ALPISTE, M.C. Evaluation of the aggressiveness of Streptomyces spp. in potato cultivars and quantification of thaxomin A produced by different species of the genus. São Paulo. 2018. Dissertação (Mestrado em Sanidade, Segurança Alimentar e Ambiental no Agronegócio)-Instituto Biológico. Potato crop has a great economic importance in many countries and its productivity has been increasing gradually, however the production is still considered low due to diseases such as potato scab caused by different species of Streptomyces. The pathogenicity mechanism of most species of this genus involves different factors, such as the Nec1 and tomatinase proteins, which aid in the evasion of host defense; and the phytotoxin thaxtomin A, responsible for the development of the lesions. The objective of this study was to evaluate the aggressiveness of 30 Streptomyces strains on potato cultivars Ágata, Asterix, Atlantic, Challenger and Markies through of pathogenicity tests on minituber slices. In addition, the work also aimed to extract and quantify the phytotoxin thaxtomin A produced by these strains and to verify the correlation between the production of phytotoxin and pathogen aggressiveness. The extracted thaxtomin was purified by thin layer chromatography (TLC) and quantified by absorbance data in a spectrophotometer. In the pathogenicity tests, the strains were more aggressive in the Challenger and Atlantic cultivars, and less aggressive in Ágata and Markies. However, variations in the aggressiveness level were observed depending on the strain and cultivar tested. In the thaxtomin A quantification experiments, the highest values were observed by the species S. europaeisabiei (IBSBF 2498) and S. scabiei (IBSBF 2242, 2250 and 2359). There was no correlation between thaxtomin A production and pathogen aggressiveness, since some strains with low phytotoxin production were very aggressive and others with high production showed low aggressiveness.