The effects of sterol-inhibiting fungicides (SIFs) on the free sterol and free fatty acid composition of apple leaves of Red Delicious and Jonathan cultivars were examiried over a 2 year period. Trees were treated in mornings vs evenings throughout each season and samples collected after 24 and 72 hours after each treatment. Generally, SIFs appeared to have an effect on the free sterol content of apple leaves after 24 hours, but the concentrations of free sterol returned to normal after 72 hours in the leaves of both cultivars. Morning versus evening application had little or no influence on leaf free sterol concentrations. There were increases in unsaturated and total fatty acid concentrations in Red Delicious leaves 24 hours following applications with the SIF, etaconazole, and the non-sterol-inhibiting fungicide (NSIF), metiram, early in the season. There were also increased concentrations of linoleic, linolenic and total free fatty acids in fenarimol and triadimefon-treated Jonathan leaves 72 hours after treatment. Early in the season, the SIF, fenarimol, caused an increase in linolenic acid in both Red Delicious and Jonathan leaves 72 hours after either morning or evening applications. Generally, both the Red Delicious and Jonathan leaves exhibited a decrease in saturated fatty acids following morning application whereas, an increase in saturation following evening application. Although SIFs may have had an effect primarily on the unsaturated fatty acids, particularly linolenic acid, early in the season, particularly linolenic acid, the fatty acid composition of the leaves appeared to return to normal later in the season. Ultrastructural observations were made of Red Delicious leaves 12, 24 and 72 hours after treatment with the SIF, bitertanol. Twelve hours after treatment thylakoids of chloroplasts appeared swollen and irregular resulting in loss of integrity of the organelles. However, after 24 and 72 hours, thylakoids of chloroplasts of treated leaves were similar to the controls. Infection of bitertanol-treated Red Delicious leaves by Spilocaea pomi was also examined at the fine structural level. Nuclear envelopes were not well defined and mitochondrial matrices appeared washed-out after 12 and 72 hours post treatment. There was dissolution of normally plate-like cristae of mitochondria, accompanied by the accumulation of minute electron dense bodies around their periphery. Invaginations and proliferations of the plasmalemma were observed as well as increased vacuolization of the cell. Further electron microscopic observations were made of the in vitro conidial state of Venturia inaequalis following the application of fenarimol. Conidia treated 2 hours with the fungicide for had necrotic areas throughout the cytoplasm. The plasmalemma was not well defined, and appeared to be degrading. Increased vacuolization was observed as were numerous lipid bodies and multivesicular complexes (MVCs) which contained vesicles of varying electron densities. Structural integrity of the organelles was such that they were difficult to discern. After 12 hours, the entire fungal cell was necrotic accompanied by the degradation of the cell wall. Detection of a selected number of SIFs in apple leaf tissue using bioassay procedures were also evaluated. It was found that the leaf disk and leaf extract bioassays examined in this study were ineffective in determining the presence of SIFs in apple leaves. / Ph. D.