Sclerotinia seen in canola, aphid populations rise in soybean crops

Manitoba Insect & Disease Update for August 16

Sclerotinia stem rot has been observed in scattered canola fields through the canola disease survey. In most fields levels are low, but incidence was about 50 per cent in a field in the Carman area.

Blackleg symptoms are easiest to identify when stems are cut at the base. The perfect time to scout is at harvest, so bring your clippers along and jump off the swather or combine from time to time.

Soybean aphids continue to be a concern in some fields, with insecticide applications having occurred. In some soybean fields natural enemy populations seem to be building in response to the soybean aphids. Management decisions are further complicated in some fields as the soybean move towards the R6 growth stage.

High levels of bertha armyworm larvae continue to be reported from some canola fields in western Manitoba.

Sclerotinia stem rot in canola

The canola disease survey is continuing or wrapping up across the province. Levels of blackleg, sclerotinia stem rot, aster yellows, and verticillium wilt have all been observed. Most disease levels are low overall due to the lack of moisture throughout the growing season and timely fungicide applications for diseases like sclerotinia stem rot. Higher levels of sclerotinia stem rot have been observed in canola fields in the Carman area, with one field showing more than 50 per cent incidence of affected plants. The general rule of thumb is that yield loss equals 0.5 x incidence. Therefore, if a field is showing approximately 50 per cent incidence, yield loss could be estimated around 25 per cent. However, this estimate changes depending on the severity of the disease.

Soybean aphids: Growth stages most susceptible

The R1 (beginning bloom) to R5 (beginning seed) growth stages are the most susceptible to feeding by soybean aphids. Once plants move into the R6 (full seed) stage, soybean aphids will have less of an impact on yield. In the R6 stage, pods contain green seeds that fills the pod cavity at one of the four uppermost nodes on the main stem. Once the seeds are filling the pod cavity full yield potential has occurred, and soybean aphids are no longer an economic threat to the crop.

The University of Wisconsin has a good guide to soybean stages and soybean aphids (PDF).

To read the full Manitoba Insect & Disease Update, visit the Manitoba Agriculture website.

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