LEAFLET AND CANOPY PHOTOSYNTHESIS OF PEANUT AFFECTED BY LATE LEAF-SPOT

Late leaf spot, induced by Cercosporidium personatum (Berk. & Curt.) Deighton, is the most important foliar disease affecting peanut (Arachis hypogaea L.) in Florida and neighboring states. The disease first occurs as necrotic lesions on peanut leaflets and subsequently induces leaflet abscission. Field experiments were conducted to study the effects of late leaf spot on leaflet and canopy photosynthesis. Fully expanded leaflets at the tip of the main stem were tagged and leaflet photosynthesis was measured during the subsequent weeks with a portable photosynthesis system. Leaflet photosynthesis was reduced linearly with the increase in the percentage of necrotic leaf area. Reduction in light utilization due only to necrotic leaf area did not explain completely the reduction in leaflet photosynthesis. Regression analysis suggested that 15% necrotic leaf area contributed to a 65% reduction in the photosynthesis of infected leaflets. It appears that there was an effect on leaflet photosynthesis due to host cells invaded or affected by the pathogen. A chamber made of aluminum frame covered with mylar film was used to estimate canopy photosynthesis over a land area of 0.56 m2. At the canopy level, the effect of reduction in leaflet photosynthesis was negligible compared to the effect of disease-induced defoliation. The decrease in leaf area index was the major component involved in the reduction of canopy photosynthesis because of the effect of late leaf spot. Canopy photosynthesis was inversely proportional to total disease severity, which is an expression of both defoliation and necrotic area.