Diversity of kresoxim-methyl sensitivities in baseline populations ofVenturia inaequalis

Edeine B1 produced by Brevibacillus brevis reduces the virulence of a plant pathogenic fungus by inhibiting mitochondrial respiration

Plant pathogenic fungi cause serious diseases, which result in the loss of crop yields and reduce the quality of crops worldwide. To counteract the escalating risks of chemical fungicides, interest in biological control agents to manage plant diseases has significantly increased. In this study, we comprehensively screened microbial culture filtrates using a yeast screening system to find microbes exhibiting respiratory inhibition activity. Consequently, we found a soil-borne microbe Brevibacillus brevis HK544 strain exhibiting a respiration inhibitory activity and identified edeine B1 (EB1) from the culture filtrate of HK544 as the active compound of the respiration inhibition activity. Furthermore, against a plant pathogenic fungus Fusarium graminearum, our results showed that EB1 has effects on multiple aspects of respiration with the downregulation of most of the mitochondrial-related genes based on transcriptome analysis, differential EB1-sensitivity from targeted mutagenesis, and the synergistic effects of EB1 with electron transport chain complex inhibitors. With the promising plant disease control efficacy of B. brevis HK544 producing EB1, our results suggest that B. brevis HK544 has potential as a biocontrol agent for Fusarium head blight.IMPORTANCEAs a necrotrophic fungus, Fusarium graminearum is a highly destructive pathogen causing severe diseases in cereal crops and mycotoxin contamination in grains. Although chemical control is considered the primary approach to control plant disease caused by F. graminearum, fungicide-resistant strains have been detected in the field after long-term continuous application of fungicides. Moreover, applying chemical fungicides that trigger mycotoxin biosynthesis is a great concern for many researchers. Biocontrol of Fusarium head blight (FHB) by biological control agents (BCAs) represents an alternative approach and could be used as part of the integrated management of FHB and mycotoxin production. The most extensive studies on bacterial BCAs-fungal communications in agroecosystems have focused on antibiosis. Although many BCAs in agricultural ecology have already been used for fungal disease control, the molecular mechanisms of antibiotics produced by BCAs remain to be elucidated. Here, we found a potential BCA (Brevibacillus brevis HK544) with a strong antifungal activity based on the respiration inhibition activity with its active compound edeine B1 (EB1). Furthermore, our results showed that EB1 secreted by HK544 suppresses the expression of the mitochondria-related genes of F. graminearum, subsequently suppressing fungal development and the virulence of F. graminearum. In addition, EB1 exhibited a synergism with complex I inhibitors such as rotenone and fenazaquin. Our work extends our understanding of how B. brevis HK544 exhibits antifungal activity and suggests that the B. brevis HK544 strain could be a valuable source for developing new crop protectants to control F. graminearum.

Molecular and Biochemical Characterization of Field Resistant Isolates of Glomerella cingulata to Pyraclostrobin in China

Glomerella leaf spot (GLS) caused by Glomerella cingulata is a destructive disease that results in severe defoliation and fruit spots in apples worldwide. The compound of pyraclostrobin and tebuconazole was registered in 2018 in China to control GLS. In 2020, the high-level resistance of G. cingulata to pyraclostrobin was found in the field in Shandong Province, with a resistance frequency of 4.8%. Except for a significant decrease in virulence, there was no fitness penalty in mycelial growth, sporulation, and stress tolerance of G. cingulata associated with the resistance to pyraclostrobin. No cross-resistance was detected between pyraclostrobin and tebuconazole or bromothalonil. The point mutation GGT (G) → GCT (A) at codon 143 in the Cytochrome b (Cytb) gene was identified in the pyraclostrobin-resistant isolates. Molecular docking analysis suggested that G143A significantly alters the affinity of pyraclostrobin to the Cytb protein. Based on the point mutation (G143A) in the Cytb gene, a cleaved amplified polymorphic sequences method was developed to detect pyraclostrobin resistance in G. cingulata populations. Results of this study will provide valuable information for the scientific management of GLS.

