Sensitivity of western Canadian <i>Pyrenophora teres</i> f. <i>teres</i> and <i>P. teres</i> f. <i>maculata</i> isolates to propiconazole and pyraclostrobin

<p><i>Pyrenophora teres</i> f. <i>teres</i> (<i>Ptt</i>) and <i>Pyrenophora teres</i> f. <i>maculata</i> (<i>Ptm</i>), the causal agents of the net and spot forms of net blotch of barley, respectively, can be managed in western Canada with fungicides containing propiconazole and pyraclostrobin. Given the potential for development of fungicide resistance, the sensitivity of a collection of <i>Ptt</i> and <i>Ptm</i> isolates to propiconazole and pyraclostrobin was evaluated using microtitre plate bioassays. The concentration of propiconazole needed to inhibit fungal growth by 50% (EC₅₀) was 1.5 mg L⁻¹ for <i>Ptt</i> and 2.3 mg L⁻¹ for <i>Ptm</i>, while the EC₅₀ of pyraclostrobin was 0.015 mg L⁻¹ for <i>Ptt</i> and 0.024 mg L⁻¹ for <i>Ptm</i>. Subsequently, 39 <i>Ptt</i> and 27 <i>Ptm</i> isolates were screened with discriminatory doses of 5 mg propiconazole L⁻¹ and 0.15 mg pyraclostrobin L⁻¹. Inhibition of growth as a result of propiconazole was 12–95% for <i>Ptt</i> and 48–92% for <i>Ptm</i>; growth inhibition as a result of pyraclostrobin was 40–100% and 24–100%, respectively. Two <i>Ptt</i> isolates were insensitive to propiconazole, while one <i>Ptm</i> isolate was insensitive to pyraclostrobin. The latter also showed decreased sensitivity to propiconazole. The identification of net blotch isolates insensitive to these fungicides emphasizes the need for farmers to employ integrated crop management strategies to avoid fungicide resistance build-up.</p

Virulence of <i>Pyrenophora teres</i> populations in western Canada

<p>Net blotch, caused by <i>Pyrenophora teres</i>, is an economically important disease of barley. The pathogen has two morphologically similar but genetically distinct forms: <i>P. teres</i> f. <i>teres</i> (<i>Ptt</i>) and <i>P. teres</i> f. <i>maculata</i> (<i>Ptm</i>), which cause net form net blotch (NFNB) and spot form net blotch (SFNB), respectively. The virulence of a collection of 39 <i>Ptt</i> and 27 <i>Ptm</i> isolates collected from western Canada was evaluated by inoculating these isolates onto sets of barley differential hosts. One week following inoculation, the second and third leaves of each plant were rated for disease severity on scales of 1–10 (for <i>Ptt</i>) or 1–9 (for <i>Ptm</i>). Plants rated <5 and ≥5 were scored as resistant and susceptible to <i>Ptt</i>, respectively, while plants rated 1–3 and >3 were scored as resistant and susceptible to <i>Ptm</i>. The experiment was repeated. Cluster analysis revealed 16 and 13 distinct pathotype groups, respectively, among the 39 and 27 representative <i>Ptt</i> and <i>Ptm</i> isolates. The barley differentials CI 5791 and CI 9820 were resistant to all isolates of <i>Ptt</i> except one, whilst the differential CI 9214 was resistant to all isolates of <i>Ptm</i> except two. Therefore, the differential lines CI 5791 and CI 9820, for <i>Ptt</i>, and CI 9214, for <i>Ptm</i>, can still be considered as potentially useful sources of resistance for Canadian barley breeding programmes.</p