Early and late blight are the most important foliar diseases of tomato and potato crops in Brazil. Decision support systems (DSS) are important tools in reducing the large amount of fungicides applied to suppress disease intensity. Systems developed for early or late blight were validated in two cropping seasons under Brazilian conditions. For tomato early blight, FAST, CUFAST, and TOMCAST systems were compared in the spring–summer (September–December) (SS) 2002 and summer–autumn (January–April) (SA) 2003. In both seasons, area under disease progress curve (AUDPC) values for FAST (123.2 and 340.3, respectively) and CUFAST (64.0 and 359.2, respectively) did not differ from calendar-based treated plots (82.5 and 225.7, respectively). Plots treated according to TOMCAST were sprayed once and AUDPC values (369.9 and 697.8) in SS and SA seasons, respectively, did not differ from the control plots (397.5 and 836.0, respectively). In SS, four fungicide sprays were recommended by CUFAST and FAST, whereas five were set by fixed calendar treatment; there were no differences in yield loss among treatments. In SA, FAST, CUFAST and calendar, recommended 4, 9, and 11 sprays, respectively. Higher yields were recorded in plots sprayed according to the calendar system compared to control plots, but there were no significant differences between the DSS and the calendar system. For potato late blight, BLITECAST, SIMCAST, NegFry, and Wallin systems were compared in the summer (December–March) and in the autumn (March–June). Despite no late blight developing in the summer, all systems recommended fungicide sprays. In the autumn–winter, late blight was severe but was reduced by fungicide applications according to all DSS, but the number of sprays was similar to the calendar. The AUDPC values in control plots (1193.5) were higher than BLITECAST (19.5), SIMCAST (97.7), NegFry (193.1), Wallin (69.7), and calendar (63.5) but there were no differences among AUDPC values in plots treated according to DSS. The NegFry and Wallin systems recommended five and the calendar six sprays. All DSS were as effective as the calendar treatment in reducing AUDPC. FAST and Wallin were the most effective systems in forecasting tomato early blight and potato late blight, respectively. Decision support systems are potentially useful tools for integrated management of both diseases in Brazil.