Black dot is a blemish disease of potato caused by the fungus Colletotrichum coccodes (Wallr.) Hughes. with worldwide distribution. It occurs in Africa, Asia, Australia, Europe, North America, South America, and Central America. C. coccodes has no known sexual cycle, but genetic exchange is possible through vegetative conjugation. The determination of VCG among fungal isolates is usually based on the complementation tests with nitrate nonutilizing (nit) mutants which enables researchers to compare strains of pathogens. Eight VCGs have been identified for the Europe/Israel population, seven for the North American population, and six for the Australian population. Variation in aggressiveness, morphological traits, and physiological traits have been detected among the different C. coccodes VCGs. AFLP markers have been used also to study relationships within and among North American VCGs. With this method, five VCGs were separated from C. coccodes isolates, coinciding almost completely with the six VCGs of nit mutants. Additionally, a relationship of specific AFLP bands to corresponding VCGs was reported using AFLP analysis. The objectives of this study were: to develop VCG-specific sequence characterized amplified regions (SCAR) markers, to study the population biology of C. coccodes of North America using the AFLP method, and to study genetic diversity of the global populations of C. coccodes. A total of 88 isolates representing the different C. coccodes VCGs were analyzed via the AFLP method to generate SCAR markers. A total of 47 primers were designed and evaluated in PCR reactions. Only one primer, AGb6F/R, with 156 bp amplification PCR product was found to be specific for NA-VCG6 and NA V CG 7 of C. coccodes. For the second objective, 210 loci were generated and used to cluster the isolates into their NA-VCGs and to test the genetic structure of the North American population of C. coccodes. C. coccodes isolates recovered from potato plants were assigned to fourNA-VCGs: NA-VCGl, NA-VCG2, NA-VCG4/5, and NA-VCG6/7. No isolates tested belonged to NA-VCG3. NA-VCG2 was the dominant group in the population (n=238) and was the most frequently detected NA-VCG among states, fields, farms, and plants. However, in several instances there was more than one NA-VCG recovered from the same plant, field, farm, and state, indicating variability within the C. coccodes population in United States. Genetic differentiation among the nine states was 0.331, and overall gene flow for the 366 isolates was (Nm =1.01). For the genetic diversity of the global C. coccodes population, 855 isolates were studied. Three primer pairs were used and generated 210 loci. Based on this study, the population of C. coccodes exists as one large population with four main groups (NA-VCGl/3; NA-VCG2; NA-VCG4/5; and NA-VCG6/7). NA-VCG5 was the most common VCG globally followed by NA-VCG2. Among the five regions studied, there was relatively low gene diversity (0.222). The overall gene flow (Nm) was 1.25, meaning that one or more individuals are exchanged among the five regions each generation and that the populations will gradually become similar. Most of the variation among the five geographic regions originated from within population differentiation. Among VCGs of C. coccodes, there was high VCG differentiation (GsT=0.463), meaning there is a differentiation among the different C. coccodes VCGs and a population structure exists. AFLP analysis proved to be valuable in differentiating and studying the global population of C. coccodes.