Impact of Preceding Crops On Incidence and Severity of Disease in Canola.

A long-term crop rotation study was initiated in 2000 to determine the impact of previous crops on blackleg and sclerotinia in canola. Six crop rotations involving canola, spring wheat, barley and flax were established at Minot, ND. Every crop in each rotation is included every year to help explain the effect of individual years. Canola is present every year, every other year, and every three or four years.  The study is being conducted using conventional tillage.  In general, sclerotinia testing from 2000 to 2009 indicated low to moderate disease risk.  In all years, sclerotinia incidence has been less than 25% (and usually less than 5%) with no significant correlation to rotation or fungicide treatment. Our observations indicate that sclerotinia disease risk is more likely dependent on environmental conditions than on rotation.  Blackleg incidence has been high in some years with a trend toward increasing blackleg over time, especially where canola is grown more frequently in the rotation.  However, blackleg severity has generally remained low.  There has been no obvious correlation between blackleg severity and crop rotation.  We believe blackleg severity may not have increased much over individual growing seasons for two reasons: (1) we plant varieties that are moderately-resistant or resistant to blackleg, and (2) the study is being conducted under a conventional tillage system, which inhibits disease survival.  At this point, there have been no canola yield differences between fungicide-treated and untreated plots, nor have there been yield differences between rotations.  This study will be continued through 2011 when each rotation will have completed at least three cycles.