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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect and Disease Update

August 16, 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

• A few sunflower fields are showing damage by downy mildew.
• Recent rainfalls have increased the risk of sclerotinia in dry beans and sunflowers.
• Some high aphid populations were detected on wheat south of Riding Mountain National Park.
• Soybean aphid populations have been increasing in some areas of the southern Interlake and eastern Manitoba. Lots of natural enemies are present in some of these fields as well; the level of increase in soybean aphid populations should be monitored.

Recent Insect and Plant Pathogen Activity

Canola

Canola disease survey: A disease survey in canola is continuing. Some observations include:
a. Some fields with R or MR rated varieties for blackleg are showing 50% blackleg incidence – an indication of changing pathogen and helped by root maggot damage.
b. Sclerotinia appears to have been controlled well by most growers. There were a few fields with patches of 15% or higher disease.
c. Alternaria pod spot is only trace in most fields.

Sunflowers

Downy mildew: A few sunflower fields are showing bad damage due to downy mildew. Downy mildew is seed borne and also spread by wind during wet weather. In figure 1, the short plants have been infected with downy mildew. The plant with the brown stem that has broken has sclerotinia stalk rot.

 

Downy mildew and sclerotinia in sunflowers

Sclerotinia in sunflowers may increase due to the recent rainfalls.

Pulse Crops

Pathogens in dry beans: More Sclerotinia is showing up in beans. The recent 2-3 days of heavy rains have increased the risk of more Sclertotinia.
Some low lying areas have had Rhizoctonia infections. The browning on the root in figure 2 is Rhizoctonia.

Rhizoctonia in beans

Soybean aphids: Soybean aphid populations generally remain low and well below the economic threshold. However, populations have been increasing in some fields in the Beausejour area and southern Interlake. The economic threshold for soybean aphids is when there are on average at least 250 aphids per plant and the population is increasing, and the plants are in the R1 (beginning bloom) to R5 (beginning seed) growth stages. The following link shows a soybean aphid scouting card, which has information on scouting and thresholds for soybean aphids, as well as common natural enemies of soybean aphids:
http://www.soybean.on.ca/aphidinfo/uploads/soybean_aphid_scouting_card__english.pdf


Cereals

Aphids: A field of wheat in the Onanole area had economical levels of aphids, resulting in the field being treated with insecticide. The primary species appeared to be English grain aphids. As well as the green form, English grain aphids can also have purple and red forms, as was the case in this field.

General

Cabbage butterflies: There are a lot of white butterflies around currently, the most common of which is the cabbage butterfly, Pieris rapae. The females have two black spots on the wings and the males only one spot. Although the butterflies are quite common around many types of plants and crops, the larvae, known as imported cabbageworms, only feed on plants in the cabbage family (Brassicaceae). The larvae are primarily a concern on vegetables such as cabbage, broccoli, cauliflower and other cruciferous vegetables. Although imported cabbageworms will feed on canola, we have not seen them do significant damage to canola in Manitoba.

Cabbage Butterfly

Imported Cabbage Worm on Cabbage

 

Surveys and Forecasts

Grasshopper Survey: A reminder that counts for the grasshopper survey are done during August, when the majority of grasshoppers are in the adult stage. Agronomists and farmers who would be interested in estimating grasshopper numbers in the fields they are in and have this information included in the survey are encouraged to see the survey protocol for more details of the survey and where to send data. Estimates of grasshopper levels can be collected during regular farm visits. The grasshopper survey protocol is located at: https://www.gov.mb.ca/agriculture/crops/insects/fad95s00.html

Insect Identification Quiz

Question: What is the insect on the corn cob in this picture?

Answer: This is a species of sap beetle. The scientific name of this species is Glischrochilus quadrisignatus. In total there are 45 known species of sap beetles in Manitoba. The species in the photo is quite common in Manitoba this year and the adult beetles feed on ripe, damaged, and decomposing plant materials. They will bore into many types of ripe or overripe fruit and vegetables. They can also be found on kernels of corn initially injured by other insects or birds, although when populations are high they can be found under the husks of otherwise healthy ears of corn. Attraction to undamaged corn may be provided by the moist, fermenting pollen on the corn silk. They may prevent the natural healing of minor wounds or cracks in fruits and vegetables. Insecticides will not give effective control of sap beetles.