Photosynthetic consequences of Marssonina leaf spot differ between two poplar hybrids

•  In foliage of two hybrid poplars, clone DN-34 (Populus deltoides × P. nigra) and clone NM-6 (P. nigra × P. maximowiczii), we examined relationships between photosynthesis and severity of leaf spot induced by Marssonina brunnea f. sp. brunnea, a common disease of many tree species in the Populus genus with the potential to affect growth. •  Gas exchange was measured on asymptomatic and diseased foliage in monoculture stands of each clone. The equation Y = (1 - x)^β was used to characterize the relationship between relative photosynthesis (Y) and percent leaf spot (x), where β represents the ratio between functional impairment and measured lesion area. •  Leaf photosynthesis was strongly and negatively correlated with leaf spot severity in both hybrids, and β-values indicated that photosynthetic impairment extended beyond visibly damaged leaf tissue. However, large differences in β between hybrids indicated differential photosynthetic consequences for a given leaf spot severity. For each hybrid, values of β were positively related to photosynthetic photon flux density incident upon the leaf during gas exchange measurement. •  Declines in leaf photosynthesis appeared to result from a disruption of the photosynthetic apparatus by the invading pathogen. However, specific causes for the differential photosynthetic responses of the two hybrids to disease remained elusive.