I. Occurrence and evidence for the use of nitrogen as a control measure

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Abstract
The occurrence of blind seed disease of ryegrass in New Zealand has declined over the last 20 years. During 1948–1960, more than 70% of ryegrass seed samples tested were infected by the pathogen, and mean infection levels per seed line, although fluctuating according to the season, averaged 12%. Levels of over 20% were recorded in 1950, 1952, and 1958. Since then, both the percentage of infected samples and the mean infection level per sample have declined, and during 1976–1978,27% of seed samples tested carried the pathogen, with a mean infection level per seed line of 4.0%. During this time, the only major change in management technique for ryegrass seed production has been the use of nitrogenous fertilisers to boost yields. Unpublished New Zealand work and the literature on blind seed disease and nitrogen is reviewed, and a link between the increasing use of nitrogenous fertilisers by New Zealand seed producers and the decrease in the occurrence of blind seed disease is proposed.