Foliar pathogens reduce thenetphotosynthetic ratein infected leaves by reducing green leaf area, but also through an effect on photosynthesis of the remaininggreen leaf tissue. Phomopsis stem canker (Phomopsis/Diaporthe helianthiMunt.-Cvet) produces toxin metabolites,which cause necrotic lesions on sunflower leaves, withthefirst signs of infection being visible at the chloroplast level. The leaf senescenceand plant wilting that this fungus also induces has been documented.However,itsimpacton photosynthesis of asymptomatic leaf area has not been studied on sunflower.The aims of the present study were (i) to determine whether the net photosynthetic rate(NPR) of the green leaf area (green LA)was reduced by Phomopsis, and (ii) to characterize the relationship between disease severity and net photosynthesisin sunflower leaves. Two contrasting genotypeswere investigated: a cultivarresistant(R)to phomopsis (cv.Bollil) and a susceptible(S)one (cv.Alhaja).A greenhouse experiment was conducted at INRA, Auzeville (France). One leaf per plant was inoculated with a monopycniospore culture of Phomopsison 7 plants for each cultivarat star bud stage. Corresponding plants without leaf innoculation were used for control samples. Net CO2assimilation ratewas measuredon three areas of the leaf with a LI-COR 6400twice a week fromthestar bud stage to the end of flowering. Leaf lesion sizesweremeasuredregularlyin order to calculate the rate of necrosis progression from the infection point (tip of the leaf).Lesion spread wasmonitoredon the3 chosenleafareas: close to the infection point, atthe middle of the leaf, and opposite tothe infection point.The proportion of necrotic tissue and the surrounding chlorotic areawas measured. On control leaves, NPRwasnot significantly different betweenthe 3 leaf areas investigated, cv.Alhaja having a higher net photosynthetic ratethan cv.Bollil. Asleaves became older(30 daysfrom fully leaf expansion), a 50% decrease in NPRwas observed without any visible sign of senescence. The impact of Phomopsison leaf photosynthesis occurredearlier and was stronger incv.Alhaja than incv.Bollil. The rate of necrosis progression significantly differed betweensunflower cultivars:11.5mm.day-1for cv.Alhaja(susceptible), and6.9mm.day-1for cv.Bollil(resistant). At the scale of the measured areas, the decrease in NPRwas greater than would have been expected from the fraction of necrotic LA,and appeared genotypic dependent. Incv.Alhaja, a fraction of necrotic LAof 0.1 reduced the relativeNPRto0.12, while fractions between 0.25 and 1 completely halted photosynthesis.Incv.Bollil, a fraction of necrotic LAof 0.1 reduced the relative NPRto 0.67, with a complete photosynthesis inhibitionwhen the necrotic area fraction rangedbetween 0.78 and 1.Using the concept of virtual lesion, amathematicalmodel was builtto represent therelationship betweenaffected tissue areas and necrosis areasat the whole leaf scale.The distance between the visual necrosis front and the virtual necrosis front was found to be 15.7mm for Alhajaand 10.2mm for Bollil. This study quantified the relationships between phomopsis leaf injuryand photosynthesis reduction. The reduction of NPR in asymptomatic leaf area may be result from stomatal and mesophyll limitations of C02assimilation and/or an increase of respiration. The results will be integrated in an epidemiologicalsub-modelin the SUNFLO model(Casadebaig et al., 2011). Until now, yield losses were calculated using the proportion of stems with a girdling spot but leaf attacks wereonlyroughly characterized. This study improved the understanding and prediction of yield loss caused by phomopsis stem canker.