Vorkommen und Standortbezüge von Phytophthora-Arten in geschädigten Eichenbeständen in Nordwestdeutschland (Niedersachsen, Nordrhein-Westfalen und Schleswig-Holstein)
In three Northwest-German states (Lower Saxony, Northrhine-Westfalia, Schleswig-Holstein) 47 oak stands showing symptoms of oak decline were sampled. The sampling sites were chosen to represent roughly the distribution of oak forests in NW-Germany with their typical soil conditions. Soil samples taken from 5-30 cm depth out of the upper rhizosphaere of 407 oaks were tested, using Jung's baiting method, for the occurrence of Phytophthora spp. In the same soil samples pH (CaCl2), Ca/Ake and clay content were determined. The presence of Phytophthora spp. near at least one tree per stand was found in 22 stands (47 %). From the total of 407 sampled trees 51 (13 %) yielded Phytophthora. Of the 22 Phytophthora-positive stands 10 showed isolation rates of 8,3-12,5 %. 10 more stands showed rates of 20-50 % and 2 stands yielded Phytophthora from 63 % and 75 % of sampled trees. The species isolated with their relative frequency in sampled trees were: P. quercina sp. nov. Jung et al. (55 %), P. cambivora (24 %), P. europeaea sp. nov. Jung et al. (22 %), P. syringae (6 %), P. citricola and P. psychrophila sp. nov. Jung et al. (each 2 %). The total frequency exceeds 100 % because several trees hosted more than one species. The isolation frequency of Phytophthora spp. on sampled oak trees in NW-Germany was less than half as high as in Bavaria found by Jung et al. (2000). Crown condition of sampled trees was not significantly related to the occurrence of Phytophthora spp. in their rhizosphaere. This might be due to interference of other agents involved in oak decline (insect defoliators; Agrilus spp.). Isolation rates of Phytophthora spp. correlated significantly to Ca/AKe, pH(CaCl2) and clay content of the upper rhizosphaere soil, determined for each tree. In most stands, yielding no Phytophthora or low isolation rates (8-13 %), the soil showed Ca/AKe values below 20 %, pH(CaCl2) around 3,5-3,8 and low clay contents of 10-15 %, rarely 20 %. High isolation rates of 50, 63 and 75 % respectively occurred only in 3 stands with Ca/AKe-values of 40 to nearly 80 %, pH(CaCl2) above 4 and high clay contents (approximately 40-50 %). In the upper rhizosphaere soil. It is concluded, that the occurrence of Phytophthora spp. in most oak stands, except on gley-sols, may be mainly controlled by the degree of calcium saturation in connection with the clay content of the upper rhizosphaere soil. Results of the investigation show, that in NW-Germany soils of oak stands rarely reach sufficiently high calcium saturation and clay content to allow optimal development of Phytophthora spp. It is therefore assumed, that root damage caused by these organisms may not be an important factor in oak decline in NW-Germany, except for relatively rare situations of oak forests on very rich sites. Even there the importance of P. quercina is reduced by the fact, that such sites are in general not used preferably for oak, but for mixed broadleaf stands of Fagus, Acer, Fraxinus, and Prunus, which are more resistant to Phytophthora quercina (Jung et al. 2000).
