Alfalfa weevil still at high levels, root rot in soybeans reported

Manitoba Insect & Disease Update for June 28

Alfalfa weevil continues to be reported at high levels in some alfalfa fields.

Cereal leaf beetle and thrips are noticeable in some fields of small grain cereals, but at below economical levels. Initial tests of levels of parasitism of cereal leaf beetle larvae show quite high levels in the Central region, with samples from other regions soon to be tested. Thanks to those agronomists who have provided field locations, or collected and sent in samples of cereal leaf beetle larvae to be tested for parasitism. See last weeks update for details on collecting and submitting cereal leaf beetle samples to determine level of parasitism.

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Low levels of thistle caterpillars have been noted. Although these will feed on sunflowers, the level of feeding is not normally economical, and they can do a lot of feeding to thistles.

Bacterial blight in oats has been detected in the Red River Valley. As temperatures increase plants will typically grow out of the symptoms.

Loose smut has been detected in a wheat field near Niverville.

Phytophthora root rot (PRR) in soybean has been reported in the Red River Valley. This disease thrives under wet conditions and can cause yield loss depending on incidence. Typically PRR affected plants will prematurely senesce and not set seed.

Bacterial Blight in Oats

Bacterial blight in oats has been reported in various areas of the Red River Valley. This disease thrives under cool and moist conditions. Typical symptoms symptoms include water soaked areas that eventually turn necrotic. The affected areas often form longitudinal blotches or stripes. As temperatures warm up the plants are able to grow out of the symptoms. Yield loss is not generally a concern with this disease.

Loose Smut in Wheat

There has been a report of smut in wheat from the Niverville area. This disease is also favoured by cool humid/wet conditions. Smut is typically managed through using resistant varieties and seed treatments. The variety where symptoms were observed is Carberry. Symptoms of smut typically show up at the same time as head emergence. In the affected field, incidence was low overall with scattered plants showing up across the field.

Phytophthora Root Rot in Soybean

There have been multiple reports of Phytophthora root rot (PRR) in soybean in the Red River Valley. In at least one case, the incidence is reported as up to 30 per cent of plants affected. PRR is caused by an oomycete and thrives under water logged conditions. Symptoms can show up in patches in the field (low spots) or as scattered plants. Severely infected plants will completely senesce, but the leaves will stay attached. PRR is common in areas of the province that have had a history of soybean production and in years where there are water-logged conditions.

Scouting for thrips in barley

Barley thrips can at times get to economic levels in barley, but populations can be quite variable between and within fields. What should be monitored is levels when the flag leaves are first visible until the heads are completely emerged from the boot.

Economic thresholds have been established; the threshold (thrips/stem) = (Cost of control / expected dollar value per bushel)/ 0.4.

This often works out to around 4 adults per stem, but because the value of barley can vary greatly depending on use and market, it is good to use this simple formula to determine the economics of potential management. Insecticides are only going to provide value if applied before heading is complete, if levels of thrips are high. Also note that barley thrips may be more numerous near protected field margins than other areas of the field.

Agronomists, Farmers, Farm Production Extension Specialists, Extension coordinators, and others scouting crops: Please remember to send in reports of insects or plant diseases over the growing season so we can make these updates as complete as possible, and alert farmers and agronomists where and to what degree insects and pathogens are of concern or being controlled. Information can be sent to: John Gavloski (entomologist) at [email protected] (phone: 204-750-0594) or Holly Derksen (plant pathologist) at [email protected] (phone: 204-750-4248).

Visit the insect pages on the Manitoba Agriculture website at

Click here to view the complete Manitoba Insect & Disease Update on the Manitoba Agriculture website.

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