Abstract
The acclimation in relationship with virulence cost was analysed for seven Plasmopara halstedii (sunflower downy mildew) isolates including five progeny isolates of several races descending from two parental isolates of races 100 and 710. Aggressiveness criteria were analysed in one sunflower inbred line showing a high level of quantitative resistance. Isolates of races 100 and 3xx were characterised with shorter latent period and higher sporulation density than isolates of races 7xx. All isolates showed high percentage infection values and caused a large reduction in seedling size except for one isolate involved in dwarfing. The seven isolates were divided, according to their virulence and aggressiveness, into two main groups as more aggressive isolates of the 100 and 3xx races which do not overcome the sunflower differential host D3, and less aggressive isolates of 7xx races which can overcome D3. Consequently, the 100 and 3xx avirulent races had a virulence cost measured by differences in aggressiveness (from 45.5 to 76.3%) compared to 7xx virulent races carrying unnecessary virulence gene.
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