Evaluation of Tactics for Managing Resistance of Venturia inaequalis to Sterol Demethylation Inhibitors

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

BACKGROUND

Myclobutanil is one of the most widely used demethylation inhibitor (DMI) fungicides for the management of apple scab, caused by Venturia inaequalis. Strains of V. inaequalis resistant to myclobutanil have been reported across the world. Tebuconazole, another DMI fungicide, has been proposed as an alternative to myclobutanil, and the extent of cross-resistance with myclobutanil therefore needs to be evaluated. The sensitivity to tebuconazole and myclobutanil of a total of 40 isolates was determined. Half the isolates came from an isolated orchard which had never been sprayed with fungicides and half from orchards sprayed regularly with myclobutanil, but still with disease control problems. The progeny of a tebuconazole resistant (R) × sensitive (S) V. inaequalis cross were analyzed in order to improve understanding of the genetic control of tebuconazole sensitivity.

RESULTS

There is cross-resistance between myclobutanil and tebuconazole (r = 0.91; P < 0.001). Sensitivity to tebuconazole of the progeny of a R × S cross varied quantitatively in a pattern which implied at least two gene loci differing between the parental strains. In addition, the asymmetric distribution of the sensitivity in the progeny implied possible epistatic effects.

CONCLUSION

Resistance to myclobutanil and tebuconazole is strongly correlated. At least two genes are involved in the control of tebuconazole resistance in V. inaequalis.

Dynamics of Fungicide Sensitivity in Venturia effusa and Fungicide Efficacy under Field Conditions

Venturia effusa, which causes pecan scab, has developed resistance to fungicides that were once effective. Over 2 years, laboratory-based sensitivity of fentin hydroxide (TPTH) and tebuconazole in V. effusa and their efficacy under field conditions were compared. Leaf and nut scab were assessed on pecan trees receiving 10 applications of TPTH, tebuconazole, azoxystrobin, azoxystrobin plus tebuconazole, TPTH plus tebuconazole, or no fungicide (NTC) per year. Sensitivity of V. effusa on leaflets collected from treated and nontreated trees was assessed in June and September, respectively. The mean relative germination (RGe) on TPTH at 30 µg/ml was 10.9 and 40.9% in 2016 and 4.2 and 0.6% in 2017. Mean relative growth (RGr) on tebuconazole at 1 µg/ml was 45.5 and 34.6% in 2016 and 69.3 and 56.3% in 2017. In both years, leaf and nut scab were significantly lower on trees treated with azoxystrobin, azoxystrobin + tebuconazole, or TPTH + tebuconazole when compared with NTC and tebuconazole-treated trees. Compared with the NTC, tebuconazole did not significantly reduce leaf scab in 2017 or nut scab in either year, indicating that an RGr value between 34.6 and 69.3% is likely to result in a control failure on tebuconazole-treated trees. Although better activity was expected, TPTH reduced scab with RGe values between 0.6 and 40.9%. These results are valuable for developing fungicide sensitivity thresholds to better predict fungicide performance.

Tank Mixing Fungicides for Effectiveness Against Eastern Filbert Blight of Hazelnut

Hazelnut (Corylus avellana) production in Oregon primarily occurs on cultivars susceptible to Anisogramma anomala, the causal agent of eastern filbert blight (EFB). Management of EFB involves planting resistant cultivars, removal of cankered limbs, and the application of fungicides. Tank mixes of demethylation-inhibiting (DMI; Fungicide Resistance Action Committee [FRAC] group 3) or quinone outside inhibitor (QoI; FRAC group 11) fungicides with chlorothalonil (FRAC group M5) at full or reduced rates were evaluated for effectiveness against A. anomala. The use of chlorothalonil in a mix with a DMI or QoI fungicide was an effective treatment for EFB even if each component of the mix was at half the labeled rate. Different liquid or dry formulations of chlorothalonil were equally effective in a tank mix for EFB control. The combination of propiconazole (FRAC group 3) tank mixed with trifloxystrobin (FRAC group 11) was not effective, whereas trees treated with propiconazole tank mixed with pyraclostrobin (FRAC group 11) resulted in significantly fewer EFB cankers compared with nontreated trees. When using tank mixes for EFB management, DMI fungicides should remain at full rates while mixing with a half-rate of chlorothalonil. In contrast, QoI fungicides and chlorothalonil could both be used at half-rates and still maintain acceptable EFB control. Tank mixing chlorothalonil with fungicides at risk of resistance development can help maintain consistent EFB control and should help prevent or delay the emergence of fungicide-resistant A. anomala isolates.

