Different Crop Rotations Suppress Soil-Borne Fusarium oxysporum in Monoculture Soil via Modulating Distinct Physicochemical and Microbial Mechanisms

Long-term continuous cropping often leads to soil-borne pathogen enrichment, and reducing pathogen abundance in continuously cropped soils is an important control measure. In this study, three rotation crops—carrot (C), garlic (G), and bok choy (B)—were introduced into potato pathogen-infested soils. The effects of different systems on pathogen abundance, soil physicochemical properties, and soil microbial abundance were investigated to preliminarily clarify mechanisms by which crop rotation suppressed pathogen enrichment. The results showed that all rotation systems significantly reduced soil pathogen abundance (Fusarium oxysporum, Fo). Among the rotation systems, carrot rotation achieved the greatest Fo reduction and exhibited the strongest increase in soil pH, followed by garlic rotation, while bok choy rotation had the weakest effect. Carrot rotation significantly increased soil bacterial abundance over other treatments. Moreover, crop rotation effectively suppressed soil pathogen enrichment by increasing soil pH and bacterial abundance. Importantly, carrot rotation enhanced soil pathogen-suppressive enzyme activities and the abundance of antagonistic bacteria in the soil. In contrast, garlic root exudates directly inhibited the pathogen, while bok choy and carrot root exudates promoted pathogen growth. These findings demonstrated different rotation crops exhibit distinct pathogen suppression mechanisms. Carrot rotation may indirectly suppress soil pathogen enrichment by increasing the abundance of antagonistic bacteria and enhancing antifungal enzymes, whereas garlic rotation may directly inhibit the pathogen via root exudates. This study provides practical guidance for growers to select optimal rotation crops and design rational continuous cropping systems.