Disease control strategies in barley, particularly against powdery mildews and rusts, are continually eroded in their effectiveness as the pathogen adapts to overcome them. This includes both application of crop protectants and 'ecological' approaches such as the use of cultivar mixtures, which select more complex powdery mildew races than the component monocultures. Knowledge of the mildew population structure and how it changes in response to control measures will help to devise strategies for minimising pathogen adaptation.
The success of mixtures of cultivars expressing major genes for resistance is dependent on a pathogen population where a substantial proportion does not express virulence matching all the host resistance genes. Even then, mixtures will tend to select for complex isolates which overcome all the resistance genes. If the population comprises few genotypes or only simple genotypes, then, provided these do not include virulence towards all the resistance components in the mixture, disease reduction is likely to be highly effective. However, if the population is highly heterogeneous and complex isolates occur, a little disease reduction may occur early in the epidemic but is likely to be eroded as complex pathotypes virulent on all components, are selected.
Disease control based on the use of resistance elicitors, which are being developed by several agrochemical companies and research groups including ourselves at SCRI may be less prone to erosion by pathogen population selection. This is because they act by triggering the plants defence mechanisms enabling them to express resistance better upon actual challenge by a pathogen. It is assumed that they act non-specifically against all pathotypes, but it is also known that they cause differential expression of resistance in different cultivars of barley.
We study pathogen populations to determine both the mechanisms of adaptation [2, 4, 5, 6, 8, 12, 14, ], and changes that occur in the field in practice [1, 3, 7, 9, 10, 11, 13, 15, 16].
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