Most of the Prairies are under high risk for sclerotinia disease in canola, according to an analysis of weather data and soil moisture in the top 10 centimetres.
Early planted canola is now flowering in many areas. Farmers should be scouting for apothecia in fields that were planted with canola or another sclerotinia-susceptible crop last year.
The germination of the sclerotinia sclerotia requires about 10 days of moist soil above field capacity. When they germinate, sclerotia produce apothecia that release spores, which attack canola and cause stem rot.
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Spores are released by the apothecia at about the same time as canola comes into flower.
Sclerotinia apothecia have now been observed in many fields in central and southwestern Manitoba and are likely to occur in other locations where the soil surface has been moist for 10 days or more.
Continued wet weather will determine which areas are at risk from sclerotinia.
Fungicide must be applied before symptoms of stem rot are visible, at the 20-30 percent bloom stage of the crop.
The objective of the fungicide application is to cover as many petals as possible. The crop should be sprayed at its yellowest, the time when the maximum number of flowers are open.
Infected petals
That causes the maximum number of fungicide-covered petals to fall into the canola canopy (lower leaf axils, leaves and shoots), taking the fungicide along. Infection of the canola plant will only take place from sclerotinia-infected petals.
To check bloom stage, sample several plants over the field and assess the number of open flowers.
Find the main stem, pull off the secondary branches, and count only the open flowers on the main stem.
At 30 percent bloom, an Argentine canola crop will have about 20 flowers on the main stem.
A polish variety will have 14 to 16 flowers.
It usually takes a crop two to four days to go from first flower to 10 percent bloom. At 30 percent bloom, a field is said to be fully flowered, with the maximum number of flowers open at one time.
Do not apply fungicide if the stem rot infection is already evident, since it is too late to have any effect.
Petal test kits and in-field diagnostic kits are commercially available. These enable a grower to determine the cost effectiveness of sclerotinia control.
A sclerotinia stem rot checklist can also assist in predicting stem rot disease levels. The list considers yield potential, previous disease presence and current weather conditions. Ask a provincial agricultural representative for a copy of the checklist.
Co-operation and funding for the sclerotinia program is supported by Agrometeorological Centre of Excellence, Environment Canada, DuPont Canada and Manitoba Agriculture.
New maps are available Mondays and Thursdays at www.gov.mb.ca/agriculture/
news/ace/sclerotinia/sclerotinia.html.
