Crop advisor casebook: Are cutworms collapsing this canola crop?

A Crop Advisor's Solution from the October 16, 2018 issue of Grainews

The bare patches in this canola field were not only located on knolls, 
but throughout the field.
Genevieve Shukin. photo: Supplied

On June 19, Ryan, a Saskatchewan producer who farms south of Canora, called me to diagnose a problem in his canola crop. While applying the first pass of glyphosate on the field, Ryan discovered some bare patches — he thought he may have a cutworm problem.

Ryan’s farm is situated on light, sandy soil with rolling topography, on which he grows wheat, oats and canola. When scouting fields in this region, I have found cutworms primarily on knolls in loose, light soil. However, when inspecting this canola field, I found the bare patches weren’t only located on knolls but throughout the field.

In addition, the canola plants, which were at the three- to four-leaf stage, were still attached to the roots and not cut off below the soil surface, which is characteristic of cutworm damage. Usually, cutworms can be found at or below the soil surface in the evening or at night because they surface to feed. However, we found no evidence of cutworm activity or cutworms in this field. Thus, another factor was responsible for the bare patches.

Seedlings in the patchy areas had topplbed over, wilted and died. 
Others appeared normal, but had decaying roots. photo: Supplied

Although the seedlings in the patchy areas weren’t cut off at the soil’s surface, they had toppled over, wilted and died. Some emerged seedlings appeared normal above ground; however, when we checked the roots of these plants, they were decaying—with rot moving into the hypocotyl (young stem). Also, the hypocotyl was partially or completely girdled with decay and shrivelled.

These symptoms all pointed in one direction, and had nothing to do with insect pests.

Crop Advisor’s Solution: The lowdown on seedling disease complex

The prolonged cold, dry soil conditions the region had been experiencing that spring was conducive to seedling disease complex. Ryan’s canola plants had developed seedling blight, wirestem and post-emergent damping-off. The seedling blight was scattered uniformly throughout the field, and occurred in patches — particularly on the knolls and slopes.

In addition, Ryan had seeded the canola plants slightly deeper than normal in an attempt to seed into moisture. Deep seeding and tight canola rotations also favour seedling diseases caused by soil-borne fungi.

Seed and seedling losses tend to be highest under cold, wet conditions, or when the seedbed is not firmly packed under dry, cool conditions. Seedling disease complex is often a problem in the northwestern prairies, with prolonged low soil temperatures.

Specifically, cold, damp soils provide favourable environments for Pythium spp.; whereas, loose, cold, dry and well-worked soils are conducive to Rhizoctonia solani. Wet, heavy soils provide favourable environments for Fusarium spp.

In-crop, there are no control measures for seedling disease complex. However, to control seedling blight and damping-off before seeding, producers should sow seed with high germination into a firm seedbed and pack dry soil to bring moisture up to the seed.

The optimal seed depth is 1.5 to 2.5 cm. Seeding deeper, to reach moisture, increases the time during which the seedling is dependent on the seed’s food reserves and, thus, the time the seedling is susceptible.

Maximize conditions favouring rapid germination and establishment by sowing seed into warm soil and correctly applying fertilizers. Inadequate or unbalanced fertility levels favour seedling disease. In addition, excessive fertilizer placed with the seed increases seedling problems, particularly phytotoxicity. Thus, reduce the amount of fertilizer placed with the seed.

Be sure to scout fields during the four weeks following seeding, or up to the four-leaf stage. If the crop is patchy or large areas die out, plant compensation may not be enough, and reseeding may be the best option for affected areas of the field — provided surface moisture and soil temperatures will allow for shallow seeding and rapid emergence of the reseeded crop.

In Ryan’s case, since the seedling losses were uniform throughout the plant stand, the surviving canola plants compensated by growing larger, without any significant yield loss at harvest. I also outlined some additional changes Ryan could make in seasons to come.

For example, seedling blight and damping-off can be minimized by using higher seeding rates, shallow seeding into warmer soils and good seed to soil contact, which will allow the crop to establish rapidly.

Seeds that emerge and grow quickly are less likely to succumb to seedling diseases. By the two- to four-leaf stages, the hypocotyl portion of the root develops a waxy coating called suberin, which helps seedlings withstand further infections. Additionally, a registered fungicide seed treatment with multiple active ingredients can minimize the threat of seedling disease complex.

Genevieve Shukin, BSc (Ag), works for Richardson Pioneer Ltd. in Canora, Sask.

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