Reading Sclerotinia Risk Maps
The risk map will be available this year starting on June 18th. Maps showing the estimated risk of development of Sclerotinia stem rot on canola will be produced daily for the duration of the flowering period. When reading the maps growers should be aware that the estimated risk applies only to canola crops that are entering the flowering period or are already flowering. This risk is calculated considering the effect of environmental factors. A risk calculator that combines the effect of weather variables, cultural practices, and history of Sclerotinia is also available. While this information is intended to advise growers when conditions may be favorable for disease development, the final decision on whether to spray a fungicide or not should include economic considerations such as the potential yield of the field and the potential market price for the commodity. Growers are encouraged to look for apothecia in their fields before making spraying decisions, especially if their fields are located in areas with intermediate or high risk.
Early detection is critical to avoid losses due to blackleg
Blackleg infections that take place when canola plants have less than four leaves are more likely to result in yield losses than those happening when plants are older. Growers should scout their fields for symptoms of symptoms of early infection on cotyledon leaves starting two weeks after planting. These lesions may be irregular in shape and have a light brown color. Numerous pycnidia which look like tiny grain of black pepper may be produced in the dead tissues especially if humid conditions prevail for several days. The fungus will move from the cotyledon leaves into the stem but will do so without showing further symptoms of infection. In this way, infected plants that have already dropped their cotyledon leaves may look healthy.
As the plants continues developing, symptoms on true leaves may also appear. Usually these lesions are not produced by the same fungus that caused the cotyledon lesions but by spores that arrived at later time. Nevertheless, their presence in plants with less than four true leaves is an indication that blackleg may be a problem that requires fungicide applications. A list of fungicides registered for use in canola against blackleg can be found in NDSU extension publication 2016 Fungicide Guide (Canola-Rapeseed).
Managing SSR with fungicides
Considering fungicide applications to manage Sclerotinia stem rot?
If you decide to make a fungicide application, remember that the best time to spray your product is when the plants are between 30% and 50% bloom. In general, plants are at 20% bloom when they have approximately 15 open flowers on main stem. This stage is reached usually within six days from the time the first flower opens and is heavily influenced by temperature. Warmer temperatures usually speed up the flowering process. Plants at 30% bloom have 20 or more flowers open on main stem; at this stage, there is little to no petals dropped and no pods are forming. This stage could be reached within 2 or three days from 20% bloom. When plants reach 50% bloom, they usually have at least 20 flowers open on the main stem and lateral branches have started to flower already. At this stage the fields reached their maximum color intensity. At that stage pods are evident. Fungicide applications after plants have reached the 50% bloom stage usually do not provide an economic return.
For an illustrated guide of canola bloom stages please see NDSU extension publication