Phytophthora and Oak Decline - Impact on Seedlings and Mature Trees in Forest Soils

Sammanfattning: This thesis investigated the occurrence of soilborne species of the plant pathogenic genus Phytophthora in southern Swedish oak forests and their possible involvement in southern Swedish oak decline. The emphasis was on the impact of P. quercina on Quercus robur in acid forest soils. Several different methods, including screening for Phytophthora in oak forest soils, greenhouse experiments with seedlings grown in soils inoculated with Phytophthora, and field studies of mature oaks growing in Phytophthora-infested stands, were used. The results showed that Phytophthora species occur in southern Swedish oak forests. The most frequently recovered species was P. quercina, which was found in 10 of 32 investigated oak stands. In addition, P. cactorum and P. cambivora (mating type A2) were recovered from one stand each. The soil conditions at the sites from which Phytophthoras were recovered ranged from mesic sediments to moraines, with clayey to silty textures and with pH(BaCl2) in the rhizosphere soil between 3.5 and 5.0. In greenhouse studies, P. quercina and P. cactorum were found to infect and cause substantial damage to roots of Q. robur seedlings grown in acid forest soils under a mesic water regime. The presence of the natural soil microflora did not hinder the pathogens from infecting and damaging the roots. In addition, significant differences in live fine-root length were found between healthy mature oaks and declining mature oaks growing in stands infested with P. quercina. No such difference was found between trees growing in non-infested stands. The impact of the pathogen on mature oaks seemed to depend on tree vitality, site and climatic conditions. Despite the significant reductions in live fine-root length of both seedlings and mature oaks, few differences in above-ground growth (measured only in seedlings) and leaf nutrient concentrations were detected. However, it seems likely that continuous root infections, with subsequent replacement of roots, will deplete the carbohydrate stores in the plant and cause reductions in the production of new root and shoot tissue as well as in the production of secondary metabolites. This may result in an increased susceptibility of the tree to further pathogen infections as well as to other stress factors. A weak association was found between the occurrence of P. quercina and the vitality of oak stands (determined from estimates of crown defoliation). The impact of soil type on carbon allocation patterns in plants and its consequences for the extent of root damage caused by P. quercina is discussed, as well as the influence of abiotic and biotic factors on the aggressiveness of Phytophthora, susceptibility of oak and subsequent disease development.