Purple blotch caused by Alternaría porri, has an impact on garlic plantings. Furthermore, the severity of this disease on local garlic varieties can be greater than 60 %. This led to the use of a biological control, involving agricultural waste biomass of cabbage leaves as a form of alternative control against this disease. Also, these biological waste biomasses were chosen because their fermentation process produces antimicrobial compounds, which inhibits pathogen growth. This research aimed to analyse the effectiveness of fermentation extract of cabbage leaves, in order to control purple blotch on garlic, the level of concentration needed, and the frequency of the most effective applications. Furthermore, it used a completely randomized design for in vitro testing, and the non-factorial randomized complete block model for in vivo analysis, which involved 8 treatments. The results showed that the fermentation extract with a concentration of 200 mL.L−1, effectively inhibited 100% of colony growth, while the bi-weekly application of this product at 300 mL.L−1, effectively suppressed the disease incidence of purple blotch, by up to 74.69 %.