Impact of pecan leaf blotch on gas exchange of pecan leaves.

Andersen, P. C., Aldrich, J. H., and Gould, A. B. 1990. Impact of pecan leaf blotch on gas exchange of pecan leaves. Plant Dis. 74:203-207. The influence of pecan leaf blotch infection (caused by Mycosphaerella dendroides) on net CO 2 assimilation rate, conductance, transpiration rate, intercellular CO 2 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 CO 2 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 CO 2 assimilation rate versus leaf conductance to CO 2 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. Leaf fungal diseases contribute greatly to reductions in photosynthetic efficiency and often induce premature defoliation of deciduous fruit crops in the fall. Downy spot (caused by Mycosphaerella caryigena Demaree & J. R. Cole) (11), powdery mildew (caused by Microsphaerapenicillata (Wallr.:Fr.) Lev.) (8), and pecan scab (caused by Cladosporium caryigenum (Ellis & Langl.) Gottwald [= Fusicladium effusum G. Wint.]) (7) infections have been shown to reduce gas exchange of pecan (Carya illinoensis (F. A. Wagenheim) K. Koch) leaves. Net CO2 assimilation of greenhouse-grown and greenhouse-inoculated pecan seedlings declined in a 1:1 ratio to the proportion of leaf surface infected with powdery mildew (8) or scab (7). By contrast, natural infection by downy spot in the field (11) resulted in reductions in net CO2 assimilation rate of two or more times the percentage of infected leaf surface. The maintenance of high photosynthetic rates in the fall can be important to the current and the subsequent season's nut production (12,16,24,25). Pecan leaves retain high levels of chlorophyll and net CO2 assimilation until leaf abscission (1), which occurs after the first hard freeze if the trees have received proper insect and disease control management. Pecan nutlets are strong assimilate sinks during kernel development (August-October) (3,22). A