Crop isolation, not extended rotation length, reduces blackleg (Leptosphaeria maculans) severity of canola (Brassica napus) in south-eastern Australia

Due to the large increase of canola production in Australia, current blackleg cultural control recommendations ( extended rotation length and isolation distance from canola stubble) are not adhered to by farmers in many canola-producing regions. Canola crops are increasingly being sown in short rotation and, in many instances, adjacent to paddocks containing canola stubble. In this study, the level of disease in commercial canola crops was determined for different rotations and distances from canola stubble. There was a strong relationship between the presence of canola stubble from the previous year (6-month-old stubble) and distance to current canola crops, but no relationship between the presence of older (18-42 month old) stubble and distance to current canola crops. Blackleg severity was highest where canola crops had been sown adjacent to 6-month-old canola stubble, with the level of blackleg severity decreasing markedly in the first 100 m. Disease severity then generally declined up to 500 m. Plants 500-1000 m from 6-month-old stubble had similar levels of blackleg infection. Blackleg severity was similar between canola crops sown into 18-month-old canola stubble ( short rotation) and crops sown into paddocks that had no history of canola for at least the previous 3 years ( long rotation). Based on these findings, we recommend that canola crops should be sown at distances greater than 100 m and preferably 500 m from last season's canola stubble, rather than extending rotation length between crops.