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Field experiments were carried out in 1982—83 and 1983—84 in Northern and Central Italy (3 locations) in order to investigate the effect of different levels of seed infection of Pyrenophora graminea on disease incidence and yield losses in 'Perga' winter barley. Six levels of natural seed infection, assessed by the deep-freezing blotter method have been compared in 8 row-plots, 7.5 m long, arranged in randomized blocks with 4 replications. The percentage of infected plants and tillers has been recorded in all locations and the yield in two of them. A highly significant correlation was found between seed infection, plant infection, tiller infection and yield reduction. Major ratios found were: infected seeds/infected seedlings 1:0.4, infected tillers/yield loss 1:0.9, infected seeds/yield loss 1:0.3. The threshold of seed infection at which production was not significantly different from the healthy control was 14%. Therefore seed treatment is advisable, under the conditions of Northern and Central Italy, when the percentage of seed infection in commercial seed lots is above this level. A tolerance near zero is recommended in prebasic and basic seeds. [ABSTRACT FROM AUTHOR]