Abstract Brown rot is the main disease of stone fruits in Brazil, but the susceptibility of peaches to brown rot at different stages of development in the field has not been studied under subtropical conditions. This information is relevant to guide the management of the disease. The objective of this research was to determine the influence of inoculating peaches with Monilinia fructicola at different stages of development on the infection and progress of brown rot at postharvest. Two experiments were carried out: one ex vivo with two cultivars and the other in the field for two seasons. Peaches were inoculated at different sizes for both experiments. In the field, peaches were bagged to avoid natural infection, and M . fructicola inoculum was monitored. The ex vivo incidence of the disease was lower at pit hardening than at other fruit stages for both cultivars. The incidence of brown rot for peaches attached to the trees increased with fruit ripening. Conversely, the time for symptom expression was reduced according to peach diameter. Peaches inoculated with a diameter smaller than 2 cm showed a lower incidence of brown rot and longer periods for disease expression than fruit inoculated near harvest. In conclusion, in areas with high inoculum in the orchard, a common condition in the subtropics, the grower must prevent infection at all stages of fruit development, thus avoiding losses during marketing.