A Two-Phase Resistance Response of Venturia inaequalis Populations to the QoI Fungicides Kresoxim-Methyl and Trifloxystrobin

Abstract

The class of fungicides acting as respiration inhibitors by binding to the Qo center of cyto-chrome b (QoIs) are in wide use for the management of apple scab caused by Venturia inaequalis. In order to assess responses of V. inaequalis populations to treatments with QoIs, sensitivities of isolates were determined for germinating conidia or for mycelial colonies developing from germinating conidia. Under both test conditions, inhibitory potencies of kresoxim-methyl and trifloxystrobin were largely equivalent. V. inaequalis populations treated with QoIs in a commercial and an experimental orchard both responded with significant shifts toward declining QoI sensitivities. However, the population responses were quantitative in nature, and highly resistant isolates indicative of a cytochrome b target site mutation were not detected. V. inaequalis populations from both orchards investigated also were fully resistant to sterol de-methylation-inhibiting fungicides (DMIs) such as fenarimol and myclobutanil, but isolate sensitivities to QoIs and DMIs were largely unrelated. Performance tests with kresoxim-methyl and trifloxystrobin at the experimental orchard diagnosed as DMI-resistant revealed that the quantitative shift toward declining QoI sensitivities did not constitute the status of practical QoI resistance. In contrast to these quantitative responses, emergence of qualitative QoI resistance was documented for V. inaequalis in an orchard in North Germany, which had been treated intensively with a total of 25 QoI applications over four consecutive seasons. Isolates retrieved from the orchard were highly resistant to both kresoxim-methyl and trifloxystrobin and were characterized as G143A cytochrome b mutants. The results indicated that the paths of QoI resistance can be both quantitative and qualitative in nature. A similar phenomenon has not been described before. Circumstantial evidence suggests that the quantitative phase of V. inaequalis population responses to QoIs might be succeeded by a quantitative selection of highly resistant G143A target-site mutants.

Venturia inaequalis resistance in local Spanish cider apple germplasm under controlled and field conditions

“…Host resistance is the most preferred method for the sustainable control of apple scab and fire blight, and is gaining an increasing momentum as the options for chemical control become severely limited due to new regulations and to the emergence of pathogen populations showing resistance to chemical fungicides and bactericides (Schnabel and Parisi 1997;Schnabel and Jones 2001;Norelli et al 2003;Koller et al 2004;Brun et al 2008). The production of traditional cider in Spain is based on local cultivars, whose level of resistance to these diseases is unknown.…”

“…The first cases of resistance to benzimidazoles were detected in those countries where these fungicides were most intensively used, and various resistant strains to IBE were found several years ago in France and other European countries (Schnabel and Parisi 1997), and later in commercial apple orchards in Indiana, USA (Quello et al 2010). Also, V. inaequalis strains resistant to sterol demethylation inhibitors were isolated already nearly a decade ago (Schnabel and Jones 2001;Koller et al 2004;Ma and Michailides 2005). In spite of that, control of apple scab in many countries still relies heavily on the use of these fungicides.…”

Genus Monilinia on pome and stone fruit species

“…However, strobilurine, azoxystrobine and pyraclostrobine fungicides are considered to be high risk fungicides in terms of resistance development (Amiri et al, 2008). Actually, reduced sensitivity of the pathogens to these groups of substances has been registered in a number of apple orchards and vineyards (Gullino et al, 2004;Köller et al, 2004), but not for fruit rot causal agents on peaches (Amiri et al, 2008). Other examples of new products with different modes of action are fenhexamid and boscalid.…”

Site-Specific Risk Factors for Ray Blight in Tasmanian Pyrethrum Fields

“…Fungicides included in the current management recommendations are from the strobilurin, carboxamide, and demethylation inhibitor groups. Rapid directional selection toward fungicide resistance has been reported for strobilurin (5,12,16,19) and carboxamide fungicides in other pathosystems (1,20,21,42). These fungicides are considered medium-to-high risk for the selection of resistant strains by the Fungicide Resistance Action Committee.…”