Tobacco blue mould observed in Iran since 1962 has caused extensive damage (above 60%) during the first years of the epidemic (1963-64), and also from 1969 to 1973, due to the emergence of virulent strains. In 1974-1991, with the exception of 1978, little damage was incurred and production loss was stable at around 3%. Since 1992 (1992 to 1994), important losses (up to 15%) were again observed in areas of susceptibility to this parasite, which lead to control on the field with frequent treatments based on metalaxyl-mancozeb. Studies on the behaviour of the resistant varieties of the CORESTA trap collection, on the field and in vitro, did not demonstrate a significant increase in the aggressiveness of the parasite, whereas climatic variation was at the same time considerable. For the three months of May, June, July, which constitute the developing season for tobacco as well as for the parasite, we had for years 1992-94 a drop in temperatures, an increase in rainfall and R.H., and less hours of sunshine. On the contrary, in 1995 we suffered no loss from the parasite, climatic variation went the opposite direction, with high temperatures, a long drought, decreased R.H. and more sunshine in comparison with the past 30 years' averages. Once again we came to the conclusion that the main climatic parameters strongly influence the host and parasite physiology and that if one could forecast the climatic variations a week ahead, one could warn growers of the need to treat tobacco. Thus one could : 1) Apply quickly the needed treatments and avoid losses; 2) Save the expenses of unneeded treatments; 3) Preserve the environment and ecosystem; 4) Reduce the pesticide residues on tobacco.