The effects of two phenoxy postemergence herbicides, 2-(2-methyl-4-chlorophenoxy)propionic acid (MCPP) and 2-(2,4,5-trichlorophenoxy)propionic acid (2,4,5-TP), on the physiology of Poa pratensis and on pathogenesis by Drechslera sorokiniana were investigated. Concentrations of nonstructural carbohydrates were generally decreased in all leaves of P. pratensis plants growing in soil to which these herbicides had been applied. Decreased concentrations of sucrose and total sugars correlated with increased leaf spot severity. It was hypothesized that decreasing sugar concentrations could have resulted in increased enzyme induction that enhanced pathogenesis by D. sorokiniana. The decrease of the concentrations of some amino acids in response to herbicide application correlated with increased leaf spot severity. The effects of the phenoxy herbicides on concentrations of sugars and free amino acids in leaves of P. pratensis are similar to changes in these compounds in senescing leaves. This is consistent with the hypothesis that factors promoting leaf senescence enhance D. sorokiniana leaf spot;A toxic extract was prepared from cultures of D. sorokiniana grown on Czapek Dox broth. The toxic extract caused leaf yellowing when applied to leaves of P. pratensis, but did not cause a rise in endogenous ethylene in the leaves. Incubation of toxic extract-treated plants at 233 mbar alleviated chlorophyll loss. It was hypothesized that naturally low levels of endogenous ethylene may predispose tissue to toxic extract-induced chlorosis, or that the toxic extract may increase the sensitivity of the tissue to ethylene and result in chlorosis. Leaves of herbicide-tolerant P. pratensis plants treated with the phenoxy herbicides had a chlorophyll loss that was not visible. This loss of chlorophyll also was alleviated by incubation at 233 mbar. Application of the toxic extract to leaves of herbicide-treated plants resulted in leaf yellowing. Incubation of the plants at 233 mbar alleviated the yellowing, but the chlorophyll content remained below control levels. It seems that the combined effect of the toxic extract and phenoxy herbicides is a diffuse loss of chlorophyll that is not ethylene related and cannot be alleviated by reduced atmospheric pressure.