An evalution of Corynespora leaf spot disease (CLSD), the devastating disease of rubber foliage in Indonesia, has been carried out in an effort to anticipate its development. It is possible that the disease will be a serious threat for rubber cultivation sometime in the future. There have been severe effects on some susceptible clones, ie RRIC 103, KRS 21, RRIM 725 and PPN 2058, since outbreaks early in the 1980s. CLSD is always present on rubber plantations from year to year irrespective of the time of year, especially in the case of the large-scale recommended clones such as GT1 and RRIM 600. Symptoms of disease have been commonly found on these clones in almost all areas in which they are cultivated although the decreased yield effect of CLSD on them is not yet clear. Field trials aimed at screening clonal planting materials relative to their susceptibility to the disease indicate that clone GT1 together with clone RRIM 600 are now more severely affected than clone RRIM 725 which is already known to be susceptible. The Corynespora cassiicola fungus, the casual agent of CLSD, has a tendency to be quite adaptive to new areas in which clones such as GT1 and RRIM 600 are planted on a large scale. There is no doubt that this pathogenic fungus has high genetic variability and adaptability as is demonstrated by its pathogenicity and the characteristics of isolates cultured in an artificial medium.