Characterizing hormesis and the in vitro effects of sub-lethal fungicide exposure in sclerotinia homoeocarpa

Abstract

Dollar spot, caused by Sclerotinia homoeocarpa F.T. Bennett, is the most economically important disease of turfgrass. Dollar spot reduces aesthetics and playability of turfgrass, and when left uncontrolled can result in plant death and weed encroachment. Sterol demethylase inhibiting (DMI) fungicides are valuable for dollar spot control due to their broad spectrum of activity with single applications. Previous studies have shown sub-lethal concentrations of fungicides can cause plant pathogens to exhibit hormesis. The first goal of this research was to evaluate the effects of sub-lethal doses of DMI fungicides on the mycelial growth of a range of S. homoeocarpa isolates via an in vitro fungicide assay. The second goal was to examine the impact of sub-lethal doses of DMI fungicides on the production of oxalic acid in S. homoeocarpa. The third goal was to determine the effects of low doses of DMI fungicides on the disease severity of resistant and sensitive populations of S. homoeocarpa on creeping bentgrass. Sub-lethal concentrations of DMI fungicides did not cause significant increases of mycelial growth in vitro when compared to mycelial growth of untreated isolates. A method to detect oxalic acid via high performance liquid chromatography was developed in this research. Oxalic acid concentrations were greatest when isolates were treated with a high rate of DMI fungicides. In the greenhouse, area under the disease progress curve (AUDPC) was consistently increased by a sub-lethal rate of DMI fungicides when compared to untreated inoculum. A sublethal rate of DMI fungicides consistently caused increased mean disease severity as early as 21 days after treatment. Results of this study suggest sub-lethal concentrations of fungicides may induce hormesis in S. homoeocarpa, resulting in increased dollar spot pressure in creeping bentgrass.