Abstract:  Social systems play an important and increasing role in the creation of plant disease epidemics.  In an attempt to encourage the growth of a desired plant, humans transform its various characteristics and surrounding ecological conditions.  This have given rise to a periodic plant disease epidemics.  The paper uses the case of the blackleg caused by Leptosphaeria maculans epidemic in rapeseed in Canada to demonstrate that such epidemics may be seen as "normal accidents", i.e., the result of a particular set of social, natural, and technical relations.  Plant disease control practices are discussed.