Identification of Septoria glycines Isolates from Soybean with Resistance to Quinone Outside Inhibitor Fungicides

Brown spot, caused by Septoria glycines, is a common foliar disease of soybean (Glycine max). Applications of fungicide products that contain quinone outside inhibitor (QoI) active ingredients to soybean fields have contributed to the selection and development of QoI-resistant populations of S. glycines. We investigated the molecular mechanisms of QoI-resistance in these populations through targeted analysis of the cytochrome b gene. Isolates of S. glycines collected from several soybean fields over different seasons varied in sensitivity to QoI fungicides. Characterization of the cytochrome b gene revealed a mutation that changed an amino acid from glycine to alanine at codon 143 - one that is generally associated with QoI fungicide resistances. A PCR assay was developed that allowed successful discrimination of QoI-sensitive and -resistant isolates based on the G143A mutation. Results of this study demonstrated that 47.5% of S. glycines isolates tested were resistant to QoI fungicides. Accurate monitoring of this mutation will help slow the spread of QoI resistance and will be important for fungicide resistant management in this pathosystem.

Quinone outside inhibitors affect DON biosynthesis, mitochondrial structure and toxisome formation in Fusarium graminearum

Quinone outside inhibitors (QoIs) are currently extensively used agricultural fungicides. However, the application of QoIs in controlling Fusarium graminearum was rarely reported. No information is available on pharmacological characteristics of QoIs against F. graminearum, as well as their effects on DON biosynthesis. Here, we found that six QoIs exhibited an excellent fungicidal activity against F. graminearum based on mycelial growth and spore germination. ATP production assay further confirmed that QoIs decreased ATP production via inhibiting mitochondrial respiration, which contributes their fungicidal activity. Unfortunately, QoIs can stimulate DON production and up-regulate the expression of Tri5 and Tri6 genes. Additionally, acetyl-CoA, the basic precursor of DON biosynthesis, significantly increased as affected by QoIs, furtherly indicating that QoIs indeed enhance DON biosynthesis. We also found that QoIs can accelerate the formation of toxisomes and enhance the fluorescence signals of Tri-GFP labeled toxisomes, which may be due to the effect of QoIs on toxisome-related endoplasmic reticulum-remodeling. In addition, QoIs could disrupt the homeostasis of mitochondrial dynamics, resulting in the fragmented mitochondria. Finally, the simulated inoculation assay with wheat grains further verified that QoIs can stimulate DON production relative to wheat grain weight, especially relative to mycelial biomass.

Cytochrome b Mutations F129L and G143A Confer Resistance to Azoxystrobin in Cercospora nicotianae, the Frogeye Leaf Spot Pathogen of Tobacco

Azoxystrobin is the only synthetic, systemic fungicide labeled in the United States for management of frogeye leaf spot (FLS) of tobacco (Nicotiana tabacum L.), caused by Cercospora nicotianae. Though traditionally considered a minor disease in the United States, FLS has recently become yield and quality limiting. In 2016 and 2017, 100 C. nicotianae isolates were collected from symptomatic tobacco from eight counties in Kentucky, United States. Prior to azoxystrobin sensitivity testing, some C. nicotianae isolates were found to utilize the alternative oxidase pathway and, after assay comparisons, conidial germination was utilized to evaluate sensitivity in C. nicotianae as opposed to mycelial growth. Azoxystrobin sensitivity was determined by establishing the effective concentration to inhibit 50% conidial germination (EC50) for 47 (in 2016) and 53 (in 2017) C. nicotianae isolates. Distributions of C. nicotianae EC50 values indicated three qualitative levels of sensitivity to azoxystrobin. Partial cytochrome b sequence, encompassing the F129L and G143A mutation sites, indicated single-nucleotide polymorphisms (SNPs) conferring the F129L mutation in C. nicotianae of moderate resistance (azoxystrobin at 0.177 ≤ EC50 ≤ 0.535 µg/ml) and the G143A mutation in isolates with an azoxystrobin-resistant phenotype (azoxystrobin EC50 > 1.15 µg/ml). Higher frequencies of resistant isolates were identified from greenhouse transplant (4 of 17) and conventionally produced (58 of 62) tobacco samples, as compared with field-grown tobacco (<4 weeks prior to harvest; 4 of 62) or organically produced samples (1 of 7), respectively. Together, these results suggest that resistance to azoxystrobin in C. nicotianae occurs broadly in Kentucky, and generate new hypotheses about selection pressure affecting resistance mutation frequencies.

