Crop Advisor’s Casebook: What happened to these canola roots?

A Crop Advisor's Solution from the November 7, 2017 issue of Grainews

The canola plants were wilted, with roots that came to a gentle point. 
These damaged plants were randomly distributed throughout the field.
Cara Goldamer. Cara Goldamer works for Richardson Pioneer Ltd. 
in Vulcan, Alta. photo: Supplied

Dennis farms 3,400 acres of malt barley, wheat, yellow peas and canola near Vulcan, Alta. With canola samples in tow, he visited our office in early June. The plants he showed me were wilted, but had retained their green colour. However, it was the roots that caught my attention. They looked like they’d been put in a pencil sharpener.

Dennis explained while he was spraying an in-crop herbicide on his canola field, he noticed some of the plants were knocked over. When he exited the sprayer to have a closer look, he found the plants were also wilted, with roots that came to a gentle point.

I wasn’t sure what had caused the symptoms from looking at the samples he’d brought in, so we decided to check his crop in situ.

When I arrived at the field, I noticed a few of the canola plants had fallen over. These were wilted, but still green, like the samples Dennis brought to the office. The canola in the field was at the three-leaf stage. When I pulled an affected plant out of the soil, the crown of the root looked like it had been sharpened. These damaged plants were randomly distributed through-out the field.

“Could it be cutworms or, perhaps, wind damage?” asked Dennis.

We had just experienced some extreme wind events throughout the region. However, wind damage could be ruled out because if wind had knocked these plants over, I’d expect to see severe damage to the leaves as well, such as tears and shredding.

As for cutworms, we dug into the soil looking for larvae or evidence or cutworm activity, such as chewing damage to the leaves or cut stems. After a good look, we found no cutworms or signs of feeding activity. In addition, cutworm damage occurs in patches, usually on south-facing slopes and light-textured soils, such as hilltops, with more crop residue.

While we were scouting for insects, I also looked for wireworm damage. Wireworm larvae like to feed on germinating seeds or young seedlings, and often shred plant stems. When this happens, sometimes the central leaves die, but the outer leaves can stay green. Eventually, the damaged plants wilt and die.

However, we were not dealing with wireworm damage in this instance because the canola stems weren’t shredded and there was no evidence of feeding on seeds or seedlings. Plant stands of wireworm-infested fields also appear patchy, not random, as was the case in this field. Thin, patchy, canola stands can be attributed to poor germination, while the real cause is wireworm activity.

If canola plants are found lying on the ground and insects are suspected, cutworms usually cut off the entire plant below the soil surface, whereas wireworms may shred stems, but seldom cut off plants.

I took some photographs and dug up a few more plants to bring back to my colleagues. After examining the photos and plants, and doing a little research, we had our diagnosis.

Crop Advisor’s Solution: Brown girdling root rot can take a canola plant down

After some research on canola diseases, we agreed the symptoms in Dennis’ canola crop were almost identical to those for brown girdling root rot (BGRR).

Thought to be caused by the fungus Rhizoctonia solani, BGRR is the most severe of all root rots in canola in Western Canada. The brown lesions on the canola plant’s tap root girdles the root and pinches it off, giving it that “sharpened” appearance.

Although no economically feasible chemical control options are available for this fungus, the following management practices can help reduce the incidence and severity of the disease:

  • Healthy crop rotation
  • Good host weed control
  • High-quality seed with high seed vigour and germination rates
  • Proper stand establishment
  • Plant varieties less susceptible to the disease
  • Provision of optimum nutritional requirements

In Dennis’ case, he didn’t need to tweak his rotation as he already allowed four years between his canola crops. In addition to canola, his rotation included cereals and a pulse crop. He also does a good job controlling most weeds, however, he tends to save costs when he thinks the input isn’t worth the investment.

When it comes to getting his crop off to a good start with rapid, uniform emergence, Dennis uses high-quality seed from his retailer with high vigour and germination rates. Furthermore, Brassica napus varieties are less susceptible to BGRR than B. rapa varieties. Dennis also does a good job on his crop fertility planning, making sure to provide enough nitrogen as well as other macro- and micronutrients to reach his target yields.

Since Dennis already followed most of the best management practices to reduce BGRR infection in his field, the only additional suggestion I could make was for him to keep host weeds, such as stinkweed and shepherd’s purse, under control.

At the time of writing, we didn’t yet know if yield would be affected, as canola harvest hadn’t started in this region. When scouting, we found less than one per cent of his field had been affected, not to mention the canola plant compensates in situations with thin stands by branching out.

In the following years, Dennis should continue his four-year crop rotation, use effective chemicals and best management practices for applications to control host weeds, ensure a balanced fertilizer plan for the field, and plant the best variety — with the most resistance to BGRR — for his farm.

Because you never know what’s coming at you that season, be prepared for anything, and do the best you can to follow good management practices on your farm.

Cara Goldamer works for Richardson Pioneer Ltd. in Vulcan, Alta.

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