We don’t know all the reasons that sunflowers seem to do so poorly after canola, but the most likely culprit is sclerotinia. Sclerotinia kills or greatly reduces sunflower yields in three ways, throughout the season – basal rot (early), mid-stalk (mid-summer) and head rot (late summer). Canola is also a host for the disease and sclerots from a previous year infection would be present and germinating in very close proximity to the sunflower plants. This would provide prime opportunities to cause multiple plants to be infected at different times in a growing season. Yields reported to MASC (MB Agricultural Services Corp.) in the Harvest Acreage Reports from 1998 to 2007, show sunflowers after canola yielding 87% as compared to the overall average sunflowers yield.
Other issues contributing to the yield reduction may also include common insects between crop species grown in succession, herbicide carryover, soil moisture availability and nutrient availability.
For more information on growing sunflowers see: http://www.gov.mb.ca/agriculture/crops/oilseeds/bgd01s01.html
Specifically on Sclerotinia in sunflowers: http://www.gov.mb.ca/agriculture/crops/diseases/fac31s00.html
Information on Re-cropping Restrictions for Residual Herbicides see the Guide to Field Crop Protection at: http://www.gov.mb.ca/agriculture/crops/cropproduction/pdf/gcp2012/herbicide.pdf