Divergent Defense Strategies Induced by Variovorax soli and Microbacterium phyllosphaerae: Benefits for Strawberry Growth and Powdery Mildew Resistance

Strawberry powdery mildew, caused by Podosphaera aphanis, severely impacts the yield and quality of strawberry fruit and necessitates sustainable control strategies. Here, we assessed the growth and hormone content of strawberry seedlings (Fragaria × ananassa ‘Hongyan’) to evaluate the abilities of four plant growth-promoting bacteria (PGPB), Azospirillum brasilense, Phytobacter diazotrophicus, Variovorax soli, and Microbacterium phyllosphaerae, in a 42-day consecutive culture. Among all treatments, our results showed that the strains V. soli and M. phyllosphaerae increased the seedling biomass by 25.6–25.8% and decreased the disease index by 30–32.5% compared to the control. Moreover, the strain M. phyllosphaerae could produce 22.32 μg·mg−1 indole-3-acetic acid (IAA) after 24 h in a consecutive culture system and still maintain the IAA at 8.12 ng·mg−1 in seedlings. In addition, the salicylic acid (SA) and jasmonic acid (JA) hormones of the seedlings increased by 293% and 35.3% after inoculating with V. soli and M. phyllosphaerae, respectively. Further analysis of the coordination between plant hormones and plant growth showed that the accumulation of SA might have had a positive relation with biomass, whereas JA was negative under powdery mildew infection. These findings highlight that the V. soli and M. phyllosphaerae strains could offer the dual benefits of growth promotion and different defense strategies against powdery mildew for sustainable strawberry production.