Sensitivity of Venturia inaequalis to Trifloxystrobin and Difenoconazole in Uruguay

Abstract

The sensitivity of Venturia inaequalis to trifloxystrobin and difenoconazole was studied in Uruguay. Populations of V. inaequalis were collected from apple orchards with different histories of trifloxystrobin use. Sensitivity of populations to trifloxystrobin was analysed using a method for testing spore germination published by FRAC, using a discriminatory concentration of 2.0 mg a.i./l. Resistance to trifloxystrobin was widespread in the region of commercial apple production with resistance detected in all orchards examined, the incidences ranging from 3% to 95%. The highest frequencies were found in orchards with the most intensive use of Quinone outside inhibitors (Q o I) fungicides. Sensitivities of isolates of V. inaequalis to difenoconazole were assessed at five concentrations using a mycelial growth assay on isolates (33 isolates per orchard) from one non-commercial (baseline orchard) and two commercial orchards having differing histories of difenoconazole use. Populations in both commercial orchards exhibited reduced sensitivities to difenoconazole compared to the baseline orchard. Resistance factor (RF) values of 6.6 and 11.74 were measured in the orchards with moderate (up to 4 sprays per season) and intensive use (more than 5 sprays per season) of difenoconazole, respectively. A single-assessment concentration (SAC) was identified for assessing difenoconazole sensitivity of V. inaequalis isolates by fitting linear regressions between log 10 EC 50 and relative growth (RG) of the isolates at each fungicide concentration testing, and comparing the goodness-of-fit of the regression lines. Comparable results were obtained based on EC 50 values and RG values at a SAC of 0.05 mg of active ingredient per litre (a.i. per l). Populations from both commercial orchards differed from the baseline population, in that isolates with RG ≥70 were present at substantial levels in the former but absent from the latter.

Cross‐resistance between myclobutanil and tebuconazole and the genetic basis of tebuconazole resistance in Venturia inaequalis

“…Other authors found similar distributions of sensitivity to myclobutanil in baseline populations 1,24 . Some studies used one dose rather than a range in the sensitivity tests 22,28,29 . Using a single discriminatory dose is helpful where the sensitivity distribution is clearly bimodal, because the classification of an isolate is unchanged as long as the cut‐off is kept between the two modes.…”

“…worked with the diameters of single spore colonies, as in the present study. However, they and many other in vitro studies 6,23,27,28 used growth of the colony relative to the control (RG), whereas we used the actual mean growth at each concentration of chemical as response. We consider this preferable because dividing the growth by that of the control both introduces the error associated with the control into every measurement and loses a degree of freedom in the error estimate.…”

Antifungal Activities of Bacillus subtilis Lipopeptides to Two Venturia inaequalis Strains Possessing Different Tebuconazole Sensitivity

“…However, the repeated use of single-site chemical fungicide, such as demethylation inhibitor fungicide (DMIs), has rapidly led over time to the development of V. inaequalis strains with reduced sensitivity to triazoles fungicides (Köller et al, 1995; Gao et al, 2009; Xu et al, 2010; Villani et al, 2015). Besides, many cases of resistant strains of V. inaequalis have been recorded worldwide: in north and south America (Hildebrand et al, 1988; Köller et al, 1991, 1995; Braun and McRae, 1992; Carisse and Pelletier, 1994; Mondino et al, 2015), in Europa (Kunz et al, 1997; Gao et al, 2009; Xu et al, 2010) and in Asia (Shirane et al, 1996; Vijaya Palani and Lalithakumari, 1999). In organic farming, apple scab can be managed using sulfur or copper.…”

Efficient Control of Apple Scab with Targeted Spray Applications

“…The inclusion of dodine in spray programs is important since there is a low risk of resistance development by V. inaequalis. Furthermore, the new formulation of dodine is not associated with fruit russeting, an issue commonly observed after applications with previous formulations of the fungicide [12]. The limitation of the use of the systemic fungicides cyprodinil, dodine and difenoconazole is highly recommended because of the resistance developed by the pathogen [16].…”

“…Spray programs may include protectant fungicides with multisite activity, which affect primarily spore germination, such as dithianon (quinone class), captan (phthalimide class) and Bordeaux mixture (copper-based), or systemic fungicides, which are absorbed by the plant and affect fungal growth, such as dodine (guanidine class), difenoconazole (demethylation inhibitor group; DMIs), pyraclostrobin (quinone-outside inhibitors; QoIs) and the anilinopyrimidine cyprodinil [11]. However, among all fungicides used against apple scab, a complete loss of efficacy in practice has been observed only in the case of the QoIs [12][13][14]. A recent survey in major apple growing areas in Greece, including the area of the trial site, revealed for the first time the occurrence of V. inaequalis strains with differential multi-drug resistance to the fungicides pyraclostrobin, dodine, difenoconazole, boscalid and cyprodinil [15].…”