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USE OF MULTILOCUS SELECTION MODELS IN BARLEY POWDERY MILDEW FOR EVALUATING STRATEGIES OF DISEASE MANAGEMENT


USE OF MULTILOCUS SELECTION MODELS IN BARLEY POWDERY MILDEW FOR EVALUATING STRATEGIES OF DISEASE MANAGEMENT

H OSTERGARD
Plant Biology and Biogeocheniistry Department, PBK-301, P.O. Box 49, DK-4000 Roskilde, Denmark

Background and objectives

Genetic host resistance based on simple inherited race-specific resistance genes is an important tool for management of many diseases. The use of specific resistance genes over large areas results in the increase in frequency of the corresponding virulence genes. In order to help breeders and farmers to deploy resistance genes most effectively, virulence surveys are used to monitor the pathogen populations. What additional information can be obtained from population biology and multilocus selection models of variation in virulence genes?

Methods

Changes over time in multilocus virulence genotype frequencies of an aerial spore pool in an isolated geographical area were modelled for an airborne pathogen [1]. The basic model assumes that cultivars with different resistance genes are sown at the same time, over specified areas, and that spores are randomly dispersed on the host genotypes according to area. Selection on the virulence genotypes takes place when a new crop is emerging, and on a specific host only spores with the corresponding virulence genes are able to grow. The pathogen reproduces asexually with no mutations. The frequency of multilocus virulence genotypes in the aerial spore pool at the end of a growing season can then be expressed as a ftmetion of the frequencies at the beginning of the growing season and the area of different host genotypes. The total amount of disease can be estimated from the number of spores in the initial aciial spore, pool and their average fitness on the crop. This basic model has been extended 'm different ways to include sexual reproduction [2], to consider cultivar mixtures, to include overlapping growing seasons (spring and autumn sown crops) and to consider quantitative host-pathogen interactions [1].

Results and conclusions <1b>

The modelling has given an overall understanding of the population dynanu'cs of pathogens for which selection due to host plant resistance genes is the most important evolutionary force. The model has predicted the generation of gametic disequilibria among virulence genes, positiveor negative, depending on the use of different resistance genes in different cultivars. This prediction was largely in accordance with observations in Danish populations of barley powdery mildew[l]. The generated gainetic disequilibria result in changes in frequencies of unnecessary virulence genes due to hitch hiking, in difference in virulence gene frequencies between spores in the aerial pool and on specific host plant genotypes, and they influence the interpretation of complexity of multilocus virulence genotypes. For virulence surveys, the modelling has stressed the importance of assaying multilocus virulence genotypes and of sampling at the right time in relation to crop emergence. Further, the model has enabled different resistance gene diversification schemes to be compared, e.g., different uses of resistance genes in autumn and spring sown cultivars.

References 1. Hovmoller MS, Ostergdrd H, Munk L, 1997. In: The Gene-for-Gene Relationship 'm PlantParasite Interactions (Eds Crute IR, Holub EB, Burdon JJ), CAB International, pp. 173-190.
2. Ostergdrd H, Hovmoller MS, 1991. Plant Pathology 40, 166-177.