Antifungal activity of essential oils on two Venturia inaequalis strains with different sensitivities to tebuconazole

The antifungal activity of seven essential oils (eucalyptus, clove, mint, oregano, savory, tea tree, and thyme) was studied on Venturia inaequalis, the fungus responsible for apple scab. The composition of the essential oils was checked by gas chromatography-mass spectrometry. Each essential oil had its main compound. Liquid tests were performed to calculate the IC₅₀ of essential oils as well as their majority compounds. The tests were made on two strains with different sensitivities to tebuconazole: S755, the sensitive strain, and rs552, the strain with reduced sensitivity. Copper sulfate was selected as the reference mineral fungicidal substance. IC₅₀ with confidence intervals were calculated after three independent experiments. The results showed that all essential oils and all major compounds had in vitro antifungal activities. Moreover, it was highlighted that the effectiveness of four essential oils (clove, eucalyptus, mint, and savory) was higher than copper sulfate on both strains. For each strain, the best activity was obtained using clove and eucalyptus essential oils. For clove, the IC₅₀ obtained on the sensitive strain (5.2 mg/L [4.0-6.7 mg/L]) was statistically lower than the IC₅₀ of reduced sensitivity strain (14 mg/L [11.1-17.5 mg/L]). In contrast, for eucalyptus essential oil, the IC₅₀ were not different with respectively 9.4-13.0 and 12.2-17.9 mg/L for S755 and rs552 strains. For mint, origano, savory, tea tree, and thyme, IC₅₀ were always the best on rs552 strain. The majority compounds were not necessarily more efficient than their corresponding oils; only eugenol (for clove) and carvacrol (for oregano and savory) seemed to be more effective on S755 strain. On the other hand, rs552 strain seemed to be more sensitive to essential oils than S755 strain. In overall, it was shown that essential oils have different antifungal activities but do not have the same antifungal activities depending on the fungus strain used.

The Detection and Characterization of QoI-Resistant Didymella rabiei Causing Ascochyta Blight of Chickpea in Montana

Ascochyta blight (AB) of pulse crops (chickpea, field pea, and lentils) causes yield loss in Montana, where 1.2 million acres was planted to pulses in 2016. Pyraclostrobin and azoxystrobin, quinone outside inhibitor (QoI) fungicides, have been the choice of farmers for the management of AB in pulses. However, a G143A mutation in the cytochrome b gene has been reported to confer resistance to QoI fungicides. A total of 990 isolates of AB-causing fungi were isolated and screened for QoI resistance. Out of these, 10% were isolated from chickpea, 81% were isolated from field peas, and 9% isolated from lentil. These were from a survey of grower's fields and seed lots (chickpea = 17, field pea = 131, and lentil = 21) from 23 counties in Montana sent to the Regional Pulse Crop Diagnostic Laboratory, Bozeman, MT, United States for testing. Fungicide-resistant Didymella rabiei isolates were found in one chickpea seed lot each sent from Daniels, McCone and Valley Counties, MT, from seed produced in 2015 and 2016. Multiple alignment analysis of amino acid sequences showed a missense mutation that replaced the codon for amino acid 143 from GGT to GCT, introducing an amino acid change from glycine to alanine (G143A), which is reported to be associated with QoI resistance. Under greenhouse conditions, disease severity was significantly higher on pyraclostrobin-treated chickpea plants inoculated with QoI-resistant isolates of D. rabiei than sensitive isolates (p-value = 0.001). This indicates that where resistant isolates are located, fungicide failures may be observed in the field. D. rabiei-specific polymerase chain reaction primer sets and hydrolysis probes were developed to efficiently discriminate QoI- sensitive and - resistant isolates.