Overexpression of the CYP51A1 Gene and Repeated Elements are Associated with Differential Sensitivity to DMI Fungicides in Venturia inaequalis

The involvement of overexpression of the CYP51A1 gene in Venturia inaequalis was investigated for isolates exhibiting differential sensitivity to the triazole demethylation inhibitor (DMI) fungicides myclobutanil and difenoconazole. Relative expression (RE) of the CYP51A1 gene was significantly greater (P < 0.0001) for isolates with resistance to both fungicides (MRDR phenotype) or with resistance to difenoconazole only (MSDR phenotype) compared with isolates that were resistant only to myclobutanil (MRDS phenotype) or sensitive to both fungicides (MSDS phenotype). An average of 9- and 13-fold increases in CYP51A1 RE were observed in isolates resistant to difenoconazole compared with isolates with MRDS and MSDS phenotypes, respectively. Linear regression analysis between isolate relative growth on myclobutanil-amended medium and log10 RE revealed that little to no variability in sensitivity to myclobutanil could be explained by CYP51A1 overexpression (R(2) = 0.078). To investigate CYP51A1 upstream anomalies associated with CYP51A1 overexpression or resistance to difenoconazole, Illumina sequencing was conducted for three isolates with resistance to difenoconazole and one baseline isolate. A repeated element, "EL 3,1,2", with the properties of a transcriptional enhancer was identified two to four times upstream of CYP51A1 in difenoconazole-resistant isolates but was not found in isolates with the MRDS phenotype. These results suggest that different mechanisms may govern resistance to different DMI fungicides in the triazole group.

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

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.

Do some IPM concepts contribute to the development of fungicide resistance? Lessons learned from the apple scab pathosystem in the United States

One goal of integrated pest management (IPM) as it is currently practiced is an overall reduction in fungicide use in the management of plant disease. Repeated and long-term success of the early broad-spectrum fungicides led to optimism about the capabilities of fungicides, but to an underestimation of the risk of fungicide resistance within agriculture. In 1913, Paul Ehrlich recognized that it was best to 'hit hard and hit early' to prevent microbes from evolving resistance to treatment. This tenet conflicts with the fungicide reduction strategies that have been widely promoted over the past 40 years as integral to IPM. The authors hypothesize that the approaches used to implement IPM have contributed to fungicide resistance problems and may still be driving that process in apple scab management and in IPM requests for proposals. This paper also proposes that IPM as it is currently practiced for plant diseases of perennial systems has been based on the wrong model, and that conceptual shifts in thinking are needed to address the problem of fungicide resistance.

Mixtures as a fungicide resistance management tactic

We have reviewed the experimental and modeling evidence on the use of mixtures of fungicides of differing modes of action as a resistance management tactic. The evidence supports the following conclusions. 1. Adding a mixing partner to a fungicide that is at-risk of resistance (without lowering the dose of the at-risk fungicide) reduces the rate of selection for fungicide resistance. This holds for the use of mixing partner fungicides that have either multi-site or single-site modes of action. The resulting predicted increase in the effective life of the at-risk fungicide can be large enough to be of practical relevance. The more effective the mixing partner (due to inherent activity and/or dose), the larger the reduction in selection and the larger the increase in effective life of the at-risk fungicide. 2. Adding a mixing partner while lowering the dose of the at-risk fungicide reduces the selection for fungicide resistance, without compromising effective disease control. The very few studies existing suggest that the reduction in selection is more sensitive to lowering the dose of the at-risk fungicide than to increasing the dose of the mixing partner. 3. Although there are very few studies, the existing evidence suggests that mixing two at-risk fungicides is also a useful resistance management tactic. The aspects that have received too little attention to draw generic conclusions about the effectiveness of fungicide mixtures as resistance management strategies are as follows: (i) the relative effect of the dose of the two mixing partners on selection for fungicide resistance, (ii) the effect of mixing on the effective life of a fungicide (the time from introduction of the fungicide mode of action to the time point where the fungicide can no longer maintain effective disease control), (iii) polygenically determined resistance, (iv) mixtures of two at-risk fungicides, (v) the emergence phase of resistance evolution and the effects of mixtures during this phase, and (vi) monocyclic diseases and nonfoliar diseases. The lack of studies on these aspects of mixture use of fungicides should be a warning against overinterpreting the findings in this review.

