Monilinia fructicola (Wint.) Honey, and Rhizopus nigricans Erh., the causal organisms of brown rot and Rhizopus rot, respectively, have long been recognized as the two major causes of post-harvest rots on peach and nectarine fruits. Fruit losses of as much as 50% may occur during storage, transport, and marketing from these two diseases if effective fungicide treatments are not used. Application of fungicide suspensions as post-harvest dips have been reported to reduce losses from post-harvest rots considerably, however, more effective treatments are needed. Tests were made of three fungicides: benomyl [methyl-l-(butylcarbamoyl)-2-benzimidazole carbamate], Topsin-M [dimethyl-4,4'-ophenelenebis (3-thioallophanate) , and Botran (2,6-dichloro-4-nitroanaline) against the two post-harvest rot fungi, M. fructicola and R. nigricans, on peach and nectarine fruits and in vitro. Fruits were inoculated with an equal concentration of spores of both fungi, then treated, by dipping in the different fungicide suspensions at 0, 4, 8, and 12 hour intervals after inoculation. Three concentrations of benomyl or Topsin-M (300, 450, and 600 ~g/ml) were used in combination with corresponding concentrations of Botran (600" 900, and 1200 Ug/ml). Number of decayed fruits increased with time of treatment after inoculation up to 8 hours then leveled off. Lesion progression, however, was slower with all fungicide treatments as compared to those of the untreated check fruit. Percent of decayed fruit remained lower when treatments were applied within 4 hours after inoculation. Later application times were ineffective and fruits showed approximately the same level of infection as controls. Treated nectarines appeared to be more susceptible to the post harvest rots than peaches. This may be due to lower residues of the chemicals remaining on the smooth skinned fruit. Benomyl and Topsin-M were fungitoxic to M. fructicola in vitro but neither was effective against R. nigricans. Botran alone was fungistatic to both R. nigricans and M. fructicola, but often lost effectiveness after 96 hours. Combinations of benomyl plus Botran, or Topsin-M plus Botran, were fungitoxic to M. fructicola and fungistatic to R. nigricans. Botran alone and Botran in combination with benomyl or Topsin-M inhibited germination of M. fructicola and R. nigricans spores more effectively than benomyl or Topsin-M alone. There were no major significant differences in control provided by treatments containing benomyl or Topsin-M. Treatments using Botran with benomyl or Topsin-M did not effectively control post harvest rot when applied later than 4 hours after inoculation. In vitro tests, however, indicated either a synergistic or additive effect of the fungicide combinations against R. nigricans. / Master of Science