AbstractPopulations of Aberystwyth S23 and S24 ryegrass tillers were individually labelled and harvested at intervals to follow their life histories and productivity in response to the effect of infection with crown rust at two levels of nitrogen fertilization.The number of tillers in a population was reduced as a result of infection with crown rust. Increased rates of tiller death due to the pathogen occurred throughout the population age‐range and were most pronounced in the youngest tillers. Population changes were evident following infection and continued to be apparent in the following spring. The disease caused alterations in sward age structure, leading both to a higher proportion of older tillers in the more rust‐damaged treatments and to a decrease in the weight of the herbage produced. It is concluded that rust infection of a sward may lead to changes in population structure with long‐term yield effects, in addition to the immediate direct yield reductions commonly reported. The significance of these results to the ecological and agronomic balance of grassland is disc