Eur. J. For. Path. 26 (1996) 253-272 © 1996 Blackwell Wissenschafts-Verlag, Berlin ISSN 0300-1237 Isolation, identification and pathogenicity oi Phytophthora species from declining oak stands By T. JUNG, H. BLASCHKE and P. NEUMANN Addresses of authors: T. JUNG and H. BLASCHKE, Lehrstuhl fur Forstbotanik, Univ. Munchen (LMU), Hohenbachernstr. 22, D-85354 Freising, Germany; P. NEUMANN, Dept. de Sciences Biologiques, Univ. de Montreal C. P. 6128, Succursale A, Quebec H3G 3J7, Canada Summary Within the scope of a research project on the condition of roots of declining oaks {Quercus robur, Quercus petraea), samples of fine roots and surrounding soil, specimens of stripe cankers on the stem base, and samples of stream water were examined for the presence of Phytophthora species using both baiting methods and selective agar media. At 27 sites in Germany (Bavaria, Rheinland-Pfalz, Schleswig- Holstein), Switzerland, Hungary, Italy and Slovenia the following species were isolated (mainly from soil): Phytophthora citricola, Phytophthora cactorum, Phytophthora cambivora, Phytophthora gon- apodyides, Phytophthora undulata, a species with affinity to Phytophthora drechsleri, and two additional species with close affinity to the Phytophthora cactorum group. Moreover, Pythium group P, Pythium anandrum, Pythium chamaehyphon, and many other Pytnium species that have not yet been identified could be recovered. In a soil infestation test most isolates induced diebaek of long root tips and necrotic lesions in the root cortex and at the root collar of Quercus robur seedhngs. All Phytophthora species tested and Pythium group P caused cortical necrosis after stem inoculation of young Quercus robur trees. It could be shown in vitro that Phytophthora gonapodyides and Pythium group P were able to produce a wilting toxin. Nitrogen input and climatic changes are discussed as predisposing factors for root damage observed m the field. 1 Introduction Since the early 1980s stands of pedunculate oak {Quercus rohur L.) and sessile oak (Q. petraea (Matt.) Liebl.) in Central Europe have been attacked by a chronic progressive decline. Oak decline has also occurred in Eastern Europe since the 1960s and 1970s and in the Mediterranean region since the mid-1980s (LEONTOV^C and CAPEK 1987; MARCU 1987; OLEKSYN and PRZYBYL 1987; VARGA 1987; RAGAZZI et al. 1989, 1993; SIWECKI and LIESE 1991; Luisi et al. 1993). Above-ground symptoms include the formation of leaf clusters, an increased occurrence of twig abscission and epicormic shoots, diebaek of branches and parts of the crown, leading to a general deterioration of the branching system and a greater transparency of the crown. Furthermore reduced size, yellowing and wilting of leaves, necrotic patches and stripes in the bark and the phloem of the trunk, and stripe cankers at the root collar, often accompanied by dark exudations, formation of tyloses in the vessels, discolouration of sapwood and cracking and loosening of the bark can also be detected (SKADOW and TRAUE 1986; SCHUTT and FLEISCHER 1987; HAMMERLI and STADLER 1989; HEUKAMP 1989; GALOUX and DUTRECQ 1990; KOWALSKI 1991; SCHILL and FLEISGHER 1991; SiWEGKi and LIESE 1991; ANONYMOUS 1993; Luisi et al. 1993; BLASGHKE 1994a,b). Although visible crown symptoms indicate waterstress and malnutrition of diseased oaks, there have been only a few investigations of root status. NAVEKE and MEYER (1990) in Lower Received: 9.11.1995; accepted: 9.4.1996 U. S. Copyright Clearance Center Code Statement: 0300-1237/96/2605-0253 $11.50/0