Unexpected temporal patterns of the soil-borne fungal plant pathogen Macrophomina phaseolina in untilled grasslands

Quantifying seasonal shifts in soil-borne fungal plant pathogen (SFPP) abundance is key to understanding impacts on plant communities. While SFPP patterns are well-studied in agricultural systems, species-specific temporal patterns in untilled, native ecosystems remain almost entirely overlooked. Here, we examined how the agriculturally important SFPP Macrophomina phaseolina responds to seasonal shifts in soil temperature and moisture in nine untilled, native grassland communities in Southcentral Kansas, U.S.A. Using microsclerotia density as a measure of M. phaseolina abundance, we quantified temporal variation over 16 months across two growing seasons. Macrophomina phaseolina occurred in all nine grassland communities, but two unexpected outcomes emerged. First, abundance varied substantially during the growing season without a clear association with warmer and drier soils, contradicting results from agricultural and laboratory studies. Second, abundance increased much earlier in the growing season than expected, suggesting that low soil temperature does not limit M. phaseolina growth. We discuss plausible explanations for these differences including land use, plant diversity, and phenological differences between agricultural and grassland systems. These unexpected findings highlight the need to reevaluate assumptions derived from agricultural contexts and provide insight into the impact of M. phaseolina on plant communities in both native and agricultural systems.