Abstract: The occurrence,development,latent period,pathogen's life cycle of leaf spot caused by Corynespora cassiicola in Guizhou fluecured tobacco were studied in the field.The results showed that:1) the disease developed very slowly by the late June(5 to 6 weeks after transplanting);from the end of June to mid July,in persistent rainy days,the disease developed rapidly while source of infection was rich,it developed gradually if the source of infection was not enough;2) the disease index obviously decreased with the increase of distance from disease focus;3) from the end of May to mid July,the latent period of leaf spot was 16 to 33 hours when the tobacco plant was kept moisture for 38 hours after inoculating;4) when the bigger spots on the leaves inoculated in the ends of May and June and the early August were sampled,kept moisture for 2 days and then examined under microscopy;it was found that on the samples with a few spots,spores appeared in 11,8 and 7 days after inoculating,respectively.