Determining frequencies of avirulent alleles in airborne inoculum of Leptosphaeria maculans using molecular assays

Control of blackleg (phoma stem canker) disease relies on crop management, fungicides and breeding for disease resistance. Proteins encoded by these resistance genes recognize avirulence (Avr) gene products in the fungus during invasion of the plant. However, these Avr genes can mutate resulting in fungal isolates becoming virulent, thus avoiding recognition by the plant. Widespread use of cultivars containing resistance genes provides strong selection for isolates that have mutated forms of Avr genes. This selection results in increased frequency of virulent isolates and accordingly, resistance genes become less effective. Molecular tools are now available to assess changes in frequencies of virulent isolates in populations of L. maculans. This paper describes the development of such tools for assessing changes in frequencies of virulent isolates within airborne (ascospore) pathogen inoculum.