Sensitivity of Mycosphaerella pinodes to Pyraclostrobin Fungicide

Mycosphaerella blight, caused by Mycosphaerella pinodes, is a destructive disease of field pea that is managed using foliar fungicides. Strobilurin fungicides have been used in western Canada for disease management since 2003. To assess the baseline sensitivities of M. pinodes isolates to the strobilurin fungicide pyraclostrobin, the effective concentration to reduce mycelial growth by 50% (EC₅₀) was determined for 70 isolates collected prior to 2003 from Alberta, Saskatchewan, North Dakota, and Washington State. Each of these isolates was sensitive to pyraclostrobin, with EC₅₀ values ranging from 0.03 to 0.29 mg liter^-1. The pyraclostrobin concentrations required to reduce conidia germination by 50% was lower, ranging from 0.008 to 0.041 mg liter^-1. In all, 324 isolates collected in 2010 and 2011 were tested for high levels of insensitivity by examining mycelial growth using a discriminatory dose of 5 mg liter^-1. Nineteen isolates were highly insensitive to pyraclostrobin, with EC₅₀ values of 80 to 216 mg liter^-1. Conidia of these isolates germinated when exposed to a discriminatory dose of 0.1 mg liter^-1. Insensitive isolates infected field pea plants treated with pyraclostrobin but sensitive isolates did not. The identification of insensitive isolates indicates that insensitivity may be emerging in the pathogen population.

Heteroplasmy of the cytochrome b gene in Venturia inaequalis and its involvement in quantitative and practical resistance to trifloxystrobin

Quantitative (partial) and qualitative (complete) resistance responses to quinone outside inhibitor (QoI) fungicides have been documented for the apple scab pathogen Venturia inaequalis. Resistance monitoring efforts have traditionally focused on the detection of qualitative resistance based on a single point mutation, G143A, within the cytochrome b (cyt b) gene. In order to better understand the role of heteroplasmy of the cyt b gene in the QoI resistance response for isolates and populations of V. inaequalis, an allele-specific quantitative polymerase chain reaction was developed to quantify the relative abundance of the A143 (resistant) allele in 45 isolates of V. inaequalis with differing in vitro resistance responses to the QoI fungicide trifloxystrobin. Although a high relative abundance of the A143 allele (>62%) was associated with isolates with high resistance responses (50 to 100% relative growth on trifloxystrobin-amended medium), heteroplasmy of the cyt b gene was not the primary factor involved in isolates with moderate resistance responses (29 to 49% relative growth). The relative abundance of the A143 allele in isolates with moderate resistance to trifloxystrobin rarely exceeded 8%, suggesting that other resistance mechanisms are involved in moderate resistance and, therefore, that the Qol resistance response is polygenic. In research orchards where QoI fungicides failed to control apple scab (practical resistance), field trials were conducted to demonstrate the link between practical resistance and the abundance of the A143 allele. Relative abundance of the A143 allele in these orchard populations exceeded 20% in 2011 and 2012. Similarly, of the eight additional commercial orchards screened in 2011, the relative abundance of the A143 allele always exceeded 20% in those with QoI practical resistance. Although heteroplasmy of the cyt b gene did not entirely explain the response of isolates with moderate resistance to QoIs, the relative abundance of A143 in orchard populations of V. inaequalis helps to explain the point of emergence for practical resistance to trifloxystrobin across several orchard populations with differing production histories.

