Mating of Phytophthora ramorum: functionality and consequences

Phytophthora ramorum (Werres, De Cock, Man in’t Veld), which causes “sudden oak death” in the United States and dieback and leaf necrosis in ornamental plants (mainly Rhododendron and Viburnum) in Europe, is a heterothallic species with two mating types, A1 and A2 (Werres and others 2001, Rizzo and others 2002). Molecular studies on the population of P. ramorum isolates revealed a structure composed of three clonal lineages: one lineage from Europe (EU1) and two lineages from North America (NA1 and NA2) (Elliott and others 2009). Initial pairing studies revealed geographical separation of mating type isolates: all EU1 isolates were of A1 type whereas all NA1 and NA2 isolates were of A2 type (Werres and others 2001, Brasier 2003). However, since 2003 a few rare reports of A2 mating type isolates in Europe and reports of A1 mating type isolates in the United States (U.S.) have been made. In Europe, three EU1 A2 isolates were reported in Belgium (Werres and De Merlier 2003; Heungens, personal communication). In North America, some EU1 A1 isolates were reported in U.S. nurseries (Hansen and others 2003, Grunwald and others 2008). These findings suggest the potential for crossing between both mating types. However, attempts to produce oospores in vitro with classical methods were difficult compared to other heterothallic species (Werres and Zielke 2003, Brasier and Kirk 2004), therefore suggesting a weak functionality of the sexual system in P. ramorum. In a previous study, some critical parameters such as the gelling agent quality, the nutrient source, or the spatial arrangement of the two mating types in the Petri dish were optimized to produce a large amount of oospores in vitro. A particular EU1 A1 strain was found to be a better mating partner than other EU1 A1 strains when paired with some European (EU1) or American (NA1) A2 strains (Boutet and others 2009). The aims of this study were to investigate the functionality of P. ramorum sexual reproduction and to evaluate the characteristics of single-oospore isolates.