Sugars altered fungal community composition and caused high network complexity in a Fusarium wilt pathogen-infested soil

Despite the quantitative dominance of sugars within root exudates and their ecological importance in regulating plant disease development, it is not well understood how specific sugars influence the fate of fungal pathogens, the fungal community composition and, in particular, the fungal interactions in soil. In this study, a microcosm incubation experiment was conducted by adding four low-molecular-weight sugars to a Fusarium wilt pathogen-infested natural soil (i.e., Low-FO soil) and the soil further receiving Fusarium wilt pathogen inocula (i.e., High-FO soil) to understand the changes in fungal community composition and fungal interactions. Despite living in soils where multiple microbes coexist, after the addition of sugar, Fusarium wilt pathogen was selectively enriched, and sugar allowed it to sustain its dominance over time. Concurrently, the fungal richness became lower, and the fungal community composition was altered throughout 42 days of incubation. The Humicola-affiliated OTU600 showed a more rapid biomass increase than this pathogen after the addition of sugars in the Low-FO soil at some time points, and also increased over time in the High-FO soil. The community network in sugar-added soils was more complex and connected than in those without added sugar, indicating greater fungal interactions and niche-sharing. The Fusarium wilt pathogen formed positive or no connections with the keystone taxa in sugar-spiked networks in almost all cases. This suggests that the keystone taxa may have promoted or not constrained the wilt pathogen, representing a potential mechanism enabling this pathogen to vigorously proliferate after the addition of sugar.