By Top Crop Manager
by Anastasia Kubinec, MAFRD Oilseed Crop Specialist
July 31, 2015 - Sclerotinia in sunflowers can be frustrating and devastating, especially in the form of headrot. Headrot is very weather related. It needs rainfall to wet soils stimulate sclerotia to produce apothecia mushrooms and ascopores. Ascospores once on disk petals, need prolonged wetness to allow the infection and growth of the fungus on the petals and spread into the sunflower head tissue.
It takes approximately 14 days after a "ground soaking" rainfall for the mushrooms to appear and produce ascospores. In the past week (July 20th-26th, 2015), many areas in Manitoba received significant rainfall events which could be the start of the lifecycle to cause sclerotinia head rot in sunflowers.
Weather data can be used in combination with a risk calculator to determine if a fungicide is needed. You can find the MAFRD calculator here http://www.gov.mb.ca/agriculture/business-and-economics/financial-management/pubs/calculator_sclerotiniadecisiontool_sunflowers.xls
It has only been in the past couple of years that sunflower growers have had fungicide registered to control headrot. With limited use of use, control has been not always been what was expected. The fungicides available are protectant and work to protect plant when disease infection potential present. If there was infection prior to application, or, if pressure remains high after application, control may be less than expected.
National Sunflower Association of Canada has put out a bulletin talking about sclerotinia headrot control in sunflowers for 2015.