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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect and Disease Update

June 28, 2010                   

Compiled by: John Gavloski, Entomologist, Manitoba Agriculture, Food and Rural Initiatives
                    Phone: (204) 745-5668; Fax: (204) 745-5690, and
                    Vikram Bisht, Plant Pathologist, Manitoba Agriculture, Food and Rural Initiatives
                    Phone: (204) 745-0260; Fax: (204) 745-5690

To report observations on insects or plant pathogens that may be of interest or importance to farmers and agronomists in Manitoba, please send messages to the above contact address.

To be placed on an E-mail list so you will be notified immediately when new Manitoba Insect and Disease Updates are posted, please contact John Gavloski at the address or numbers listed above.

Summary

• Apothecia of sclerotinia are abundant in some areas.
• Root maggots larvae are being noted in some canola fields.
• Leaf spot diseases have been noted in many cereal fields

Recent Insect and Plant Pathogen Activity

Broadleaf Crops

Sclerotinia: Apothecia of sclerotinia are being easily found in cereal fields, which had Sclerotinia susceptible crops (eg sunflower, canola and soybean) in 2009. There is a very high probability that ascospores will be reaching this year’s susceptible crops from nearby cereal crops. Timely sprays are going to be important. Along Highway # 3 there are many canola fields which have reached 40-50% flowering and should have been sprayed by now, if these fields were near 2009 sclerotinia infected fields.

The canola crop in the province varies from pre-bloom to >50% bloom; and could be susceptible to sclerotinia for a long time. Rains with strong winds help spread the ascospores over long distances.

Survey for developing a sclerotinia forecast program: A prairie-wide canola survey is being conducted for a Sclerotinia forecast program. Along with others, MAFRI is also participating in sample collection. Canola petals are being sampled at 25-30% flowering; followed by another sample collected 7-14 days later. These petals will be tested for the presence of Sclerotinia infection. If some growers wish to collaborate and would like us to collect samples from their fields, please contact Vikram Bisht (204-745-0260).

Blackleg in canola: Blackleg infection is also being reported from some canola fields.

Root Maggots in canola: There have been some reports of high levels of root maggots in canola. In one field near Rossendale, the root maggots were noticed after some of the plants appeared wilting. In this particular case the field is in canola for the second year in a row, which may have helped to increase the risk. There are five species of flies (all in the genus Delia) whose larvae feed on the roots of canola, and broadly get categorized as root maggots. The figure below shows a root maggot on a canola root.


Figure 1.

Diamondback moth: Although some larvae can be found in canola fields, there are still no reports of high or economical levels of larvae.

One of the questions that some were asking last week was should canola growers be tank mixing in an insecticide, when applying fungicides, if diamondback moth trap counts were high in their area. As emphasized in previous updates, high adults counts do not mean there will be high levels of larvae – so there is a good chance the insecticide is only increasing the cost of production without providing many benefits (and potentially harming some of the beneficial insects that may be present). Decisions regarding diamondback moth need to me made based on larval counts, not adult counts. Larvae are easy to scout for by tapping plants, and if you are having trouble finding the larvae the insecticide is not needed. Economic thresholds are about 10-15 larvae per ft of canola in fields not yet flowering, and 20-30 larvae per ft of canola in plants with flowers and pods.

Forecasting programs for insects like diamondback moth and bertha armyworm are a tool to help emphasize insects that need extra attention when scouting crops. For both diamondback moth and bertha armyworm, intelligent decisions on the economics of control can only be made based on larval counts. Using data from trap counts to make management decisions would be a counter-productive use of the trapping program.

Cereals

Leaf spot diseases in cereals: Leaf spot diseases of cereals, mainly tan spot, are now reaching the flag leaf in many fields. Fields with tight rotations and with cereal stubbles not completely buried may have more than normal leaf spotting this year. Some rust spots have been reported in Portage and west – still minor.

In the Boissevain area, one winter wheat field had flag leaves showing >50% covered with leaf spots (mainly tan spot).

Armyworms: There have been a few reports of true armyworm (Pseudaletia unipuncta) feeding in wheat in the northeast region of North Dakota near Grand Forks. This is something to watch for, particularly when scouting cereal fields, although they will also feed on some broadleaf crops. So far their have been no reports of armyworms in Manitoba, other than an adult moth that ended up on the sticky insert of one of our traps for diamondback moth (see May 31st update).

 

Insect Identification Quiz

Question: I found a few of these caterpillars feeding on the pansies in my yard on the weekend. What are these caterpillars? Do they potentially feed on any crop plants? And for bonus mark, do they overwinter in Canada?

Answer: The caterpillar in the photo is a variegated fritillary. Aside from pansies, their food plants also include violets and flax. Flax is the only crop I have seen or heard of them feeding on, although feeding is usually not economical. Something to watch for though if you are scouting flax. They do not overwinter in Canada. In some years populations that are quite noticeable migrate in, while in other years they are hard to detect.