Photosynthetic Consequences of Late Leaf Spot Differ between Two Peanut Cultivars with Variable Levels of Resistance

Late leaf spot (LLS) caused by Cercosporidium personatum (Berk. & Curt.) Deighton reduces leaf CO2 assimilation rate (A(sat)) and accelerates leaf defoliation, which together lead to major reductions in peanut (Arachis hypogaea L.) yield worldwide. This study was conducted to determine whether differences in photosynthetic response to LLS severity exist among peanut cultivars of differing resistance. Field experiments were conducted in 2008 and 2009 to study the effects of LLS on A(sat) of tagged leaf cohorts, and photosynthetic response of similar age leaves to LLS in peanut cultivars with more (York) and less (Carver) quantitative resistance. A nonlinear model, y = (1 - x)(beta) was used to analyze A(sat) data, where y is relative A(sat), x is measured visual lesion area, and beta represents the relationship between virtual and visual lesion area. Progression of LLS severity on leaf cohorts was slower in York than in Carver. However, the reduction in A(sat) with leaf cohort age was similar across the cultivars. This paradox could be explained by a higher beta value in York (4.6) than in Carver (3.6), indicating a greater relative reduction in A(sat) beyond the necrotic lesion area in York. This greater reduction in A(sat) in York compared to Carver was most closely related to a reduction in maximum carboxylation velocity. Results indicated that future efforts to improve LLS resistance should include sustaining A(sat) in response to LLS infection along with slower disease progress.