Governing principles can guide fungicide-resistance management tactics

Fungicide-resistance management would be more effective if principles governing the selection of resistant strains could be determined and validated. Such principles could then be used to predict whether a proposed change to a fungicide application program would decrease selection for resistant strains. In this review, we assess a governing principle that appears to have good predictive power. The principle states that reducing the product of the selection coefficient (defined as the difference between the per capita rate of increase of the sensitive and resistant strains) and the exposure time of the pathogen to the fungicide reduces the selection for resistance. We show that observations as well as modeling studies agree with the predicted effect (i.e., that a specific change to a fungicide program increased or decreased selection or was broadly neutral in its effect on selection) in 84% of the cases and that only 5% of the experimental results contradict predictions. We argue that the selection coefficient and exposure time principle can guide the development of resistance management tactics.

The usefulness of fungicide mixtures and alternation for delaying the selection for resistance in populations of Mycosphaerella graminicola on winter wheat: a modeling analysis

A fungicide resistance model (reported and tested previously) was amended to describe the development of resistance in Mycosphaerella graminicola populations in winter wheat (Triticum aestivum) crops in two sets of fields, connected by spore dispersal. The model was used to evaluate the usefulness of concurrent, alternating, or mixture use of two high-resistance-risk fungicides as resistance management strategies. We determined the effect on the usefulness of each strategy of (i) fitness costs of resistance, (ii) partial resistance to fungicides, (iii) differences in the dose-response curves and decay rates between fungicides, and (iv) different frequencies of the double-resistant strain at the start of a treatment strategy. Parameter values for the quinine outside inhibitor pyraclostrobin were used to represent two fungicides with differing modes of action. The effectiveness of each strategy was quantified as the maximum number of growing seasons that disease was effectively controlled in both sets of fields. For all scenarios, the maximum effective lives achieved by the use of the strategies were in the order mixtures ≥ alternation ≥ concurrent use. Mixtures were of particular benefit where the pathogen strain resistant to both modes of action incurred a fitness penalty or was present at a low initial frequency.

Mancozeb: Past, Present, and Future

This feature article reviews the broad-spectrum fungicide mancozeb. Introduced in 1962, it still plays a significant role in the world fungicide market. Mancozeb possesses a number of key attributes that have contributed toward its development into a globally important tool in modern chemical-based plant disease management. These attributes are discussed from the perspective of both public and private research.

Sensitivity Distributions of California Populations of Colletotrichum cereale to the DMI Fungicides Propiconazole, Myclobutanil, Tebuconazole, and Triadimefon

The baseline sensitivity of a California population of Colletotrichum cereale (turfgrass anthracnose) to the sterol demethylation inhibitor (DMI) fungicide propiconazole was determined using an in vitro assay with known reproducibility. The 50% effective dose (ED₅₀) values for propiconazole for a nonexposed, baseline population ranged from 0.025 to 0.35 μg/ml with a mean of 0.14 μg/ml. Examination of two DMI-exposed populations indicated an approximate increase of 6.5× in mean ED₅₀ values. In vivo testing of two isolates with ED₅₀ values of propiconazole at 0.15 and 0.90 μg/ml indicated reduced control for the less sensitive isolate by propiconazole at rates ≤38 μg/ml. It was determined that single discriminatory dose testing in vitro with propiconazole at 0.50 μg/ml could differentiate sensitive and resistant isolates. Using this dose, six additional populations were tested and DMI-exposed populations were found to be three to nine times less sensitive compared with the baseline population. Two populations were assayed for sensitivity to myclobutanil, tebuconazole, and triadimefon. Mean ED₅₀ values for a nonexposed population were 0.72, 0.082, and 5.6 μg/ml, respectively; for a DMI-exposed population, mean ED₅₀ values were 3.8, 0.35, and 18 μg/ml, respectively. This work provides information on the development of DMI resistance in populations of C. cereale in California and methodologies for future resistance monitoring for this pathogen.

