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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect and Disease Update

June 20, 2011                   

Compiled by: John Gavloski, Entomologist, Manitoba Agriculture, Food and Rural Initiatives
Phone: (204) 745-5668; Fax: (204) 745-5690
Holly Derksen, Plant Pathologist, Manitoba Agriculture, Food and Rural Initiatives
Phone: (204) 750-4248; 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.


 

Recent Insect and Plant Pathogen Activity

Cereals

Armyworms: Larvae of armyworms (Mythimna unipuncta) are starting to be found in some fields in Central Manitoba. Small numbers of larvae were found in fields of perennial ryegrass near Homewood and winter wheat near Graysville late last week, and adult moths noticed near Oakville. Armyworms do not overwinter in Manitoba but blow in. Larvae will feed on a variety of crops, but seem to be most troublesome in cereal crops and forage grasses when numbers get large. Figure 1 is of a young armyworm larvae found last week in perennial ryegrass near Homewood. Figure 2 is the adult moth of the armyworm. Note the distinctive white spot in the middle of each forewing.

armyworm larva armyworm adult
Fig. 1. Armyworm larva Fig. 2. Armyworm adult


Note that this is not the bertha armyworm, which can be a concern on canola.

Tan spot in wheat: Spring and winter wheat crops throughout the Carman/Morden area are showing symptoms of tan spot. Conditions this year are proving conducive for many fungal pathogens. Disease pressure along with previous, current, and upcoming environmental conditions should be considered when deciding on fungicide applications.

Fusarium Risk Maps: Winter wheat in some parts of the province is nearing anthesis (flowering), the stage at which it is most susceptible to Fusarium Head Blight (FHB). The FHB risk maps for the 2011 field season will be posted this week and will be available online:
https://www.gov.mb.ca/agriculture/crops/fhb/index.html

Cereal Leaf Beetle: In 2009 and 2010, cereal leaf beetle, which is a new insect in Manitoba that will feed on cereal crops, was found in the Swan River area. We are still not sure of the full distribution of this insect in Manitoba, and we are trying to get a better idea of where it may be present.

When sampling fields of cereal crops this summer, watch for cereal leaf beetle and report any suspected findings. Note that there is another leaf beetle in Manitoba that can be confused with cereal leaf beetle. Figure 3 below is the cereal leaf beetle (Oulema melanopus) , a potential pest of cereal crops. Figure 4 is a species called Gastrophysa polygoni, which has some similar characteristics to cereal leaf beetle but is not a pest species. We have found several specimens of Gastrophysa polygoni in the Carman area in the past few years. They feed on several weed species, and in cereal fields are known to feed on knotweeds or docks. In Saskatchewan they have been found to feed on wild buckwheat. If you find any of these insects and want clarification as to the species, send in your samples and we can help you with this.

cereal leaf beetle gastrophysa polygoni
Fig. 3. Cereal Leaf Beetle Fig. 4. Gastrophysa polygoni

Taking some samples with a sweep net is a good way to look for the beetles. The adult beetles for both species are small (4-5 mm) so you are looking for quite small beetles. This can be done as soon as the wheat is tall enough to sweep. Also examine plants for the presence of the beetles or their larvae. Figure 5 is a photo of a cereal leaf beetle larva.

larva of cereal leaf beetle
Figure 5. Larva of cereal leaf beetle

Dry beans

Root rot: A dry bean field near Morden has shown symptoms of root rot/damping-off. There were no visible emergence problems in the field, but some of the plants developed symptoms post-emergence including loss of root development and lesions on the stem near the soil line. It appeared the infection was caused by Rhizoctonia solani, one of the main root rot pathogens in bean. The crop diagnostic lab is confirming this result. Post-planting control options for damping-off are limited. Proper crop rotation and practices that promote root growth are methods of cultural control. Seed treatments are also available to help control the disease, but they are only effective during the seedling stage of the crop.

 

General Crop Scouting

Powdery Mildew: In many of the wetter areas powdery mildew infections have been spotted. Powdery mildew is an obligate parasite that infects cereals, grasses, legumes, fruits, vegetables, and trees. This disease flourishes in wet and humid conditions such as those present in many crop canopies across the province this year. Keep an eye out for the development of this disease especially if wet conditions persist.

 

Surveys and Forecasts

Diamondback moth monitoring: Diamondback moth (Plutella xylostella) was first reported this year in a trap near Carman during the week of May 2-8, then more generally during the week of May 9-15th. There has been an increase in the number of moths caught in some of the traps in the eastern region over the past few weeks. Higher catches of note are 108 from a trap near Beausejour for the week of June 6-12th, and 94 from a trap near Sewell from June 13-19th.

Table 2. Highest cumulative trap counts for diamondback moth in Manitoba as of June 20, 2011

Location

Cumulative
Trap count

Beausejour

175

Sewell

162

Thalberg

55

Carman

55

Plum Coulee

48

The full data set for adult counts of diamondback moth can be viewed at: https://www.gov.mb.ca/agriculture/crops/insects/db/pdf/2011diamondbackdata.pdf

Bertha armyworm monitoring: Data is just starting to come in from some of the bertha armyworm traps that have been set up. Emergence of adults is just starting and numbers so far are very low. Data from the bertha armyworm monitoring program and a chart on how to interpret the data are available at:
https://www.gov.mb.ca/agriculture/crops/insects/bertha/index.html