Physiological Consequences of Late Leaf Spot on Peanut Cultivars of Differing 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 physiological response to LLS exist among peanut cultivars and how these differences would translate to yield variations. Field experiments were conducted over two years at Citra, FL evaluating two peanut cultivars with more (York) and less (Carver) quantitative resistance to LLS grown under fungicide sprayed and non-sprayed conditions. Disease severity based on canopy lesion area was reduced by 30% in York compared to Carver. No additive effects of combining the resistant cultivar with fungicide were seen, as fungicide use increased yield by 364 kg ha^-1 for both cultivars. Yield was more strongly related to disease severity based on canopy lesion area than to the Florida scale.  Yield improvement with York was not as closely related to disease severity with only 6% gain in pod yield in York over Carver. To explain this paradox, A_sat was studied at leaf cohort level. Progression of LLS severity on leaf cohorts was slower in York compared to Carver. However, the reduction in A_sat with leaf cohort age was similar across the cultivars. These similar reductions in A_sat could be explained by a greater relative reduction in photosynthetic capacity beyond the necrotic lesion area in York compared to Carver at a given disease severity. This greater reduction in A_sat in York was most closely related to a reduction in chlorophyll content and could help explain small yield benefits in York over Carver. Results from this study indicated that future efforts to improve LLS resistance should include sustaining A_sat in response to LLS infection along with slower disease progress.