Occurrence of QoI Resistance and Detection of the G143A Mutation in Michigan Populations of Venturia inaequalis

Plant Disease ◽  
2011 ◽  
Vol 95 (8) ◽  
pp. 927-934 ◽  
Author(s):  
Kimberley E. Lesniak ◽  
Tyre J. Proffer ◽  
Janna L. Beckerman ◽  
George W. Sundin
Keyword(s):  
Spore Germination ◽  
Control Strategies ◽  
Venturia Inaequalis ◽  
Apple Scab ◽  
Pathogenic Fungi ◽  
Qoi Fungicides ◽  
Widespread Occurrence ◽  
Scab Control ◽  
High Level ◽  
Qoi Resistance

Control strategies for Venturia inaequalis rely heavily on chemical fungicides. Single-site fungicides such as the quinone-outside inhibitors (QoI) have been used in Michigan apple orchards for more than 11 years. In 2008, we sampled eight commercial orchards in the Fruit Ridge growing region of Michigan in which apple scab control failures were observed on ‘McIntosh’ apple following applications of kresoxim-methyl or trifloxystrobin. QoI resistance was assessed in 210 total isolates (a total of 17 orchards) using a spore germination assay and in 319 isolates using a polymerase chain reaction (PCR) assay to detect the G143A mutation located within the V. inaequalis cytochrome b gene (CYTB). The G143A mutation is known to confer high-level QoI resistance in plant-pathogenic fungi. QoI resistance was confirmed in 50 and 64% of the isolates tested with the spore germination and PCR assays, respectively, and there was a 97% concordance observed between the assays. In 2009, we sampled and examined an additional 1,201 V. inaequalis isolates from 64 orchards in Michigan and 86 isolates from four baseline sites in Ohio. All of these isolates were assayed for the G143A mutation and it was detected within 67 and 0% of the Michigan and Ohio isolates, respectively. Our results indicate the widespread occurrence of QoI resistance in Michigan commercial orchard populations of V. inaequalis. Loss of QoI fungicides further limits the arsenal of fungicides available to commercial apple growers for successful scab management.

scholarly journals Prevalence and Stability of Qualitative QoI Resistance in Populations of Venturia inaequalis in the Northeastern United States

Plant Disease ◽  
2014 ◽  
Vol 98 (8) ◽  
pp. 1122-1130 ◽  
Author(s):  
Zachary A. Frederick ◽  
Sara M. Villani ◽  
Daniel R. Cooley ◽  
Alan R. Biggs ◽  
Jessica J. Raes ◽  
...  
Keyword(s):  
United States ◽  
Venturia Inaequalis ◽  
Effective Means ◽  
Apple Scab ◽  
Northeastern United States ◽  
Qoi Fungicides ◽  
Quinone Outside Inhibitor ◽  
Management Paradigms ◽  
The Stability ◽  
Qoi Resistance

Quinone-outside-inhibitor (QoI) fungicides are a safe and effective means of managing apple scab caused by Venturia inaequalis. To determine the prevalence of both quantitative (partial) and qualitative (complete) QoI resistance in V. inaequalis in the northeastern United States, we sampled single-lesion conidial isolates (n = 4,481) from 120 commercial and research orchards from 2004 to 2011 with a range of exposure to QoI fungicides from none to several applications a year. In all, 67% of these orchard populations of V. inaequalis were sensitive to QoI fungicides, 28% exhibited QoI practical resistance, and 5% were not sensitive QoI fungicides but had not become practically resistant. Isolates with qualitative QoI resistance, conferred by the G143A cytochrome b gene mutation, were found in 13 of the 34 QoI-resistant orchard populations. To evaluate the stability of the G143A mutation, 27 isolates were selected from different orchard populations to represent the scope of regional populations. These isolates were subcultured continuously in the presence or absence of the QoI fungicide trifloxystrobin. All isolates that initially possessed qualitative resistance maintained the resistant genotype (G143A) for six transfers over 6 months in both the absence and presence of trifloxystrobin. Given the observed QoI resistance in orchard populations of V. inaequalis and the stability of the G143A mutation in individual isolates, apple scab management paradigms must encompass strategies to limit selection of QoI resistance in the sensitive orchard populations remaining in the region.

scholarly journals First Report of Practical Resistance to QoI Fungicides in Venturia inaequalis (Apple Scab) in Chile

