The soybean rust (Phakopsora pachyrhizi H. Sydow & P. Sydow) was reported in soybean (Glycine max L. Merrill) in many tropical and subtropical regions, causing significant reductions in productivity and quality of seeds (Bromfield, 1984, Hartman et al., 2005; Kawuki et al., 2004; McGee, 1992, Medina et al., 2006, Sinclair & Backman, 1989; Vale, 1985, Yang et al., 1990, Yang et al., 1991; Yorinori & Lazzarotto, 2004), with losses of up to 70% in production (Bromfield, 1976). The rust occurs in almost all soybean fields in Brazil. The states with high occurrence of the disease in 2003/04 were Mato Grosso, Goias, Minas Gerais and Sao Paulo. Considering Brazilian states in 2002/03, soybean rust caused losses of 4.011 million of megagrams or the equivalent of US$ 884.25 million, while in 2004, the losses were approximately US$ 2.28 billion (Yorinori & Lazzarotto, 2004). The success of pathogen infection depends on the sequence of events determined by spore germination, appressoria formation and penetration. Each of these events, the subsequent colonization and sporulation, are influenced by biotic factors such as pathogen-host and abiotic environment. Among abiotic factors, temperature and leaf wetness play a crucial role, especially in the monocyclic germination, infection and colonization of P. pachyrhizi in soybeans. Thus, several studies were conducted to model the effects of temperature and humidity on the disease progress for Brazilian cultivars (Vale, 1984, Vale et al., 1990) and for different cultivars adapted to other countries (Batchelor et al., 1997, Kim et al., 2005, Marchetti et al. 1975; Melching et al. 1989; Pivonia & Yang, 2004, Reis et al., 2004). According to Sinclair & Backman (1989), the range of optimum temperature for infection is 20 °C to 25 °C. Under these conditions, with the availability of free water on the leaf surface, the infection starts after 6 hours of the deposition of the spore (Marchetti et al., 1975; Melching et al. 1989; Vale et al., 1990). However, after 12 hours (Marchetti et al. 1975; Melching et al., 1989) up to 24 hours of leaf wetness (Vale et al., 1990) was more successful in establishing infection (Sinclair & Backman, 1989). Therefore, such studies are important for estimating the potential occurrence and formulate strategies to control disease in geographic regions not yet reported (Pivonia & Yang, 2005) and to investigate the potential of spreading in major producing regions throughout the months of the year (Alves et al., 2006; Pivonia & Yang, 2004).