Monitoring virulence of populations of the blackleg canvas

The blackleg fungus rapidly adapts to selection pressure since it outcrosses prolifically, producing large numbers of recombinant windborne ascospores as inoculum. When canola crops with major gene resistance are grown extensively, the frequency of virulent isolates increases rapidly, resulting in this resistance being overcome. This occurred with ‘sylvestris’-derived resistance, particularly on the Eyre Peninsula, SA in 2003.

Mechanisms used to overcome resistance can be understood by exploiting knowledge of fungal effector or avirulence (Avr) genes that are complementary to plant resistance genes (Rlm). Once avirulence genes have been identified, molecular markers can be used to determine whether an isolate is avirulent or virulent towards the corresponding resistance genes. We are  

• monitoring the blackleg population for increased virulence towards canola varieties containing the Rlm1 and Rlm6 resistance genes. This will provide information about which resistance genes will be more susceptible to disease due to changes in virulence of the blackleg population.
• mapping the fungal avirulence gene, AvrLmS, that when mutated to the virulence allele is responsible for breakdown of ‘sylvestris’-derived resistance.
• mapping a fungal gene that confers virulence to all B. juncea (Indian mustard) varieties tested. This latter fungal gene controls host species specificity rather than host variety specificity (like AvrLm1 & AvrLm6). Once these genes are identified, molecular markers will be developed to allow monitoring of the fungal populations.
• monitoring the incidence of Leptosphaeria biglobosa ‘canadensis’ on canola and mustard stubble in eastern Australia. Isolates of this fungus were recently found on B. juncea in northern New South Wales.

These projects are pursued in collaboration with Thierry Rouxel and Mylene Balesdent, INRA, Versailles, France who are characterising other avirulence (effector) genes of L. maculans.  

Selected Publications

Van de Wouw AP, Marcroft SJ, Barbetti MJ, Hua Li, Salisbury PA, Gout L, Rouxel T, Howlett BJ, Balesdent MH(2008)  Dual control of avirulence in Leptosphaeria maculans towards a Brassica napus cultivar with sylvestris-derived resistance suggests involvement of two resistance genes. Plant Pathology in press

Van de Wouw AP, Thomas VL, Cozijnsen AJ, Marcroft SJ, Salisbury PA, Howlett BJ (2008)  Identification of Leptosphaeria biglobosa ‘canadensis’ on Brassica juncea stubble from northern New South Wales. Australasian Plant Disease Notes 3: 124-8

Sprague SJ, Hayden HL, Brun H, Marcroft SJ, Pinochet X, Rouxel T, Balesdent MH, Howlett BJ (2006) Major gene resistance in Brassica napus (oilseed rape) is overcome by changes in virulence of populations of Leptosphaeria maculans in France and Australia.  European Journal of Plant Pathology 114: 33-40

Sprague SJ, Hayden HL, Marcroft SJ, Howlett BJ (2006) Breakdown of major gene resistance of Brassica napus to Leptosphaeria maculans in south eastern Australia.  Plant Disease 90: 190-8