The population genetics of phytophthora.

There are more than 60 species in the genus Phytophthora (94), and most are destructive plant pathogens. Extensive efforts are directed at the control of Phytophthora diseases each year, yet they still cause serious crop losses. For example, more than $200 million in lost production annually is attributed to Phytophthora diseases in Australia alone (47). Late blight, caused by P. infestans, probably cost United States potato and tomato growers more than $200 million during 1994: $100 million in lost crop production and $100 million in additional control measures (30). Although usually remembered for the historical role played by P. infestans during the Irish potato famine of the 1840s, Phytophthora species still pose an immediate and real threat to world agriculture. Despite the huge economic costs of Phytophthora diseases, relatively little is known about the population genetics of the causal organisms. Genetic analyses in this genus were hindered until recently by the inability to perform genetic crosses, due to homothallism, poor germination of oospores, lack of suitable markers, or, simply, the limited availability of both mating types (91). Biochemical markers were first applied to analyze populations of P. cinnamomi (77) and P. infestans (99) in the mid-1980s. Progress accelerated during the 1990s with the addition of DNA-based markers and the inclusion of more species. Although much remains to be learned, some general patterns are beginning to emerge. The purpose of this paper is to review the literature on the population genetics of Phytophthora species and to formulate testable hypotheses that can explain many of the observed phenomena. Five main areas will be considered: sources of variation, migration, genetic drift, selection, and mating systems. Reviews of the genetics, systematics, and evolution of Phytophthora species have been published recently (9,10,48,50,91) and will not be duplicated here, except as needed to clarify particular points.