Sour cassava starch production process, during the fermentation step, an acidic wastewater is generated, in this study named wastewater of cassava starch fermentation (WCSF). It has been studied the effect of WCSFs (from three industrial source: WCSF 1, 2 and 3) and patterns of organic acids (lactic, acetic, propionic and butyric acids, which were the main organic acids present in the WCSFs) singly or together on the development and growing of Botrytis cinerea, Monilinia fructicola and Colletotrichum gloeosporiodes. WCSFs showed antimicrobial effect on the three fungi studied. WCSF2 inhibited the germination of conidia from the three fungi, while WCSFs 1 and 3 inhibited conidia germination of B. cinerea and C. gloeosporiodes. The acetic, propionic, butyric and the mix acids were also effective in the reduction of conidial germination. In the analysis of mycelia growth, it has been observed that the growing was statically smaller, in relation to negative control, for all fungi in the treatments with the WCSFs and with the pattern solution of butyric acid. Which suggest that the WCSFs compounds and butyric acid are effective inhibitor for fungi and justify future studies using this wastewater.