Elevated soil nitrogen increases the severity of dieback due to Phytophthora cinnamomi

A survey of a dieback-affected urban bushland reserve on the foreshore of Sydney Harbour found that deaths of native trees and increased weed density were associated with high levels of nutrients, particularly nitrogen, and Phytophthora cinnamomi. To test the interaction between P. cinnamomi and elevated soil nitrogen, a glasshouse trial was established to determine the susceptibility of the most common native trees of the Sydney Harbour foreshore; Angophora costata, Corymbia gummifera, Eucalyptus botryoides and Eucalyptus piperita to P. cinnamomi. Under low nitrogen conditions, inoculation with P. cinnamomi reduced the health of A. costata and C. gummifera, and reduced the root biomass of E. botryoides. Angophora costata and C. gummifera displayed signs of phytotoxicity with elevated levels of inorganic nitrogen. The presence of the pathogen, in combination with elevated nitrogen reduced the health of all species, but the symptoms were more severe in A. costata and E. piperita. The results suggest that the effect of elevated nitrogen application and susceptibility to P. cinnamomi is species specific. Mitigating soil nutrient loading in urban bushland areas by redirecting stormwater flow and surface runoff away from vulnerable bushland may assist in the management of dieback caused by P. cinnamomi.