Nitrogen-fixing beneficial bacteria improve soil fertility but can be adversely affected by fungicides, which may reduce their numbers or survival. To assess the effects of Dizam 50% SC as a fungicide on useful root nodule bacteria from Rhizobium sp. (collected from roots of Vigna spp. black-eyed peas), bacteria were isolated from the plant’s root system and exposed to varying concentrations of the fungicide (1,000–12,000 mg/L). The findings revealed that the average number of bacterial colonies decreased with increasing fungicide concentrations, while the percent mortality of the bacteria also increased. There also appeared to be some variability among the tested concentrations (2,000, 6,000, and 7,000 mg/L) in terms of both the mean bacterial count per mL and percent mortality compared with the other concentrations tested. Furthermore, the findings revealed a median lethal concentration (LC₅₀) of approximately 9,359.54 mg/L; there was a statistically significant inverse relationship between fungicide concentration and bacterial colonization, and there was a positive association between fungicide concentration and percent mortality of the bacteria. Lastly, based on the collected data, it was shown that when the previously described two strains were re-cultured in media containing various concentrations of the fungicide (range: 1,000–12,000 mg/L), their respective LC₅₀ values were determined to be 75.19 and 57.36 mg/L. Therefore, these findings suggest a dose-dependent reduction in Rhizobium sp. colony formation. However, these data do not confirm genetic resistance to Dizam 50% SC, which requires verification using molecular techniques.