Occurrence, Distribution, and Polymerase Chain Reaction-Based Detection of Resistance to Sterol Demethylation Inhibitor Fungicides in Populations of Blumeriella jaapii in Michigan

ABSTRACT The intensive use of site-specific fungicides in agricultural production provides a potent selective mechanism for increasing the frequency of fungicide-resistant isolates in pathogen populations. Practical resistance occurs when the frequency and levels of resistance are great enough to limit the effectiveness of disease control in the field. Cherry leaf spot (CLS), caused by the fungus Blumeriella jaapii, is a major disease of cherry trees in the Great Lakes region. The site-specific sterol demethylation inhibitor fungicides (DMIs) have been used extensively in the region. In 2002, CLS control failed in a Michigan orchard that had used the DMI fenbuconazole exclusively for 8 years. That control failure and our observations from around the state suggested that practical resistance had developed in B. jaapii. Field trial data covering 1989 to 2005 for the DMIs fenbuconazole and tebuconazole supported observations of reduced efficacy of DMIs for controlling CLS. To verify the occurrence of fungicide-resistant B. jaapii, monoconidial isolates were collected in two surveys and tested using a fungicide-amended medium. In one survey, 137 isolates from sites with different DMI histories (no known history, mixed or alternated with other fungicides, and exclusive use) were tested against 12 concentrations of fenbuconazole, tebuconazole, myclobutanil, and fenarimol. Isolates from sites with no prior DMI use were DMI sensitive (DMI(S) = no colony growth at 0.2 mug/ml a.i.) whereas the isolates from the site with prior exclusive use showed growth at DMI concentrations 3 to >100 times higher, and were rated as DMI resistant (DMI(R)). A second survey examined 1,530 monoconidial isolates, including 1,143 from 62 orchard sites in Michigan, where DMIs had been used to control CLS. Resistance to fenbuconazole was detected in 99.7% of the orchard isolates. All isolates from wild cherry trees were sensitive and isolates from feral and dooryard trees showed a range of sensitivities. A polymerase chain reaction (PCR)-based detection method for identifying B. jaapii and DMI(R) was developed and tested. The species-specific primer pair (Bj-F and Bj-R) based on introns in the CYP51 gene of B. jaapii, and the DMI(R)-specific primer pair (DMI-R-Bj-F and DMI-R-Bj-R) based on an insert found upstream of CYP51 in all DMI(R) isolates, provided an accurate and rapid method for detecting DMI(R) B. jaapii. The PCR-based identification method will facilitate timely decision making and continued monitoring of DMI(R) subpopulations in response to management programs.

Sensitivity of Venturia inaequalis Populations to Anilinopyrimidine Fungicides and Their Contribution to Scab Management in New York

Sensitivities of Venturia inaequalis isolates to the anilinopyrimidine fungicides (APs) pyrimethanil and cyprodinil were determined for nine populations by measuring the growth of colonies formed from germinating conidia derived from single scab lesions. At the discriminatory pyrimethanil dose of 0.2 μg ml^-1, the mean relative growth range measured for eight V. inaequalis populations (n = 39 to 74) never treated with AP fungicides varied from 18.1 to 48.2, translating into an approximately sixfold difference in mean baseline sensitivities. For the composite of all 469 isolates tested, sensitivities to pyrimethanil and to the sterol demethylation inhibitor (DMI) myclobutanil were significantly correlated. When isolates were organized into subpopulations based on their sensitivities to an individual fungicide, sensitivities to both fungicides declined in parallel through the highly and moderately sensitive spectra of subpopulations, but they diverged for isolates in subpopulations least sensitive to either fungicide. The result suggested that at least one of the multiple genes conferring DMI resistance also lowered the sensitivity to AP fungicides. The relative contribution of AP fungicides to scab management was evaluated at an experimental orchard representative of the Great Lakes region of the United States. Frequencies of DMI-resistant isolates of V. inaequalis had progressed to the stage of practical resistance at the site, and the sensitivity to pyrimethanil was similar to several commercial orchard populations never treated with APs. For management programs at the experimental site involving the AP fungicides cyprodinil and pyrimethanil and conducted from 1996 to 2000, the level of fruit and terminal leaf scab control was inferior to that of nonspecific protectants such as mancozeb or captan. For the control of scab on cluster leaves, the efficacy of AP fungicides equaled the performance of nonspecific protectants. This modest contribution of AP fungicides to scab management might have been caused by a lack of the extended cool temperature conditions that were conducive to AP performance in northern Europe in previous studies, and/or by the reduced sensitivity to AP fungicides in this DMI-resistant V. inaequalis population.