Molecular characterization of pyraclostrobin resistance and structural diversity of the cytochrome b gene in Botrytis cinerea from apple

Botrytis cinerea isolates obtained from apple orchards were screened for resistance to the quinone outside inhibitor (QoI) pyraclostrobin. Of the 220 isolates tested, 43 (19.5%) were resistant to pyraclostrobin. Analysis of partial sequences of the cytochrome b gene (cyt b) in five pyraclostrobin-resistant (PR) and five pyraclostrobin-sensitive (PS) isolates showed that PR isolates harbored the point mutation leading to the substitution of glycine by alanine at codon position 143 in cyt b (G143A). Two pairs of allele-specific primers were designed based on this point mutation, and allele-specific polymerase chain reaction analysis with these primers showed that all 73 PR isolates (including 30 collected from decayed apple fruit) harbored the G143A mutation but PS isolates did not. Six pairs of primers were designed to analyze the presence of various introns in cyt b. There were six types (I to VI) of cyt b present in 247 isolates of B. cinerea collected from various apple-production areas in Washington State. Of the 247 isolates, 23 had type I cyt b containing all four introns (Bcbi-67/68, Bcbi-131/132, Bcbi-143/144, and Bcbi-164), 176 had type II cyt b containing three introns (Bcbi-67/68, Bcbi-131/132, and Bcbi-164), six had type III cyt b containing two introns (Bcbi-67/68 and Bcbi-131/132), one had type IV cyt b containing two introns (Bcbi-131/132 and Bcbi-164), one had type V cyt b containing only the Bcbi-131/132 intron, and 40 had type VI cyt b containing no introns. This is the first report of types III to VI cyt b present in B. cinerea. All 73 PR isolates did not carry the Bcbi-143/144 intron in cyt b. Of the 247 isolates tested, >90% did not carry the Bcbi-143/144 intron in cyt b, suggesting that B. cinerea populations from apple pose a high inherent risk for the development of resistance to QoIs because the presence of this intron in cyt b prevents the occurrence of G143A-mediated resistance. Analysis of genetic background based on three microsatellite primers showed that PR isolates originated from different lineages, and there was no correlation between cyt b types (I, II, and III) and the genetic background of the isolates; however, isolates carrying type VI cyt b might originate from the same lineage.

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

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.

Identification of Resistance to Multiple Fungicides in Field Populations of Venturia inaequalis

Venturia inaequalis, the causal agent of apple scab, is controlled primarily by fungicides. Long-term, extensive fungicide use has led to the development of resistance to multiple fungicides. To assess fungicide resistance, isolates of V. inaequalis were collected from Indiana and Michigan orchards. Single-spore derived isolates were evaluated by mycelium growth assays with previously determined discriminatory doses on media containing dodine, kresoxim-methyl, myclobutanil, or thiophanate-methyl. Of 195 isolates tested, 5.2, 0.7, 57.0, and 92.6% of isolates were found to be resistant to dodine, kresoxim-methyl, myclobutanil, and thiophanate-methyl, respectively. This is the first report of kresoxim-methyl field resistance in these states. Isolates resistant or shifted to a single fungicide were often found to have multiple fungicide resistance. Of all isolates tested, 38% were identified as resistant or shifted to two fungicides, and 12% were resistant or shifted to all four fungicides tested. No fitness penalty was found for isolates resistant to multiple fungicides based on a statistical analysis of mycelial growth and conidial production.

