Hormetic Effects of Thiophanate-Methyl in Multiple Isolates of Sclerotinia homoeocarpa.

Twenty-eight isolates of Sclerotinia homoeocarpa, causal agent of dollar spot disease in turf, were assessed for fungicide hormesis at sublethal concentrations of thiophanate-methyl (T-methyl). Each isolate was grown in corn meal agar amended with 11 concentrations of T-methyl (30,500 to 0.047 µg/liter), and the area of mycelial growth was determined relative to the control. Three replicates were used per concentration, and the experiment was repeated three to five times for each isolate. Reference isolates (EC50 > 20 µg/liter), with no prior history of T-methyl exposure, were highly sensitive and not stimulated by low doses. Likewise, no stimulation was observed in two highly sensitive isolates (EC50 > 30 µg/liter) that had been preconditioned by exposure to T-methyl, or in four T-methyl-tolerant isolates. Seventeen (81%) preconditioned T-methyl-tolerant isolates (EC50 = 294 to1,550 µg/liter) had statistically significant growth stimulation, in the range of 2.8 to 19.7% relative to the control. These results support that hormesis (low-dose stimulation, high-dose inhibition) is a common dose response in preconditioned S. homoeocarpa, particularly in response to subtoxic doses of T-methyl.