Immune system responses of creeping bentgrass putting greens to plant health products

Abstract

Creeping bentgrass (Agrostis stolonifera L.) is a cool season turfgrass species widely used on golf course putting greens. However, it is often challenging to manage in summer due to its susceptibility to summer bentgrass decline (SBD). Weakened turf stands are more susceptible to pathogens such as Clarireedia jacksonii (causing dollar spot). Commercial plant health products (PHP) can have physiological effects that enhance plant immunity against various biotic and abiotic stresses. This research investigates the role of a plant growth-promoting rhizobacterium (PGPR) Bacillus subtilis strain UD1022 (hereafter UD1022) and turfgrass PHPs, in promoting disease resistance, turfgrass health, and growth through field and controlled environment experiments. To this end, PHPs including UD1022, acibenzolar-S-methyl, kelp extract, fosetyl-Al, and trinexapac-ethyl were evaluated on a USGA-specification creeping bentgrass putting green over three years. The findings showed that UD1022 successfully colonized roots, and that certain PHPs could reduce the severity of summer bentgrass decline (SBD), leading to healthier putting greens. In addition, we showed that the single inoculum approach of using UD1022 may have direct antagonism against the dollar spot pathogen C. jacksonii. Direct antagonism assays revealed wild-type UD1022 to significantly inhibit the mycelial growth of C. jacksonii. Bacterial compounds like surfactin and volatile organic compounds (BVCs) showed no significant inhibitory effects, suggesting that live bacterial cells, rather than secreted compounds, are essential for the observed antagonism. Interestingly, UD1022’s direct antagonistic activity against C. jacksonii was dependent on key biofilm and non-ribosomal peptide (NRP) pathway genes. We further explored UD1022's potential to control dollar spot through induced resistance in creeping bentgrass. Plants primed with UD1022 root treatment were evaluated for defense gene expression and disease severity. Detached leaf assays were optimized and used for screening for dollar spot disease severity. Direct antagonism was found to be more effective in controlling dollar spot than defense priming alone. Overall, our work shows that the mechanistic understanding of how a benign microbiome alters and modulates the host plant’s innate immune system is important to develop field strategies for turfgrass management. In addition, the evidence that the direct antagonism and persistence of UD1022 overrides plant’s response against the pathogen confirms UD1022’s potential as a biological agent for growth and fitness of bentgrass against biotic stressors.