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We performed a 3‐year field survey in France to characterize the dynamics of sexual reproduction in Mycosphaerella linicola, the causal agent of pasmo, during the interepidemic period. Cohorts of fruiting bodies were sampled from linseed straw during the autumn and winter and carefully observed, focusing on pseudothecia, asci, and ascospores. A sequence of experimental steps corresponding to Koch's postulates confirmed in July 2014, for the first time in France and continental Europe, the widespread presence of the sexual stage of M. linicola in plant host tissues. The developmental dynamics of pseudothecia on straw, expressed as the change over time in the percentage of mature pseudothecia, was similar in all three years. Pseudothecia appeared in late summer, with peak maturity reached in October. A temporal shift, thought to be due to early autumn rainfall, was highlighted in one of the three years. These observations suggest that sexual reproduction plays a significant role in the epidemiology of pasmo in France. A resurgence of M. linicola infections in spring flax is thought to have occurred in recent years, due to the increase in the area under flax. The presence of the sexual stage of this pathogen probably increased the quantitative impact of residues of winter linseed (used for oil) and flax straw (left on the soil for retting and used for fibres) as an interepidemic "brown bridge". This case study highlights how certain parts of a disease cycle, in this case the sexual phase, can become crucial due to changes in production conditions. [ABSTRACT FROM AUTHOR]