The study was conducted to determine the critical climatic factors influencing sporulation of Mycosphaerella fijiensis and progression of black Sigatoka severity in Cavendish banana. The highest numbers of ascospores and conidia per cubic meter of air were observed during the months of August, October and December 2005. Increased production of ascospores and conidia was significantly influenced by maximum temperature. The correlation between maximum temperature especially those occurring the previous 4, 5, 6, 7 and 8 weeks and cumulative number of spores was higher than that compared between the former and amount of rainfall. Black Sigatoka severity was highly correlated with cumulative number of spores. Maximum temperature had the highest correlation with disease development. Increased disease severity appeared to be the result of the combined effects of cumulative number of spores, maximum temperature, relative humidity and amount of rainfall. The effect of leaf wetness and number of rainy days on disease severity was not significant.