Final regular Manitoba insect and disease update

Soybeans downy mildew

MAFRD has issued its last regular Insect & Disease Update for the season, but will still issue periodic reports if required. A summary is below. The full text of the Aug. 21 report is available online.


With warm conditions continuing and with rainfall deficit in many area of Manitoba, many crops are showing effects, such as poor growth on corn, sunflowers and soybeans. Plants are stunted and foliage is desiccating and leaf drop has been seen in soybeans. Diseases favoured by high humidity appear to have stopped in their tracks; but plants with root rots will now start showing up. Scattered heavy showers in parts of Manitoba may have provided some respite.

Some spider mites have been showing up along the edges of some soybean fields. Lygus bugs levels have been high in some canola fields, although many fields are now beyond the stage where Lygus bugs can be of concern. If there is brown-green mottling in the seeds in the lower pods, or complete colour change, than the crop will no longer be at risk of injury from Lygus bugs.


The Canola disease survey in now on-going for blackleg, root rots, white mould, aster yellows and alternaria pod blight. Due to changing strains of the blackleg fungus, some of the R rated varieties may show higher levels of blackleg in different regions.


Diseases: Mid-May planted fields have in general reached the R6 stage, i.e. full green seed formation in the pod. Many fields need water, indicated by top pods aborting and lower leaves with yellowing not caused by brown spot disease.

The diseases most commonly seen are brown spot, bacterial blight, downy mildew, and scattered plants of Phytophthora root rot (mostly 1-2%). In some fields, especially soybean following soybean, there is apparently a higher number of downy mildew leaf spots, many of which have matured into a brown colour, but are still showing downy growth on the underside of the leaves (Fig 1a — 1c).
Heavy scattered rains with strong winds have lodged a few fields of lush soybeans, and could create a micro-climate favourable for white mould development.
At this stage of crop development, none of these diseases or the level of severity would benefit from a fungicide spray.

Spider mites — In a few soybean fields, feeding from spider mites and yellowing of leaves has been noticed, mainly around the edges of the field. There is often a strong edge effect with spider mites on soybeans.
Spider mite populations are often higher when conditions have been hot and dry for a sustained period of time, and populations can change substantially after heavy rains. So if you have been noticing what appears to be feeding from spider mites on soybean plants, and the area has had some recent rain, make sure to look for the mites and see what the current population is like.

Soybean aphids. Low levels of soybean aphids have been showing up in more fields in southern Manitoba over the past week, but so far no fields have been reported with economical populations of aphids.

Wheat — Up to 15-25% of spikes/heads are being recorded with Fusarium head blight infection in winter wheat. This year ergot bodies can be seen in rye, barley, wheat and some grassy weeds.


Potato aphid in flax — There have been a couple of recent reports regarding aphids in flax, although it is getting late for aphids to be of economical importance in flax.

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