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Efficacy assessment of Demethylation Inhibitor (DMI) fungicides against Guignardia citricarpa, the causal agent of Citrus Black Spot (CBS)
N. THAPA (1), M. M. Dewdney (1). (1) Citrus Research and Education Center, University of Florida, Lake Alfred, FL, U.S.A.

Copper-based and strobilurin fungicides are the recommended fungicides to manage CBS, a new fungal disease in Florida. DMI fungicides could be a potential alternative mode of action for rotation against CBS. The project objectives are to determine the <i>in vitro</i> fungal population response and the baseline sensitivity to the potentially registered DMIs. Tetraconazole, propiconazole, difenoconazole, fenbuconazole, and imazalil were evaluated in a mycelial growth inhibition assay with various ranges of concentrations for each fungicide. One ml of each stock solution, or water as a control, was mixed with the media to obtain the final concentrations. Inoculated plates were incubated at 24°C, under 12 hr light/dark conditions. The plates were scanned after 2 weeks and mycelial growth area was measured with Image-J. Mycelial growth on amended media was compared to the control. There were three replicates per concentration and the whole experiment was done twice. Regression with 3-parameter sigmoidal curves was used to calculate the half maximal effective concentration (EC50). The mean EC50s for difenoconazole, propiconazole, and tetraconazole were 0.0013, 0.011, and 0.0401 mg/ml, respectively. Preliminary results for the mean EC50s of fenbuconazole and imazalil were 0.0073 and 0.1 mg/ml, respectively. Results obtained from this study will aid the management of CBS and to understand the fungal response pattern towards DMI fungicides.

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