Reduction in the in vitro sensitivity oF Drechslera tritici-repentis, isolated from wheat, to strobilurin and triazole fungicides

Abstract

Reports of failure in the chemical control of wheat yellow leaf spot led to determination of the sensitivity of Drechslera tritici-repentis (Dtr) to the fungicides quinone outside inhibitors (QoIs) and demethylation inhibitors (DMIs). The IC50 was obtained for strobilurins (azoxystrobin, kresoxim-methyl, picoxystrobin and pyraclostrobin) and for triazoles (cyproconazole, epoxiconazole, propiconazole, prothioconazole and tebuconazole), using five Dtr isolates. Seven concentrations of the fungicides were tested in the bioassay: 0.00; 0.01; 0.10; 1.00; 10:00 and 20.00 and 40.00 mg/L active ingredient (a.i.). Assays consisted of completely randomized design and four replicates. Each experiment was performed twice, using the average of the two tests for statistical analysis. The percentage inhibition data for conidial germination (QoIs) and for mycelial growth (DMIs) were subjected to logarithmic regression analysis, calculating the 50% inhibitory concentration (IC50) based on the generated equation. There was a reduction in the sensitivity of Dtr isolates to strobilurins. IC50 values ranged from 0.58 to > 40.00 mg/L. The lowest sensitivity of isolates was detected for azoxystrobin, kresoxim-methyl, picoxystrobin and trifloxystrobin. Pyraclostrobin was most efficient, showing IC50 between 0.58 and 1.03 mg/L. The IC50 ranged from 0.35 to 1.37 mg/L for epoxiconazole, from 0.49 to 1.28 mg/L for propiconazole and from 1.41 to 2.34 mg/L for tebuconazole. Prothioconazole was most potent, showing IC50 between 0.09 and 0.21 mg/L. The hypothesis that the control failure can be attributed to the reduced Dtr sensitivity to the fungicides QoIs and DMIs was confirmed.

Detection and characterization of QoI resistance inPyrenophora tritici‐repentispopulations causing tan spot of wheat in Argentina

“…In Brazil, tan spot is an important and severe disease of wheat, particularly in the southern region of the country. Tonin et al (2017) demonstrated that tan spot control failures in Brazilian fields with fungicide applications were due to a reduced sensitivity of Ptr isolates to both Qols and DMI fungicides. In the present study, a total of 82 Argentinian Ptr isolates were obtained from fields where tan spot control failure was reported, and assessed for in vitro sensitivity to three QoI fungicides.…”

“…The higher fungitoxic capacity of pyraclostrobin compared to other QoI fungicides has already been demonstrated for various pathogens including Ptr. For example, Tonin et al (2017) and Abdullah (2017) reported that among all the QoI fungicides evaluated against Ptr, pyraclostrobin was the most fungitoxic. These researchers did not find an explanation for these differences between molecules.…”

“…Resistance to QoI fungicides in Ptr was first detected in 2003 in Europe (FRAC, 2020; MacLean et al, 2017; Sierotzki et al, 2007). However, few studies reported on shifts in sensitivity of Ptr isolates to strobilurins or mutations associated with the resistance (Mehamdia et al, 2020; Reimann & Deising, 2005; Sierotzki et al, 2007; Tonin et al, 2017), while others reported that no reduced sensitivity was found (MacLean et al, 2017; Patel et al, 2012). QoI resistance in Ptr isolates has been attributed to three mutations in the cytochrome b ( cytb ) gene that lead to the substitution of amino acids conferring resistance (Sierozki et al, 2007): (a) glycine to alanine at amino acid position 143 (G143A); (b) phenylalanine to leucine at amino acid position 129 (F129L); and (c) glycine to arginine at amino acid position 137 (G137R).…”

Addition of mancozeb to the fungicide mixtures DMI + QoI and SDHI + QoI on the control of wheat leaf blights

“…Plots treated with the mixtures, without addition of mancozeb, showed severity of 18% for epoxiconazole + pyraclostrobin, 26% for fluxapyroxad + pyraclostrobin, 28% for epoxiconazole + kresoxim methyl, 34% for prothioconazole + trifloxystrobin, 34 % for azoxystrobin + propiconazole, 39% for cyproconazole + picoxystrobin, and 62% for azoxystrobin + benzovindiflupyr ( Table 1). The high severity after treatments, without addition of the protectant, evidences the difficulty in controlling leaf blights with these mixtures (10,12). This shows the reduction in sensitivity of D. tritici-repentis to DMIs + QoIs, and QoI + SDHI.…”

“…Mean control of wheat leaf blights has been less than 50%. Such low efficiency indicates that the fungicide application cost is not covered and has a negative effect on wheat grain yield (10).…”