Autumn sowing increases severity of pasmo (Mycosphaerella linicola) on linseed in the UK

Surveys and field experiments showed pasmo to be the most serious disease affecting UK winter linseed in the 1997-98, 1998-99 and 1999-2000 growing seasons. Survey data indicated that pasmo was widespread in England and Scotland, causing extensive loss of leaves and stem and capsule symptoms, on both winter and spring linseed crops. In winter linseed experiments at ADAS Boxworth and Rothamsted, when severe epidemics occurred (1997-98 and 1999-2000), control of pasmo with one or two MBC fungicide sprays increased yield. In experiments when severe pasmo epidemics did not occur (1998-99), fungicide applications did not increase yield. In all three growing seasons, large numbers of air-borne Mycosphaerella linicola ascospores were collected in the summer months. At the time when the winter linseed crop was emerging and becoming established in October/November, there were more air-borne M. linicola ascospores in 1999 than in 1998. April/May rainfall was much greater in 1998 (135 mm) and 2000 (223 mm), when severe pasmo epidemics developed by July, than in 1999 (68 mm) when disease severity in July was less. Regression analyses suggested that yield decreased as percentage area affected by pasmo on leaves or stems in July increased. The formulae relating yield loss to pasmo severity, derived from these experiments, were combined with disease survey data to estimate, retrospectively, the UK national losses from pasmo. Estimated national losses from pasmo on winter linseed, although > 50% of crops were sprayed with fungicide, were approximately 2.9M pound in 1998, 1.6M pound in 1999 and 0.37M pound in 2000 (when the area of winter linseed had decreased greatly). Estimated combined losses on winter and spring linseed were approximately 14.8M pound in 1998, 34.9M pound in 1999 and 11.0M pound in 2000.