Baseline Sensitivity of Cercospora zeae-maydis to Quinone Outside Inhibitor Fungicides

Cercospora zeae-maydis, the causal agent of gray leaf spot on corn (Zea mays), can cause severe yield loss in the United States. Quinone outside inhibitor (QoI) fungicides are effective tools that can be used to manage gray leaf spot, and their use has increased in corn production in the United States. In total, 61 C. zeae-maydis isolates collected from fields in which QoI fungicides had never been applied were tested in vitro using azoxystrobin-, pyraclostrobin-, or trifloxystrobin-amended medium to determine the effective fungicide concentration at which 50% of the conidial germination was inhibited (EC₅₀). The effect of salicylhydroxamic acid (SHAM) also was evaluated for seven isolates to determine whether C. zeae-maydis is capable of using alternative respiration in azoxystrobin-amended medium. All seven C. zeae-maydis isolates tested had significantly greater (P < 0.02) EC₅₀ values when SHAM was not included in medium amended with azoxystrobin, indicating that C. zeae-maydis has the potential to utilize alternative respiration to overcome QoI fungicide inhibition in vitro. Baseline EC₅₀ values of azoxystrobin ranged from 0.003 to 0.031 μg/ml, with mean and median values of 0.018 and 0.019 μg/ml, respectively. Baseline EC₅₀ values of pyraclostrobin ranged from 0.0003 to 0.0025 μg/ml, with mean and median values of 0.0010 and 0.0010 μg/ml, respectively. Baseline EC₅₀ values of trifloxystrobin ranged from 0.0004 to 0.0034 μg/ml, with mean and median values of 0.0023 and 0.0024 μg/ml, respectively. These baseline sensitivity values will be used in a fungicide resistance monitoring program to determine whether shifts in sensitivity to QoI fungicides are occurring in C. zeae-maydis populations.

Resistance to Pyraclostrobin and Boscalid in Populations of Botrytis cinerea from Stored Apples in Washington State

Gray mold caused by Botrytis cinerea is a major postharvest disease of apple. Pristine, a formulated mixture of pyraclostrobin and boscalid, was recently registered for use on apple. Pristine applied within 2 weeks before harvest is effective in controlling gray mold in stored apple fruit. To determine the baseline sensitivity of B. cinerea populations to these fungicides, 40 isolates from organic and 80 from conventional apple orchards where Pristine had not been used were tested for mycelial growth or conidial germination on fungicide-amended media. To monitor fungicide resistance, gray-mold-decayed apple fruit originating from orchards in which Pristine had been used were sampled from a fruit packinghouse. Isolates of B. cinerea recovered from the fruit were tested for resistance to the two fungicides. In the in vivo study in the orchards, Pristine was applied to fruit 1 day before harvest. Fruit were then harvested, wounded, and inoculated with isolates exhibiting different fungicide-resistance phenotypes. Fruit were stored at 0°C for 8 weeks for decay development. The effective concentration that inhibits mycelial growth by 50% relative to the control (EC₅₀) values for sensitive isolates ranged from 0.008 to 0.132 μg/ml (mean = 0.043, n = 116) for pyraclostrobin and from 0.003 to 0.183 μg/ml (mean = 0.075, n = 117) for Pristine in a mycelial growth assay on potato dextrose agar. The EC₅₀ values of boscalid for sensitive isolates ranged from 0.065 to 1.538 μg/ml (mean = 0.631, n = 29) in a conidial germination assay on water agar. Four isolates were resistant to pyraclostrobin, with resistance factors (RFs) ranging from 12 to 4,193. Of the four pyraclostrobin-resistant isolates, one also was resistant to boscalid (RF = 14) and Pristine (RF = 373), and two exhibited reduced sensitivity to Pristine (RF = 16 and 17). The minimum inhibitory concentration for conidial germination (for boscalid) or mycelial growth (for pyraclostrobin and Pristine) of sensitive isolates was 5 μg/ml, which is thus recommended as a discriminatory concentration for phenotyping isolates for resistance to these fungicides. Of the 56 isolates obtained from decayed apple fruit that had been exposed to Pristine, 11 (approximately 20%) were resistant to both pyraclostrobin and boscalid and 1 was resistant only to pyraclostrobin. Of the additional 43 isolates obtained from decayed apple fruit originating from an organic orchard, 3 were resistant only to pyraclostrobin, 2 were resistant only to boscalid, and 2 were resistant to both fungicides. It appeared that there was no cross resistance between pyraclostrobin and boscalid because of the existence of isolates resistant only to either pyraclostrobin or boscalid. Pristine applied at label rate in the orchard failed to control gray mold on apple fruit inoculated with the Pristine-resistant isolates. This is the first report of multiple resistance to pyraclostrobin, boscalid, and Pristine in field populations of B. cinerea. Our results suggest that the development of dual resistance to pyraclostrobin and boscalid in B. cinerea populations could result in the failure to control gray mold with Pristine.

