots or disposable booties when entering fields to prevent the spread of clubroot. When seeding canola, ensure that it is a clubroot-resistant variety.

In confirmed clubroot regions, it is recommended to use multigenic clubroot genetics. In some scenarios, some counties even require that multigenic resistant varieties are used. The new clubroot-resistant varieties have performed well and have a lot of research and breeding behind them to make canola a profitable option on the farm when clubroot is present or not.

Clubroot has been confirmed in many Alberta counties, farms and fields. Each county has a slightly different approach to clubroot management; growers and agronomists should understand the protocols set up in their home counties.

Even with the newest technologies, Dave’s crop was negatively affected by clubroot. Luckily, the clubroot in Dave’s field had a negligible effect on yield. Since there was premature shelling, the volunteers must be controlled in following years, but the majority of the field was not affected and the outbreak was mostly contained to a small area. He will continue to manage clubroot by following the clubroot management protocol that is set out by the county in which he resides.

Jaclyn Hunter works for Richardson Pioneer in Legacy Junction, Camrose, Alta.

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