Rapid evolution of introduced plant pathogens via interspecific hybridization

Plant disease epidemics resulting from introduction I of exotic fungal path ogens are a well-known phenomenon. Limited resistance in the host and exces sive aggressiveness in the pathogen (reflecting their lack of prior coevolu tion) can result in an explosive outbreak of disease. Introduction events a lso present a window of evolutionary opportunity for the pathogen. In its e ndemic location, a plant pathogen tends to be subject to routine selection constraints, favoring maintenance of a relatively stable, if fluctuating, p opulation structure over time. When introduced into a new environment, it w ill often be subject to novel or episodic selection, reflecting sudden expo sure to new biotic and abiotic influences, such as a new host population,:n ew vectors, new competitors, or a different climate. These influences provi de the potential for rapid evolution (Brasier 1995). Falling within the category of episodic selection is the sudden contact tha t can occur between closely related but previously geographically isolated pathogens as a result of introductions. Theoretically, this process present s an opportunity for rapid emergence of new or modified pathogens via inter specific gene flow (Brasier 1995). Until recently, this phenomenon has been little studied. Now Dutch elm disease, a major ecological accident of the 20th century (Heybroek 1993), is providing remarkable insights into rapid e volution of a plant pathogen outside its endemic environment. This article describes the migratory events and unusual genetic events that have occurre d in this fungus and other new examples of rapid pathogen evolution via int erspecific gene now, and discusses some of the wider environmental, evoluti onary, and quarantine implications.