Sequential changes in leaf chlorophyll content and associated gas exchange parameters in isabgol (Plantago ovata) were recorded over a period of nine days starting from initial sporulation to severe chlorosis in a highly downy mildew susceptible cultivar (Niharika). The Downy mildew caused significant reduction in chlorophyll content. The loss of chlorophyll was more severe in case of chlorophyll ‘a’ compared to chlorophyll ‘b’. Net photosynthetic rate and stomatal conductance in the diseased leaves also reduced compared to healthy ones. However, internal CO2 concentration (Ci) increased in diseased leaves. Difference between healthy and infected leaves in terms of stomatal conductance and Ci became significant at ninth day of study. In the complex situation of variable disease severity and leaf physiological stage, we found net photosynthetic rate was highly correlated (r = ~0.8) with the disease severity.