Characterizing Hormesis and the in Vitro Effects of Sublethal Fungicide Exposure on Oxalic Acid Production in Sclerotinia Homoeocarpa F.T. Bennett.

Characterizing hormesis and the in vitro effects of sublethal fungicide exposure on oxalic acid production in Sclerotinia homoeocarpa F.T. Bennett

K.G. Robertson and G.L. Miller

Dollar spot of turfgrass, caused by the fungal pathogen Sclerotinia homoeocarpa F.T. Bennett, has a large economic impact on high amenity turfgrasses. Preliminary in vitro studies found dollar spot isolates may demonstrate hormesis when exposed to a sublethal dose of a demethylation inhibiting (DMI) fungicide. Hormesis is a biphasic dose response in which a low concentration of a stressor causes an increased biological response compared to the response without the stressor. This study aimed to evaluate the in vitro effects of sublethal doses of DMI fungicides on mycelial growth of S. homoeocarpa F.T. Bennett and production of oxalic acid (OA), an important metabolite for pathogenesis. Experiments were initiated with thirty isolates ranging in sensitivities to DMI fungicides. A mycelial growth assay confirmed a broad range of sensitivity, however future research is necessary to characterize hormesis at lower, specific concentrations. For assessment of OA accumulation, 10 mm diameter mycelial plugs from six isolates were grown on Modified Melin-Norkrans (MMN) media, and shaken for 10 days at 165 rpm in HPLC grade water amended with difenoconazole, propiconazole, triadimefon, or triticonazole at concentrations of 0, 0.0002, and 20 mg ml^-1. Treatments were replicated three times. Detection of OA was conducted with ultra-high performance liquid chromatography, utilizing a C-18 column at a wavelength of 209 nm. Quantification of OA was performed in comparison to standard curves of synthetic OA concentrations ranging from 0.0375 mM to 0.25 mM. Detection of OA was variable, with accumulation (ranging from 0.0008 mM to 0.01465 mM) occurring in three of six isolates. Differences were observed in OA accumulation among isolates (P = 0.001), with largest amounts produced by an isolate sensitive to DMI fungicides. No differences were observed in OA accumulation among the four fungicides (P = 0.1244) or between concentrations (P = 0.0775).