In recent years in Australia, monogenic resistance has failed dramatically causing severe yield losses in canola cultivars reliant on this resistance source. In addition, polygenic resistance in some cultivars has been eroded. This loss of resistance is due to populations of the blackleg fungus changing in virulence under selection pressure by disease resistance genes. The literature and experience in other crops suggests that rotation in space and time of different blackleg resistance genes may reduce the frequency that resistance genes are overcome by changes in fungal populations.At two sites in south-eastern Australia, canola cultivars (each with a different blackleg resistance source) were grown in succession (2003 monogenic resistance from R rapa ssp. sylvestris; 2004 polygenic partial resistance; 2005 European winter resistance Rlml and Rlm3 genes). In 2004, 2005 and 2006 pots of canola plants each with different sources of blackleg resistance were placed onto the sites containing the three years of canola stubble. A very low level of infection was observed. Plants with polygenic, European (Rlm1 and Rlm3 genes) and Brassicajuncea all had less than 7％ mortality. Plants with monogenic resistance from B. rapa ssp. sylvestris initially had 70％ mortality; however by 2006 plant mortality had fallen to 25％. Thus rotation of canola cultivars reliant on different blackleg resistance genes may be a future method of increasing the longevity of canola cultivars and thus reducing yield loss caused by the blackleg fungus in Australia.
Marcroft Grains Pathology P/L, Grains Innovation Park, Horsham, Victoria 3400, Australia Department of Primary Industries, Horsham, Victoria 3400, Australia South Australian Research and Development Institute Naracoorte 5271 Australia Local farmer /trial coordinator School of Earth and Geographical Sciences, The University of Western Australia, Wa 6009, Australia Institute of Land and Food Resources, The University of Melbourne, Victoria 3010, Australia School of Botany, The University of Melbourne, Victoria 3010, Australia School of Plant Biology, The University of Western Australia, WA 6009, Australia