Fungicide use intensity influences the soil microbiome and links to fungal disease suppressiveness in amenity turfgrass.

Disease-suppressive soils have been documented in many economically important crops, but not in turfgrass, one of the most intensively managed plant systems in the United States. Dollar spot, caused by the fungus Clarireedia jacksonii, is the most economically important disease of managed turfgrass and has historically been controlled through the intensive use of fungicides. However, previous anecdotal observations of lower dollar spot severity on golf courses with less intensive fungicide histories suggest that intensive fungicide usage may suppress microbial antagonism of pathogen activity. This study explored the suppressive activity of transplanted microbiomes against dollar spot from seven locations in the Midwestern U.S. and seven locations in the Northeastern U.S. with varying fungicide use histories. Creeping bentgrass was established in pots containing homogenized sterile potting mix and field soil and inoculated with C. jacksonii upon maturity. Bacterial and fungal communities of root-associated soil and phyllosphere were profiled with short-amplicon sequencing to investigate the microbial community associated with disease suppression. The results showed that plants grown in the transplanted soil microbiome collected from sites with lower fungicide intensities exhibited reduced disease severity. Plant growth-promoting and pathogen-antagonistic microbes may be responsible for disease suppression, but further validation is required. Additional least squares regression analysis of the fungicides used at each location suggested that contact fungicides such as chlorothalonil and fluazinam had a greater influence on the microbiome disease suppressiveness than penetrant fungicides. Potential organisms antagonistic to Clarireedia were identified in the subsequent amplicon sequencing analysis, but further characterization and validation are required.

Importance: Given the current reliance on fungicides for plant disease control, this research provides new insights into the potential non-target effects of repeated fungicide usage on disease-suppressive soils. It also indicates that intensive fungicide usage can decrease the activity of beneficial soil microbes and lead to a more disease conducive microbial environment in turfgrass. The results from this study can be used to identify more sustainable disease management strategies for a variety of economically important and intensively managed pathosystems. Understanding the factors that facilitate disease-suppressive soils will contribute to more sustainable plant protection practices.