Azoxystrobin and soil interactions: degradation and impact on soil bacterial and fungal communities

AIMS

To provide an independent assessment of azoxystrobin effects on nontarget soil bacteria and fungi and generate some baseline information on azoxystrobin's persistence in soil.

METHODS AND RESULTS

Plate based assay showed that azoxystrobin exhibited differential toxicity upon cultured fungi at different application rates. While (14)C labelled isotopes experiments showed that less than 1% of azoxystrobin was mineralized, degradation studies revealed over 60% azoxystrobin breakdown over 21 days. PCR DGGE analysis of 16S and 18S rRNA genes from different soil microcosms showed that azoxystrobin had some effects on fungal community after 21 days (up to 84 days) of incubation in either light or dark soil microcosms. Light incubations increased fungal diversity while dark incubations reduced fungal diversity. Bacterial diversity was unaffected.

CONCLUSIONS

Significant biotic breakdown of parent azoxystrobin occurred within 21 days even in the absence of light. Azoxystrobin under certain conditions can reduce fungal soil diversity.

SIGNIFICANCE AND IMPACT OF THE STUDY

One of the few independent assessments of azoxystrobin (a widely used strobilurins fungicide) effects on soil fungi when used at the recommended rate. Azoxystrobin and metabolites may persist after 21 days and affect soil fungi.

Baseline Sensitivity of Botrytis cinerea to Pyraclostrobin and Boscalid and Control of Anilinopyrimidine- and Benzimidazole-Resistant Strains by These Fungicides

Fifty-five isolates of Botrytis cinerea collected from vegetable crops were used to determine the pathogen's baseline sensitivity to two new fungicides: boscalid, which inhibits the enzyme succinate dehydrogenase in the electron transport chain, and pyraclostrobin, which blocks electron transport between cytochrome b and cytochrome c₁. Measurement of sensitivity to boscalid was based on both inhibition of mycelial growth and spore germination, while measurement of sensitivity to pyraclostrobin was based only on inhibition of spore germination. For both fungicides, the sensitivity distribution was a unimodal curve, with a mean EC₅₀ value (effective concentration that reduces mycelial growth or spore germination by 50%) of 0.033 μg ml^-1 for pyraclostrobin and 2.09 and 2.14 μg ml^-1 for boscalid based on the inhibition of mycelial growth and spore germination, respectively. No cross-sensitivity relationship was observed between the two fungicides (r = 0.09). In addition, no cross-resistance relationship was observed between these two fungicides with other botryticides: cyprodinil, pyrimethanil, fenhexamid, fludioxonil, and iprodione. Moreover, the control efficacy of the two fungicides was tested against two anilinopyrimidine-resistant and two benzimidazole-resistant isolates, and two of wild-type sensitivity. Both pyraclostrobin and boscalid provided satisfactory control of all six isolates that was independent of the isolate sensitivity to benzimidazoles and anilinopyrimidines. In contrast, carbendazim failed to control sufficiently the benzimidazole-resistant isolates, while cyprodinil failed to provide satisfactory control of the anilinopyrimidine-resistant isolates.

