<b>Spatial structure of soil-borne fungal plant pathogens: a brief review of the evidence and a way forward using </b><b><i>Macrophomina phaseolina</i></b><b> as an example</b>

Understanding the link between spatial structure of soil-borne fungal plant pathogens (SFPP) and ecological conditions is an important first step to understanding how these organisms impact agricultural and untilled systems. A literature review revealed that most SPFF spatial patterns studies have occurred in tilled systems and relied on rudimentary statistical approaches. Here we begin to address this knowledge gap by quantifying the spatial pattern of <i>M. phaseolina</i> at multiple spatial scales in an untilled grassland system in Northeastern Kansas, U.S.A, and relate these patterns to soil and plant variables. Using microsclerotia density as a measure of <i>M. phaseolina</i> abundance, we found high spatial variability in untilled soil, even at very fine scales (7 cm). Using point pattern analysis, we found no evidence of spatial aggregation. Rather, <i>M. phaseolina</i> density was randomly distributed and did not strongly associate with any measured soil or plant variables. Our findings contrast with previous work detecting aggregation of <i>M. phaseolina</i> and other SFPP likely due to our use of more informative spatial statistics that examine multiple scales and the ecological dichotomy of untilled grassland systems and row-crop agricultural systems. Additionally, the fact that high spatial variation of <i>M. phaseolina</i> over small scales was poorly explained by plant or soil variables suggests that a better understanding of temporal variation may be necessary to identify the factors that control SFPP abundance.