Pathogenicity of Phytophthora isolates originating from several woody hosts in Bulgaria and Poland

Our aim was to examine the virulence of eight Phytophthora isolates belonging to three species (Phytophthora cryptogea, Phytophthora plurivora and Phytophthora quercina) obtained from diverse European ecosystems (in Bulgaria, Poland and Germany) towards three forest tree hosts – English oak (Quercus robur L.), Turkey oak (Quercus cerris L.) and European beech (Fagus sylvatica L.).

All plants grown from seeds in a greenhouse conditions were artificially inoculated under the stem bark with Phytophthora cultures. The tested isolates turned to be more aggressive to Turkey oaks than to English oak trees. In case of European beech, the isolates of P. cryptogea and P. plurivora exposed various virulence. The potential hazard of the introduced foreign isolates for the oak and beech forests in Poland and Bulgaria is discussed. Amongst the tested isolates, P. quercina P290 from German highly infected Bulgarian Turkey oaks; therefore, its negative potential impact on Bulgarian oak forests could be considered as high (if unintentionally introduced). Also, two Bulgarian isolates belonging to P. cryptogea and P. plurivora are risky for Polish beech forests, if exposed to the pathogen. The observed pathogenicity of the tested Phytophthora species proved their potential as important contributors to decline of valuable forest ecosystems dominated by oaks (Q. robur and Q. cerris) or beech (F. sylvatica), in both Poland and Bulgaria. We found that investigated Phytophthora pathogens could develop in the living plant stem tissues without causing any disease symptoms, which is another demonstration that phytosanitary control by simple observation of plant material is not effective.