infestans showed that heterothallic Phytophthora species are capable of producing antheridia and oogonia but are self-incompatible (Galindo and Gallegly 1960). Until recently, P. ramorum was known

to exist as three clonal lineages named NA1 and NA2 (from North America) and EU1 (from Europe) (Grünwald et al. 2009). In 2012, a fourth lineage (EU2) was reported in Europe (Van Poucke et al. 2013). Originally, P. ramorum mating type A2 was only present ABC294640 concentration in the US and mating type A1 was only present in Europe. In the US, this changed when EU1-A1 was first found in 2006 in a Californian nursery (Grünwald et al. 2008). In Europe, this also changed with the report of three A2 isolates (from 2002 to 2003) in Belgium (Werres and De Merlier 2003; Vercauteren et al. 2011). Molecular studies revealed that these ‘Belgian’ A2 isolates belong to the EU1 lineage and resulted probably from mutation or mitotic gene conversion (Vercauteren et al. 2011). Oospore production in pure culture of several heterothallic Phytophthora has been reported in response to fungicides (Groves and Ristaino 2000), long-term culture (Brasier 1972; Ko 1981), stimulation by compounds produced by root exudates (Jayasekera et al. 2007), bacteria (Mukerjee and Roy 1962), antagonistic fungi (Brasier 1975) Wnt inhibitor or compounds

used in growth media (Smart et al. 2000). Self-fertility

phenomena have also been reported in oospore progenies of P. ramorum (Boutet et al. 2010). Reversible mating type conversion has been described in Phytophthora parasitica (Ko 1981) and P. cinnamomi (Ann and Ko 1989). These changes, if they occur in nature, might increase the level of recombination within the population of the pathogen. This is especially the case for P. ramorum for which only a single compatibility Astemizole type could be introduced into a specific geographical area. The purpose of this study was to evaluate the mating type stability of the three Belgian EU1 A2 isolates maintained under different storage conditions for several years. The Phytophthora ramorum isolates used in this study are listed in Table 1. Four isolates (2531, 2533, 2545 and 2546) were isolated from saplings of Quercus robur, Q. petraea, Alnus glutinosa and Acer pseudoplatanus inoculated with isolate 2338 (under bark inoculation with a mycelium plug) in 2003. Routine cultures were carried out on V8 at 20–22°C in the dark. A subculture of isolate 2338 was transferred to JKI in 2003 and maintained in the JKI collection as a hyphal tip culture. The isolate was given a new name, BBA26/02. Isolate 3237 is derived from a subculture of isolate BBA26/02. It was sent to the CRAW in 2005 after subculturing isolate BBA26/02 on carrot piece agar (CPA, Werres et al. 2001) at JKI.