Fungicide sensitivity of South African net- and spot-type isolates of Pyrenophora teres to ergosterol biosynthesis inhibitors
Pyrenophora teres, the causal agent of net blotch of barley (Hordeum vulgare), is a foliar pathogen that occurs as two distinct types as indicated by symptom expression on differentially susceptible cultivars. P. teres f. teres produces a net-type symptom while P. teres f. maculata produces a spot-type symptom. Fungicide sensitivities (IC50 values) of 89 monoconidial isolates of P. teres to sterol-demethylation-inhibiting fungicides were determined, based on the inhibitory effect on radial mycelial growth. These isolates were evaluated in vitro to determine their sensitivity to triadimenol, bromuconazole, flusilazole, propiconazole and tebuconazole. Infected leaves displaying either net- or spot-type symptoms were sampled from four fields, with two fields representing each respective symptom type. Both net- and spot-type isolates revealed strong resistance to triadimenol, the mean IC50 value being 25.7 μg/mL. Flusilazole was shown to be the strongest inhibitor of fungal growth with a mean IC50 value of 0.71 μg/mL. Spot-type isolates showed a higher resistance than net-type isolates to all five fungicides screened. Significant differences in fungicide sensitivities were found. The overall conclusion of this study is that spot-type isolates show a higher degree of resistance to commercially used fungicides than net-type isolates.