In the absence of a gene-for-gene relationship between a pathogen and its host, knowledge about aggressiveness is crucial to characterize novel pathogen populations that potentially emerge in agricultural pathosystems. Information about pathogen aggressiveness is also critical when establishing representative panels of pathogen isolates to test host resistance and in mapping quantitative trait loci involved in the host resistance. In this study, we focused on the fungus C. gloeosporioides that causes necrosis on the aerial part of one of its host plants, Dioscorea alata, and identified the in vitro conditions required to assess fungal aggressiveness on this host. Our main purpose was to convert the necrosis area development into a unique index for quantifying pathogen aggressiveness. The 'Ag' index described here has two advantages. First, it integrates the variance of symptom evolution curves to estimate the lesion development rates (initial and secondary) and the maximal necrosis area. Secondly, the new index takes two different symptoms commonly observed when inoculating D. alata leaves with C. gloeosporioides into account, one correlated with high leaf colonisation efficiency and the other with low colonisation efficiency. The weights accorded to each symptom in the index were proportional to leaf colonisation efficiency. We propose a framework for the acquisition of this index that has been designed to be conveniently combined with the routine bioassays required to establish representative panels of pathogen isolates. The general framework for the construction of this index can be broadly applied to diseases with necrotic symptoms.