Judelson, H. S. 2007. Sexual reproduction in plant pathogenic oomycetes: biology and impact on disease. In: Sex in fungi: molecular determination and evolutionary implications (J. Heitman, J. Kronstad, J. Taylor, L. Casselton, eds.). ASM Press, Washington DC.

The sexual cycle is a significant component of the life histories of oomycetes, a group of fungus-like eukaryotes that include many devastating plant pathogens. Sexual reproduction is important not only as a source of genetic variation, but also because it creates thick-walled sexual spores called oospores. These can be important for surviving harsh environments such as freezing or dry conditions, and for resisting microbial attack. Oospores are the most durable of all oomycete propagules, and are particularly valuable to the pathogenic species which generally persist poorly as saprophytes in soil or plant debris. The relative importance of the sexual cycle in each pathosystem can vary depending on factors that include climate, host, cropping practices, patterns of pathogen migration, and the inherent biology of each species. This chapter describes the biology of sexual reproduction in phytopathogenic oomycetes and its role in disease. Examples will be provided across a spectrum of pathosystems in which the sexual cycle appears to play essential, minor, or intermediate roles. How sexuality confers long-term evolutionary advantages to oomycetes will also be discussed.