Photosynthetic Consequences of Late Leaf Spot Differ In Peanut Cultivars of Differing Resistance.

Late leaf spot (LLS; caused by Cercosporidium personatum) induced decline in leaf photosynthesis (Pn) contributes immensely towards the reduction in peanut yield worldwide. This study was conducted to determine whether differences in photosynthetic response to disease intensity exist among peanut cultivars. Field experiments were conducted at Citra, FL in 2008 and 2009 to study the effects of LLS on disease progress and severity, leaf Pn, and pod yield in two cultivars with more (York) and less (Carver) quantitative resistance to LLS grown under fungicide-sprayed and non-sprayed conditions. A non-linear model, Y= (1-X)^β was used to analyze leaf Pn data, where Y is the relative Pn, X is the visual disease severity, and β represents the ratio between virtual and visual area. Disease severity based on individually tagged leaf cohorts demonstrated the slow progression of disease in York compared to Carver, consistent with their disease resistance ratings. However, reduction in leaf Pn was similar between the cultivars despite differences in disease severity. This could be explained by a higher β value in York (4.6) compared to Carver (3.5), indicating a higher reduction in leaf Pn in the remaining green leaf area in York. However, a poor model fit in York suggested a relatively greater reduction in Pn at low compared to high disease severity. Decline in quantum efficiency did not differ between cultivars. However, maximum carboxylation velocity was impacted more severely in York. The inability of York to sustain Pn explains its mere 6% yield improvement and a similar yield reduction under non-sprayed conditions over Carver. Results indicated that Pn in the resistant cultivar was severely impacted at low disease severity, so future efforts to improve LLS resistance should include sustaining leaf Pn along with slower disease progress.