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In-season dynamics in sensitivity to azoxystrobin in the tobacco frogeye leaf spot pathogen, Cercospora nicotianae

Emily Pfeufer: University of Kentucky

<div>Reduced sensitivity to azoxystrobin fungicide in the tobacco frogeye leaf spot pathogen, <em>Cercospora</em> <em>nicotianae</em>, was documented in 2016. To determine the influence of fungicide programs on azoxystrobin sensitivity patterns in <em>C. nicotianae</em> populations, a randomized complete block trial was conducted in 2017. Fungicide programs were sequences of azoxystrobin and mancozeb applied on three dates. Symptomatic samples were taken at season outset, after each spray, and at season conclusion. Single-spore isolates were generated and spore germination was determined on 0, 0.001, 0.01, 0.1, 1, and 10 ppm azoxystrobin–amended ¼-strength potato dextrose agar with SHAM. For each isolate, the effective concentration to inhibit 50% of spore germination (EC<sub>50</sub>) was calculated based on the dose response curve. EC<sub>50</sub>s ranged from 0.04 to 0.91 ppm azoxystrobin. <em>C. nicotianae</em> populations from the untreated control and azoxystrobin-intensively treated plots had mean and median EC<sub>50</sub>s of 0.20 and 0.06, and 0.62 and 0.58 ppm azoxystrobin, respectively, after two applications. The EC<sub>50</sub> mean and EC<sub>50</sub> distribution of the azoxystrobin-intensive treated population were significantly greater than the nonexposed <em>C. nicotianae</em> population as determined by a two-sample t-test (<em>P</em> = 0.002) and a Kolmogorov-Smirnov test (<em>P</em> = 0.003), respectively. This suggests that measurable changes in <em>C. nicotianae</em> populations’ sensitivity to azoxystrobin may occur over the course of a single cropping season.</div>