Phytophthora root and stem rot (PRSR) in soybean tends to occur on poorly drained fields; however, the contribution of soil moisture on its spread is not clear. A field survey was conducted to analyze the relationship between the soil moisture condition and other soil attributes with field-to-field and intra field variation and PRSR occurrence. The field surveys were conducted in 2019 and 2020 on a drained paddy field in Sasayama and an upland field in Ayabe, where the cultivar Tanbaguro was grown. PRSR was observed in both fields and disease occurrence on soybean was tested by collecting soils under identical moisture conditions using a pot experiment. Results showed that PRSR occurred more in the Sasayama field than in the Ayabe field. Spatial variation was analyzed for the Sasayama field (10 m×100 m) by dividing it into 75 blocks. The occurrence rate was higher in the blocks with lower moisture content than the other blocks. The other factors examined were infection potential and soil properties such as soil pH, total nitrogen, total carbon, exchangeable cations, available phosphate, and stone content. The Sasayama field showed higher content of most cations and stones and greater infection potential than the Ayabe field. The spatial variation analysis for the Sasayama field showed weak positive correlations with soil magnesium and stone contents. The infection potential correlated with the total carbon and nitrogen contents. These results suggested that excess soil moisture is not necessarily the dominant factor causing the spread of PRSR in soybean and that soil fertility may be related to the infection potential.