Plant Disease ◽  
2006 ◽  
Vol 90 (3) ◽  
pp. 375-375 ◽  
Author(s):  
B. V. Sallato ◽  
B. A. Latorre ◽  
G. Aylwin
Keyword(s):  
Venturia Inaequalis ◽  
Apple Scab ◽  
Resistance Management ◽  
Qoi Fungicides ◽  
First Report ◽  
Resistance Factors ◽  
Action Committee ◽  
Scab Control ◽  
Resistant Strains

Resistance to the QoI fungicide (kresoxim-methyl) was detected in Venturia inaequalis (Cooke) Winter isolates from apple scab lesions collected from apple orchards in Chile after 5 to 6 years of commercial use of QoI fungicides. To reduce or prevent resistance development, QoI fungicides have been applied in blocks of two or three consecutive applications followed by an application of a fungicide with a different mode of action. However, the efficacy of kresoxim-methyl declined dramatically, providing almost no disease control in several orchards under high scab pressure in southern Chile, and resistance to the QoI fungicides was suspected. Conidia from at least five sporulating leaf scab lesions were collected from each of 10 and 14 orchards in 2003 and 2004, respectively. Fungicide sensitivity was determined by using a conidial germination test on potato dextrose agar acidified with 0.5 ml/liter of 95% lactic acid (APDA) containing kresoxim-methyl (Stroby 50 SC) at 0.0, 0.01, 0.1, and 1.0 μg/ml. The kresoxim-methyl concentration (0.01 μg/ml) has been used as a discriminatory dose in previous resistance studies (2). Germination was assessed for at least 100 conidia at each inhibitor concentration after 18 to 24 h of incubation at 20°C (1,2). Sensitivities were expressed as the percentage of germination relative to that on unamended APDA (1,2). These assays were repeated and all isolates with 50% effective dose (ED50) at >0.1 μg/ml were considered resistant. Isolates from eight of 10 and two of 14 orchards were resistant to kresoxim-methyl in 2003 and 2004, respectively. Relative germination varied from 42.4 to 100% for isolates from resistant orchards on 0.1 μg/ml of kresoxim-methyl, with eight isolates having ED50 at >0.1 μg/mL. Conidia from isolates obtained in 2003 also germinated on APDA amended with 1.0 μg/ml of kresoxim-methyl, with two isolates having ED50 at >1.0 μg/mL. Relative to the mean ED50 of 0.02 μg/ml of kresoxim-methyl determined for relative germination in North America baseline population of V. inaequalis (2), resistance factors for these Chilean isolates were estimated to range from 5 to 50. Conidia of resistant isolates continued to germinate in the presence of 0.1 μg/ml of kresoxim-methyl when 100 μg/ml of salicylhydroxamic acid was added to the amended APDA, suggesting that an alternative oxidase pathway was not responsible for the in vitro results obtained (2). A G143A target mutation has been detected in a V. inaequalis isolate from Chile, although no details concerning this finding were reported (Fungicide Resistance Action Committee, online publication, 2004 [ http://www.frac.info ]). The poor control of apple scab with QoI fungicides in Chilean orchards appears to be due to the development of resistant strains of V. inaequalis that occurred despite the limited use of these materials in blocks of two or three consecutive applications, as initially suggested for resistance management. Farmers have been advised to make only one application of QoI fungicide per year in a tank mixture with another type of fungicide or even to abandon the use of QoI fungicides for apple scab control. To our knowledge, this is the first report of a severe outbreak of apple scab due to the presence of populations of V. inaequalis resistant to QoI fungicides under commercial conditions in Chile. References: (1) R. B. Küng Färber et al. Pest Manage. Sci. 58:261, 2002. (2) G. Olaya and W. Köller. Plant Dis. 83:274, 1999.

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

Plant Disease ◽  
2004 ◽  
Vol 88 (5) ◽  
pp. 537-544 ◽  
Author(s):  
Wolfram Köller ◽  
D. M. Parker ◽  
W. W. Turechek ◽  
Cruz Avila-Adame ◽  
Keith Cronshaw
Keyword(s):  
Cytochrome B ◽  
Venturia Inaequalis ◽  
Apple Scab ◽  
Target Site ◽  
Site Mutation ◽  
Two Phase ◽  
Resistance Response ◽  
Population Responses ◽  
North Germany ◽  
Qoi Resistance

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.

scholarly journals First Report of QoI Resistance in Venturia inaequalis Causing Apple Scab in Apple Orchards in Turkey

