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Hail and fungicide timing, alfalfa weevil levels seen high

Manitoba Insect & Disease Update for June 14

Recent hail events in the province have left producers wondering whether or not they should be using a fungicide at the herbicide timing.

Flea beetles in canola, and cutworms continue to be the main insects of concern, although for both concern is diminishing as canola advances to stages less susceptible to feeding from flea beetles, and some cutworms larvae turn to pupae. Alfalfa weevil larvae are noticeable in some alfalfa fields.

Post-Hail Fungicides

For any of the spring crops, a fungicide applied at this time of year would be hitting the early herbicide-timing. For spring cereals, fungicides at this time can be used to help manage leaf diseases, such as tan spot and Septoria leaf blotch. Spraying a fungicide at the early timing will only protect the leaf tissue that has already emerged. When there is disease pressure, a reduction in disease severity may be realized, however yield bumps are more inconsistent.

For canola, fungicides can be used at the herbicide timing to help manage blackleg. Timing is key when it comes to this practice, as the most consistent results are when you get in early, around the 2 to 4 leaf stage. Hail (and wounding in general) makes plants more susceptible to blackleg infection, so if you are planning a post-hail fungicide application it is also important to get out there as soon as possible. Protecting the plants while the wounds are still open will provide the best results. In the absence of hail or excessive wounding, strobilurin fungicides have been shown to be more effective than triazoles at reducing disease. However, yield increases were only observed with strobilurin fungicides used at the 2 to 4 leaf stage in susceptible cultivars. Yield was not significantly affected in moderately resistant or resistant cultivars.

Overall, it is important to assess whether a fungicide application is worth it. What is the yield potential of your crop and how much of that yield has been compromised due to the hail event? A fungicide will not rescue your crop from hail damage, but can help protect your crop from the disease issues it is now more susceptible to.

Alfalfa Weevil

Alfalfa weevil have been hatching, and high and above threshold levels of larvae have been noted in some alfalfa fields in the Interlake.

In alfalfa fields for hay, if alfalfa has reached the bud or early bloom stage, immediate cutting may kill many alfalfa weevil larvae.

Alfalfa weevil monitoring tips: A suggested monitoring strategy in alfalfa hay fields is; collect 30 stems while walking in a “w” shaped pattern in the field, and place them into a white pail. Beat the stems against the side of the pail to dislodge the larvae. The economic thresholds for chemical control of alfalfa weevil are:

  • less than 30 cm crop height – one larvae/stem
  • 30 to 40 cm crop height – two larvae/stem
  • Three larvae/stem requires immediate action regardless of the height of the crop.

In alfalfa seed fields, a sweep net can be used or the percent of foliage tips showing damage can be assessed to make decisions based on the following nominal thresholds:

Foliage assessment: 35 to 50 per cent of foliage tips show feeding damage.

Larvae in sweep net: 20-30 3rd/4th instar larvae per 180 degree sweep of insect sweep net.

Insect Monitoring Programs

Bertha Armyworm: A reminder for those setting up traps to monitor adults of bertha armyworm that traps should be set up as soon as possible if they are not set up already.

Data is just starting to come in, but at this early stage is quite low, as expected. The highest count so far is 11 from a trap near Emerson.

Diamondback Moth: Diamondback moth monitoring with the pheromone baited traps has been underway since the beginning of May. Very low levels of moths were caught in traps in May and early-June. The highest cumulative trap count on June 6 was 19.  Some higher levels of moths were caught in traps the following week, however. By June 13th 5 traps had cumulative counts greater than 50, and the highest cumulative count was 125.

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 www.gov.mb.ca.

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

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