| Summary: | The effects of drought stress, <i>Phytophthora cinnamomi</i> infection and their interaction on water relations and growth were examined for 28 days on two year-old potted trees of <i>Eucalyptus obliqua</i> (L’Hér.). There were significant effects of drought stress on plant photosynthesis, stomatal conductance, biomass accumulation, plant water potential at turgor loss point and the bulk modulus of elasticity. <i>E. obliqua</i> was successfully infected but the trees showed only mild symptoms. Infection with <i>P. cinnamomi</i> led to a significant reduction in the root biomass and root-to-shoot ratio in well-watered and droughted plants but did not impact water relations. There was no observable cumulative effect of drought and <i>P. cinnamomi</i> infection. There are multiple potential reasons why <i>P. cinnamomi</i> infection did not lead to drought-like symptoms in <i>E. obliqua</i>, including short experimental duration, delayed infection symptoms, potential resistance of <i>E. obliqua</i> and a possible lower aggressiveness of the <i>P. cinnamomi</i> strain. Hence, our results indicate that <i>P. cinnamomi</i> infection will not always lead to immediate short-term symptoms, and that plants that are mildly symptomatic respond very similar to drought stress compared to non-infected trees.
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