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Effect of Chlorophenoxy Herbicides on Soluble Sugars and on Pathogenesis by Drechslera sorokiniana in Sequentially Senescent Leaves of Poa pratensis. James P. Madsen, Postdoctoral research fellow, Department of Agronomy, University of Kentucky, Lexington 400546-0091; Clinton F. Hodges, professor of horticulture and of plant pathology, Department of Horticulture, Iowa State University, Ames 50011. Phytopathology 73:1296-1299. Accepted for publication 25 April 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1296.

In studies of the four most recently formed visible leaves on shoots of Poa pratensis, severity of leaf spot of Drechslera sorokiniana increased on progressively older leaves of control, 2-(2-methyl-4-chlorophenoxy)-propionic acid (MCPP)-, and 2-(2,4,5-trichlorophenoxy)propionic acid (2,4,5-TP)-treated plants. Each progressively older leaf generally was more severely diseased on herbicide-treated plants than on control plants, except for leaf three. Total soluble sugar content of leaves of plants treated with either herbicide was less than that of controls; sucrose, glucose, and fructose constituted the loss. Inoculation of the two youngest leaves of control plants also decreased total soluble sugars; inoculation of leaf three of control plants increased sugars and had no effect on leaf four. No correlation existed between total sugar content of uninoculated control leaves and severity of leaf spot after inoculation. Inoculation of leaves of all ages on plants treated with MCPP or 2,4,5-TP induced an increase in soluble sugars, primarily in glucose and fructose. The decrease in total soluble sugars and sucrose in leaves of MCPP-treated plants correlated with increased leaf spot severity; no correlations existed for 2,4,5-TP-treated plants. Herbicide-induced changes in the soluble sugar content of sequentially developing and senescing leaves are discussed relative to the hypothesis that factors that promote senescence may enhance leaf spot severity.