Plant Disease ◽  
2016 ◽  
Vol 100 (5) ◽  
pp. 1016-1016 ◽  
Author(s):  
C. Turan ◽  
I. M. Nanni ◽  
N. Tosun ◽  
M. Collina
Keyword(s):  
Venturia Inaequalis ◽  
Apple Scab ◽  
Apple Orchards ◽  
First Report ◽  
Qoi Resistance

scholarly journals Field comparison of sterol-biosynthesis-inhibiting fungicides used alone and in combination with protectant fungicides for apple scab control

2005 ◽  
Vol 71 (1) ◽  
pp. 1-8 ◽  
Author(s):  
J. Warner
Keyword(s):  
Disease Control ◽  
Broad Spectrum ◽  
Venturia Inaequalis ◽  
Apple Scab ◽  
Field Trials ◽  
Sterol Biosynthesis ◽  
Scab Control

Several sterol-biosynthesis-inhibiting (SBI) fungicides were tested alone and in combination with a broad-spectrum protectant material in field trials for control of Venturia inaequalis. When used in a 10-day spray schedule, SBI fungicides, whether used alone or in a mixture, provided improved scab control as compared to the protectant material used alone. With bitertanol, diniconazole, and penconazole, the mixture also provided better scab control than when the SBI fungicide was used alone. However, with flusilazole and myclobutanil, no improvement in disease control occurred with the mixture as compared to the SBI fungicide alone. Fruit russeting was observed after a post-bloom application of hexaconazole and growth-regulating effects on the foliage were observed following cyproconazole and hexaconazole use. The use of SBI fungicides in mixtures to avoid or delay the development of fungus resistance is discussed.

Susceptibility of apple cultivars to apple scab and powdery mildew in Victoria, Australia

1998 ◽  
Vol 38 (6) ◽  
pp. 625 ◽  
Author(s):  
W. S. Washington ◽  
O. N. Villalta ◽  
J. Ingram ◽  
D. Bardon
Keyword(s):  
Powdery Mildew ◽  
Venturia Inaequalis ◽  
Natural Infection ◽  
Apple Scab ◽  
Podosphaera Leucotricha ◽  
Commercial Cultivars ◽  
Apple Cultivars ◽  
High Level

Summary. Twenty apple cultivars were assessed over 3 seasons for their susceptibility to natural infection with scab (caused by Venturia inaequalis) and powdery mildew (caused by Podosphaera leucotricha). It was evident that a number of important commercial cultivars are highly susceptible to leaf scab (average incidence of leaf scab: Pink Lady, 40.2% of leaves; Lady William, 37.1%; Braeburn, 20.0%; and New Jonagold, 18.7%) and powdery mildew (average incidence of leaf mildew: New Jonagold, 23.0%; Pink Lady, 17.8%). There was a high level of resistance to leaf scab in the cultivars Florina-Quérina, Redfree and Jonafree (all less than 1% incidence on leaves) and to powdery mildew in the cultivar Earlidel (no infection observed). The cultivars Jonathan, Bonza and Red Elstar showed a useful level of resistance to scab (average incidence of leaf scab between 1.6 and 3.2%), while the cultivars Red Fuji, HiEarly and Redfree all showed some resistance to powdery mildew (average incidence of mildew between 2.6 and 5.9%). Observations made on the low and variable numbers of fruit produced by some trees in their third year after planting showed that Pink Lady and Lady William were highly susceptible to fruit scab (67.6 or 57.6% scabbed fruit respectively). Twig infection by scab was also observed on shoots of the current season’s growth of Pink Lady.

scholarly journals A new apple scab risk model that integrates ascospore release infection risk and susceptible leaf area

2004 ◽  
Vol 57 ◽  
pp. 20-24 ◽  
Author(s):  
R.M Beresford ◽  
W.R. Henshall ◽  
J.W. Palmer
Keyword(s):  
Leaf Area ◽  
Risk Model ◽  
Venturia Inaequalis ◽  
Apple Scab ◽  
Black Spot ◽  
Infection Risk ◽  
Weather Data ◽  
Scab Control ◽  
Ascospore Release ◽  
Maturation Rate

A new model has been developed for assessing daytoday variation in risk of infection of apples by Venturia inaequalis the scab or black spot pathogen The model comprises three components ascospore availability wetnessbased infection risk based on Mills periods and susceptible leaf area The ascospore and wetnessbased infection risk components were adapted from previous models whereas the susceptible leaf area component is new When the model used weather data from Hawkes Bay and Nelson in spring 2003 the predicted risk incidence was determined mostly by wetnessbased infection risk but the magnitude of risk periods was greatly influenced by predicted ascospore release The susceptible leaf area component predicted a hitherto unidentified increase in infection risk after the peak in ascospore maturation rate had occurred The model is intended to assist in fungicide selection and timing for scab control in New Zealand apples but needs to be field tested before implementation

scholarly journals Overwintering of Conidia of Venturia inaequalis and the Contribution to Early Epidemics of Apple Scab

