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Yield losses caused by late blight (Phytophthora infestans (Mont.) de Bary) in potato crops in IrelandField experiments, using foliage blight susceptible cultivars, were conducted at Oak Park, Carlow from 1983 to 2007 to determine the loss in potato production caused by crop infection with Phytophthora infestans. In each of the 25 years an untreated control was compared with protectant and with systemic fungicide programmes to determine the effect of late blight on the defoliation percentage at the end of the season, the area under the disease progress curve, marketable tuber yield, total tuber yield and yield of blighted tubers. The earliest date of first recorded late blight was 22 June and the latest was 15 September, but in 15 of the 25 years, blight was first recorded between 17 July and 13 August. Disease reached epidemic proportions in all but 4 of the years. Yields varied considerably among years. The mean loss in total yield from not using a fungicide was 10.1 t/ha. Differences in yield were significant across the 25 seasons. No overall increase in aggressiveness of the pathogen could be detected over the 25-year period.