Influence of Pecan Scab on Gas Exchange and Chlorophyll Content of Pecan Leaves. Ann B. Gould, Department of Plant Pathology, Rutgers University, New Brunswick, NJ 08903. James H. Aldrich and Peter C. Andersen, University of Florida, Institute of Food and Agricultural Sciences, North Florida Research and Education Center, Monticello 32344. Plant Dis 80:317. Accepted for publication 14 November 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0317.

The influence of pecan scab (Cladosporium caryigenum) on leaf physiology of summer-flush and spring-flush leaves of Choctaw and Cape Fear pecan (Carya illinoensis) was assessed in early October 1987 and 1989. In both years, net CO₂ assimilation, conductance to water vapor, transpiration rate, and chlorophyll content of summer-flush leaves were reduced in a linear or curvilinear manner with an increase in scab infection. Relatively low estimates of scab (5%) generally resulted in a 30 to 50% decline in the above physiological variables. The effect of scab on leaf gas exchange of summer-flush leaves was best described when scab severity on the entire leaflet, leaflet veins, or leaf petioles was considered rather than scab severity solely in the sector of leaflet tissue in which physiological variables were measured. In older spring-flush leaves, gas exchange was best correlated with scab severity on entire leaves. Both the disproportionate impact of scab on leaf physiology and the apparent influence of scab lesions in areas outside the sector in which dependent variables were quantified suggest that scab exerts a systemic effect on leaf tissue of pecan