Plant Disease ◽  
2004 ◽  
Vol 88 (7) ◽  
pp. 751-757 ◽  
Author(s):  
I. J. Holb ◽  
B. Heijne ◽  
M. J. Jeger
Keyword(s):  
Venturia Inaequalis ◽  
Apple Scab ◽  
Early Spring ◽  
Shoot Length ◽  
Bud Break ◽  
Phenological Stage ◽  
Tight Cluster ◽  
The Mean ◽  
High Level ◽  
Previous Autumn

Overwintering of conidia of Venturia inaequalis associated with shoots and buds was determined, and the contribution to early spring epidemics of apple scab was evaluated during three consecutive seasons (1999 to 2001) in the Netherlands. Examinations of shoot samples collected before bud break showed that the percentage of shoots with superficial black fungal mycelia or conidia was above 65%, and the mean number of conidia on a 1-cm piece of shoot length ranged from 581 to 1,033. However, germination tests showed that the viability of conidia on shoots was less than 1.5%. No macroscopic scab lesions were detected on the scales of dormant buds. However, microscopic examinations of individual bud tissues demonstrated that the number of conidia was >3,000 per 100 buds in each year. The mean viability of conidia associated with buds ranged from 0.7 to 1.9% and from 3.7 to 10.5% for the outer and inner bud tissues, respectively. Results of field assessments at tight-cluster phenological stage showed that the percentage of infection caused by the viable overwintered conidia ranged from 0.3 to 3.8% in the various treatments. Our results indicated that conidia were unlikely to overwinter on the surface of shoots or outer bud tissues, where they were exposed to fluctuating environmental conditions, and, consequently, were unlikely to play a role in initiating an early epidemic of apple scab in the spring. However, our results indicated a risk from overwintered conidia in the inner bud tissues arising from a high level of scab the previous autumn. Therefore, orchards with high levels of apple scab, where ascosporic inoculum is much reduced, e.g., by sanitation, should be protected in early spring by means of fungicide treatment at green tip.

scholarly journals Prevalence of Myclobutanil Resistance and Difenoconazole Insensitivity in Populations of Venturia inaequalis

Plant Disease ◽  
2015 ◽  
Vol 99 (11) ◽  
pp. 1526-1536 ◽  
Author(s):  
Sara M. Villani ◽  
Alan R. Biggs ◽  
Daniel R. Cooley ◽  
Jessica J. Raes ◽  
Kerik D. Cox
Keyword(s):  
New England ◽  
Field Experiments ◽  
Venturia Inaequalis ◽  
Apple Scab ◽  
Apple Fruit ◽  
Scab Control ◽  
Dmi Fungicides ◽  
Commercial Research ◽  
Mature Apple ◽  
Single Lesion

Demethylation inhibitors (DMIs) are a class of single-site fungicides with high levels of protective and curative efficacy against Venturia inaequalis, the causal agent of apple scab. To determine the prevalence of resistance to the DMI fungicide myclobutanil, 3,987 single-lesion conidial V. inaequalis isolates from 141 commercial, research, and baseline orchard populations were examined throughout New England, the mid-Atlantic, and the Midwest from 2004 to 2013. Of these orchard populations, 63% had practical resistance, 13% had reduced sensitivity, and 24% were sensitive to myclobutanil. A sensitivity baseline for the recently introduced DMI fungicide difenoconazole was established to make comparisons with myclobutanil sensitivity in orchard populations. The mean effective concentration of difenoconazole at which mycelial growth was inhibited by 50% (EC50) was determined to be 0.002 μg ml−1 for 44 baseline isolates of V. inaequalis. From 2010 to 2013, 1,012 isolates of V. inaequalis from 37 of the 141 orchard populations above were screened for sensitivity to difenoconazole. In all, 1 orchard population had reduced sensitivity to difenoconazole, while the remaining 36 orchard populations were sensitive to the fungicide. In field experiments, difenoconazole demonstrated high levels of apple scab control on mature apple fruit, despite the fact that the population of V. inaequalis had practical resistance to difenoconazole. Although our results indicate widespread resistance to myclobutanil but not difenoconazole, due to the propensity for cross-sensitivity among DMI fungicides, growers with myclobutanil resistance should be cautious when using difenoconazole for disease management.

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