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Summary• Mycosphaerella leaf disease (MLD) is a major cause of foliage damage in Eucalyptus globulus plantations. Our study is the first to describe the physiological effects of MLD on E. globulus leaves. It involved measurements on both field and potted plants.• Changes in photosynthetic parameters in response to MLD were quantified in a study using gas exchange techniques.• There was a negative linear relationship between light-saturated photosynthesis ( A max ) and leaf-level damage from MLD. Reductions in A max were proportionally greater than might be expected from the reduction in green leaf area as a result of the disease, indicating that asymptomatic tissue also was affected by MLD. The reductions in A max were not related to increases in stomatal resistance, but were a result of reduced activity of ribulose bisphosphate carboxylase (Rubisco) and changes in the capacity for ribulose bisphosphate (RuBP) regeneration. Changes in mesophyll resistance to CO 2 were also implicated.• The effect of MLD was similar at different sites and irrespective of tree-level infection, suggesting a general leaf-level response of E. globulus to MLD. Key words:Eucalyptus globulus , light-saturated photosynthesis ( A max ), maximum rate of Rubisco activity ( V cmax ), Mycosphaerella leaf disease (MLD), potential electron transport rate ( J max ), ribulose bisphosphate carboxylase (Rubisco), ribulose bisphosphate (RuBP), stomatal conductance.