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

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.

Management of Late Leaf Spot of Peanut with Benomyl and Chlorothalonil: A Study in Preserving Fungicide Utility

Recent registration of sterol biosynthesis inhibitor and strobilurin fungicides for control of early (Cercospora arachidicola) and late (Cercosporidium personatum) leaf spot diseases of peanut (Arachis hypogaea) has renewed interest in the potential for loss of disease control due to fungicide resistance. The objectives of this study were to use the systemic fungicide benomyl, the protectant fungicide chlorothalonil, and late leaf spot of peanut as a model system to compare fungicide application strategies for fungicide resistance management. Field experiments were conducted at Tifton and Plains, GA, in 1995 and 1996 to determine the effects of alternate applications, mixtures, and alternating block applications of chlorothalonil and benomyl compared with full-season applications of two rates of chlorothalonil and two rates of benomyl alone on late leaf spot of peanut and on the proportion of the pathogen population resistant to benomyl following the various regimes. Tank mix combinations of half rates of the two fungicides and alternations of the full rates of the two fungicides provided better (P ≤ 0.05) control of late leaf spot than full-season applications of either rate of benomyl alone, and were comparable to full rates of chlorothalonil alone. Neither tank mixes nor alternating sprays prevented an increase in the relative frequency of benomyl-resistant isolates compared with other treatments in which benomyl was used. Both mixtures and alternate applications of chlorothalonil and benomyl were effective for management of leaf spot in fields where benomyl alone did not provide season-long leaf spot control.

Sensitivity to Azoxystrobin Among Isolates of Uncinula necator: Baseline Distribution and Relationship to Myclobutanil Sensitivity

Two hundred fifty-six single-conidial chain isolates of Uncinula necator were assayed for their sensitivity to azoxystrobin and myclobutanil. These isolates were collected from two sites in New York in 1999: an "organic" vineyard where no synthetic fungicides have been used (baseline population) and a commercial vineyard having a history of compromised powdery mildew control with myclobutanil (demethylation inhibitor [DMI]-resistant population). Mean coefficients of variance for a leaf disk assay used to test fungicide sensitivities were 31% for azoxystrobin and 41% for myclobutanil. Baseline ED₅₀ values ranged from 0.0037 to 0.028 μg/ml (mean 0.0097μg/ml) for azoxystrobin and from 0.0049 to 0.69 μg/ml (mean 0.075 μg/ml) for myclobutanil. A shift in the mean ED50 value for azoxystrobin to 0.018 μg/ml was observed in the DMI-resistant population; with the strongest shift observed for isolates collected from vines treated exclusively with myclobutanil (0.024 μg/ml). For the 256 tested isolates, there was a moderate, but statistically significant, correlation between azoxystrobin and myclobutanil sensitivities (R² = 0.36, P < 0.001). Tests with three other strobilurin fungicides (kresoxim-methyl, pyraclostrobin, and trifloxystrobin) indicate clear differences in the intrinsic activity of these compounds against U. necator, and the applicability of the methods developed with azoxystrobin for assays with pyraclostrobin and trifloxystrobin. Isolates from the high and low ends of the azoxystrobin sensitivity distribution (15× difference in mean ED₅₀ values) were equally controlled in planta by protectant or postinfection treatment with azoxystrobin at 250 μg a.i./ml, but postinfection application at lower rates (2.5 and 25 μg a.i./ml) resulted in a 41 and 44% decrease, respectively, in the control of the low-sensitivity isolates versus high-sensitivity isolates. The results of this study document the baseline sensitivity distribution of U. necator to azoxystrobin, provide evidence of partial cross-sensitivity between azoxystrobin and myclobutanil, and illustrate the potential selection for individuals with reduced sensitivity (quantitative range) to azoxystrobin by postinfection application and reduced rates of this fungicide.

