Pre-harvest scouting: Diseases, weeds, CSPW exit holes

August 4, 2016 - Issue 20

Diseases: Assess whether fungicide worked to manage sclerotinia stem rot in the high risk conditions this year and whether clubroot and blackleg resistance performed as expected. In the case of sclerotinia fungicide, compare treated areas to check strips or missed areas to check on performance. With fungicide providing 14 days protection, sprayed fields this year could exhibit high levels of disease because of the persistent wet weather during flowering. As an example, a single application may only reduce/inhibit sclerotinia stem rot by 50% compared to untreated. A split application may provide better protection, but it too could be significantly less than 100% this year. Read more on how to do a pre-harvest ID on the most common canola diseases.

Check suspicious weeds for herbicide resistance: Are those patches herbicide resistant and to what? Or are they just escapes?
—Herbicide resistant weeds tend to be in patches of all the same weed. Other weeds were controlled while this particular species-specific patch was not. Herbicide resistant weeds also tend to be in patches of irregular shape, starting off as one weed and going out from there as seeds shell out or the combine spreads them around. In the case of glyphosate-resistant kochia, plants are often in long lines as the resistant parent tumbled along dropping seeds. Wild oats will be in patches that spread out from the original resistant plants.
—Escapes tend to be multiple weed species. Weeds under the canopy may have missed herbicide contact, and these will usually be more than one species. Patches of escapes also tend to be in clear shapes, perhaps in arcs in headlands where the sprayer missed on a turn. Read more.

Exit holes from cabbage seedpod weevil larvae. Credit: Justine Cornelsen

Exit holes from cabbage seedpod weevil larvae. Credit: Justine Cornelsen

Cabbage seedpod weevil exit holes: Larvae will eat some seeds inside a pod then dig out, leaving an exit hole. If 25% of pods or less have exit holes, one can assume that a spray did its job. Or, if you didn’t spray, you made the right decision. If more than 25% of pods have exit holes, damage exceeded economic thresholds. A check strip would be especially helpful in determining whether a spray paid off.

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