Impact of Pecan Leaf Blotch on Gas Exchange of Pecan Leaves. Peter C. Andersen, University of Florida, Institute of Food and Agricultural Sciences, Agricultural Research and Education Center, Monticello 32344. James H. Aldrich, and Ann B. Gould. University of Florida, Institute of Food and Agricultural Sciences, Agricultural Research and Education Center, Monticello 32344. Plant Dis. 74:203-207. Accepted for publication 21 September 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0203.

The influence of pecan leaf blotch infection (caused by Mycosphaerella dendroides) on net CO₂ assimilation rate, conductance, transpiration rate, intercellular CO₂ concentration, water use efficiency, and chlorophyll concentration of Cape Fear and Choctaw pecan (Carya illinoensis) leaves was assessed. Physiological variables were related to disease severity in a linear or curvilinear manner. A leaf blotch disease severity rating of 40% resulted in declines in net CO₂ assimilation rate of 63 and 47% for spring- and summer-flush Cape Fear leaves, respectively, and 72 and 79% for spring- and summer-flush Choctaw leaves, respectively. Smaller percentage reductions occurred in leaf conductance, transpiration rate, and leaf chlorophyll concentration. Leaf intercellular CO₂ concentration increased substantially with leaf blotch infection. Plots of net CO₂ assimilation rate versus leaf conductance to CO₂ and intercellular CO₂ concentration of leaf blotch-infected and control leaves revealed that net CO₂ assimilation approached zero before complete stomatal closure and that the degree of stomatal closure was not sufficient to prevent an increase in intercellular CO₂ concentration. These data indicate that leaf blotch may reduce leaf efficiency by adversely affecting the photosynthetic apparatus.