Evidence for the Predisposition of Fungicide-Resistant Isolates of Venturia inaequalis to a Preferential Selection for Resistance to Other Fungicides

In the United States, populations of the apple scab pathogen Venturia inaequalis have progressed through three consecutive rounds of fungicide resistance development, first to dodine, then to the benzimidazoles, and most recently to the sterol demethylation inhibitors (DMIs). Analysis of extensive monitoring data have to date provided no indication of detectable cross-resistance or partial cross-resistance of V. inaequalis populations to the three unrelated classes of fungicides prior to the selection of resistant subpopulations. However, in this study, resistance to both benomyl and DMIs developed to significantly higher frequencies within the previously established dodine-resistant population than in the population sensitive to dodine. Accelerated selection of phenotypes double resistant to dodine and the DMI fenarimol was apparent over the course of distinct seasons of apple scab management with either dodine or fenarimol. The data provide evidence for an accelerated speed of resistance development among phenotypes of V. inaequalis already resistant to an unrelated fungicide. This finding represents a departure from the previous model, which assumed entirely independent rounds of resistance developments. The data indicate that phenotypes of V. inaequalis might not only be selected for the trait of fungicide resistance but also for traits allowing a more flexible response to changes in the environment where they compete.

The 14alpha-Demethylasse(CYP51A1) Gene is Overexpressed in Venturia inaequalis Strains Resistant to Myclobutanil

ABSTRACT We identified the cytochrome P450 sterol 14alpha-demethylase (CYP51A1) gene from Venturia inaequalis and optional insertions located upstream from CYP51A1 and evaluated their potential role in conferring resistance to the sterol demethylation-inhibitor (DMI) fungicide my-clobutanil. The CYP51A1 gene was completely sequenced from one my-clobutanil sensitive (S) and two myclobutanil-resistant (R) strains. No nucleotide variation was found when the three sequences were aligned. Allele-specific polymerase chain reaction (PCR) analysis indicated that a previously described single base pair mutation that correlated with resistance to DMI fungicides in strains of other filamentous fungi was absent in 19 S and 32 R strains of V. inaequalis from Michigan and elsewhere. The sequencing results and PCR analyses suggest that resistance in these strains was not due to a mutation in the sterol demethylase target site for DMI fungicides. Expression of CYP51A1 was determined for strains from an orchard that had never been sprayed with DMI fungicides (baseline orchard), and the data provided a reference for evaluating the expression of strains collected from a research orchard and from three commercial Michigan apple orchards with a long history of DMI use and a high frequency of R strains. Overexpression of CYP51A1 was significantly higher in 9 of 11 R strains from the research orchard than in S strains from the baseline orchard. The high expression was correlated with the presence of a 553-bp insertion located upstream of CYP51A1. Overexpression of the CYP51A1 gene was also detected in eight of eight, five of nine, and nine of nine R strains from three commercial orchards, but the insertion was not detected in the majority of these strains. The results suggest that overexpression of the target-site CYP51A1 gene is an important mechanism of resistance in some field resistant strains of V. inaequalis, but other mechanisms of resistance also appear to exist.

Interactive Effects of Dodine and the DMI Fungicide Fenarimol in the Control of Apple Scab

Interactive effects of a mixture of fenarimol and dodine in the inhibition of Venturi inaequalis populations and the risk that resistance will develop when the mixture is used for control of apple scab were investigated. Interactive mixture effects were determined for a composite population (n = 1007) representative of the Great Lakes apple-growing region and for a population (n = 1,295) sampled over three years from a single experimental orchard subjected to various fungicide treatments. Several interactive effects were identified: (i) subpopulations of V. inaequalis isolates resistant to dodine contained a higher proportion of isolates also resistant to fenarimol, (ii) synergistic effects were apparent for a large proportion of dodine- or fenarimol-resistant isolates when tested in mixture, and (iii) the level of fenarimol resistance was higher in the subpopulation resisting the mixture than in the subpopulation resisting fenarimol alone. Interactive effects determined under in vivo test conditions were of low relevance in the control of apple scab. The partly pleiotropic resistance of dodine and fenarimol had no apparent impact on the fenarimol sensitivities of dodine-resistant populations, and expression of synergistic mixture effects were of low or no significance under orchard conditions of scab control. Treatments of apple trees with a half-rate mixture of fenarimol and dodine selected V. inaequalis isolates resistant to the mixture. Such isolates were poorly controlled by the mixture and by each component applied alone at twice their mixture rates; therefore, mixture-resistant isolates will be selected and must be controlled by other means. The level of fenarimol resistance was high for the mixture-resistant subpopulation; therefore, increasing the mixture rate of fenarimol is unlikely to significantly improve control of these isolates. The advantage of a half-rate mixture of fenarimol and dodine in scab management resides in a substantially lower frequency of selectable isolates compared to the frequencies encountered by fenarimol and dodine applied alone.