Impact of Potassium Phosphite on Rhizosphere and Endosphere Microbiota, and Its Role in Disease Suppression in Tomato Plants

Bacterial wilt, caused by the Ralstonia solanacearum species complex, severely limits tomato production and remains difficult to control chemically. Potassium phosphite (KP) can activate plant defense and suppress pathogens, but its effects on rhizosphere microbiota during bacterial wilt are not fully understood. Here, we evaluated how soil-applied KP (0.5% w/v) affected tomato ( Solanum lycopersicum cv. Hawaii 7996) growth, disease development and rhizosphere microbial structure under R. pseudosolanacearum challenge in controlled greenhouse conditions. KP strongly suppressed wilt: 75% of untreated plants developed severe symptoms by day 14, whereas nearly all KP-treated plants remained asymptomatic, with an approximately 10⁴-fold reduction in stem pathogen load. However, repeated KP application reduced plant biomass by 20–30% after six weeks, indicating a growth–defense cost. KP also reshaped the rhizosphere microbiota, reducing bacterial richness and Shannon diversity while enriching putatively beneficial taxa, including Streptomyces and Bacillales, and decreasing opportunistic groups such as Pseudomonas and Ralstonia . Co-occurrence networks showed loss of a Ralstonia -centered hub under KP treatment. Functional prediction indicated enrichment of antimicrobial, siderophore and oxidative stress-related pathways, with reduced lipopolysaccharide-associated functions. These findings suggest that KP suppresses bacterial wilt through direct pathogen inhibition and microbiome-mediated disease suppression, highlighting its potential for microbiome-informed disease management in tomato systems.