The fuzzy rust system measures how susceptible a given region is to the occurrence of Asian soybean rust. It is a plant disease caused by the fungus Phakopsora pachyrhizi (PP), which causes its early ripening. The system comprises 11 fuzzy rules, which combine environmental, pathogen, and host conditions as input data and return the fuzzy occurrence as output variables—combining fuzzy logic and differential equations generated data that fed into the system. The favorability (environmental conditions favorable to the progress of the disease) was estimated by a fuzzy system that already exists in the literature, which combines temperature and relative humidity data. The diffusion–advection differential equation estimated the concentration of spores (pathogens). The existence of soybeans (hosts) was mapped from official records. It was tested for the state of Mato Grosso (Brazil). The tests were made for harvests from 15/16 to 19/20 in 38 locations, where 84 first occurrences were recorded throughout the harvests. Comparing the periods in which each locality registered occurrence, 81% of the localities were susceptible to the disease. Verifying the fuzzy occurrences of these 38 sites, three there was no susceptibility to the disease on any day of the harvest.