Soil-borne fungal plant pathogens (SFPP) occupy the soils of ecosystems globally, but the ecology of individual species is often not well understood. One approach to help better understand a SFPP is to examine its temporal variation and response to its environment. Research on SFPP is biased towards agriculture, but many agriculturally important SFPP inhabit adjacent untilled, native systems. One such case is the SFPP Macrophomina phaseolina. Past research has shown that M. phaseolina thrives in soil with high temperatures and low moisture. In untilled grasslands, the presence of this SFPP and its response to seasonal fluctuations in soil temperature and moisture remains unclear. To determine the dynamics of M. phaseolina in untilled systems, I established nine plots within distinct soil types and quantified the monthly response of M. phaseolina to shifting soil temperature and moisture conditions from May 2022 to May 2023. I hypothesized that M. phaseolina density would increase across the growing season, reaching its highest densities in the month of August 2022 when high soil temperatures and low soil moisture would be most favorable. Macrophomina phaseolina density was highly variable among the nine plots during the growing season and seldom reached its highest density during August 2022. During the dormant season, M. phaseolina density markedly decreased to its lowest densities before sharply increasing to unexpectedly high densities in April 2023. These findings suggest that the sporadic variation of M. phaseolina in untilled systems during the growing season contrasts with frequent assertions that soil temperature and moisture will heavily influence this SFPP’s density throughout the year. The results suggest that soil temperature and moisture play a reduced role in untilled systems, as plant community phenology and competing soil antagonists may more strongly impact M. phaseolina growth and disease development.