Change of Mating Type in an EU 1 Lineage Isolate of P hytophthora ramorum

All Phytophthora ramorum EU 1 lineage isolates tested are of A 1 mating type, except for three rare isolates from 2002 to 2003 from Belgium, which were originally assigned the A 2 mating type. In one of these isolates (2338), a switch from A 2 to A 1 mating type was observed in 2006. This observation initiated a larger study in which all cultures and subcultures of the original three EU 1 A 2 isolates, maintained in three laboratories under different storage conditions, were checked for mating type change. The A 2 to A 1 mating type switch was observed in four of seven independently maintained isolates that were derived from isolate 2338 in two laboratories, using different transfer regimes and storage conditions. Following the mating type switch to A 1 in these four derived isolates, no reversion back to A 2 mating was observed, even after up to 5 years of additional isolate maintenance and several more subculturing events. The three other isolates that were derived from isolate 2338 as well as the other EU 1 A 2 isolates collected in 2002 and 2003 and stored in the same conditions did not display such mating type change. The potential causes of the mating type conversions as well as their epidemiological implications are discussed.