Improving Fungicide-Based Management of Ray Blight Disease in Tasmanian Pyrethrum Fields

Ray blight disease, caused by Phoma ligulicola var. inoxydablis, is a serious threat to the Tasmanian pyrethrum industry. The management of this disease relies upon the strategic application of fungicides in early spring. A range of fungicides were assessed for their efficacy in controlling ray blight disease in Tasmanian pyrethrum fields, and the primary objective of this study was to increase fungicide options available to growers in different resistance groups. Fungicides were assessed under in vitro conditions, within five replicated-plot field trials over three seasons (2004 to 2006) and in single-plot trials over eight fields in 2005. In each of the field trials, regular assessments of disease intensity (defoliation severity and the incidence of stems with ray blight), stem height, and the number of flowers produced on each stem were made using stems as the primary sampling unit. Canopy reflectance at 830 nm and the Difference Vegetative Index, measured using a handheld multispectral radiometer, also were used to compare fungicide effects on green leaf area. The effect of fungicides on the dry weight of flowers, pyrethrin content within the flowers, flower maturity, and pyrethrin yield were determined. Under in vitro conditions, boscalid reduced both conidial germination and mycelial growth at concentrations of at least 0.16 μg/ml. In field trials 1 and 2 (in 2004), the premixed formulation of pyraclostrobin + boscalid (Pristine) increased pyrethrin yield by an average of 79% compared with nontreated plots over the two locations. Furthermore, in single-plot trials, pyraclostrobin + boscalid increased pyrethrin yield by 134 and 60% compared with the industry-recommended protocol (single application of azoxystrobin at 150 g a.i./ha [Amistar WG] and two additional applications of a tank mixture of difenoconazole at 125 g a.i./ha [Score] and chlorothalonil at 1,008 liters a.i./ha [Bravo 720] at 14- to 21-day intervals) and nontreated plots, respectively. In field trials 3 (in 2005) and 4 and 5 (in 2006), similar yield benefits also were produced by applying pyraclostrobin (Cabrio SC) or boscalid (Filan) alone or in combination with chlorothalonil (Bravo 720) at 1.4 liters of product per hectare, regardless of the rates of pyraclostrobin (250 and 125 g a.i./ha) and boscalid (500 and 250 g a.i./ha) used. These data were used to recommend the incorporation of boscalid to improve the fungicide-based management of ray blight disease. This decreases the number of applications of both strobilurin and triazole fungicides which have been used extensively for the management of ray blight and other diseases in Tasmanian pyrethrum fields and are prone to fungicide resistance development.

Baseline Sensitivity of Ascochyta rabiei to Azoxystrobin, Pyraclostrobin, and Boscalid

Ascochyta rabiei, causal agent of Ascochyta blight on chickpea (Cicer arietinum), can cause severe yield loss in the United States. Growers rely on applications of fungicides with site-specific modes of action such as the quinone outside inhibiting (QoI) fungicides azoxystrobin and pyraclostrobin, and the carboximide fungicide boscalid, to manage disease. In all, 51 isolates collected prior to QoI fungicide registration and 71 isolates collected prior to boscalid registration in the United States were tested in an in vitro assay to determine the effective fungicide concentration at which 50% of conidial germination was inhibited (EC₅₀) for each isolate-fungicide combination. The effect of salicylhydroxamic acid (SHAM) on conidia of A. rabiei in the presence and absence of azoxystrobin also was assessed to determine whether the fungus is capable of using alternative respiration. Five of nine A. rabiei isolates tested had significantly higher (P ≤ 0.05) EC₅₀ values when SHAM was not included in media amended with azoxystrobin, indicating that A. rabiei has the potential to use alternative respiration to overcome fungicide toxicity in vitro. EC₅₀ values of azoxystrobin and pyraclostrobin ranged from 0.0182 to 0.0338 μg/ml and from 0.0012 to 0.0033 μg/ml, with mean values of 0.0272 and 0.0023 μg/ml, respectively. EC₅₀ values of boscalid ranged from 0.0177 to 0.4960 μg/ml, with a mean of 0.1903 μg/ml. Establishment of these baselines is the first step in developing a monitoring program to determine whether shifts in sensitivity to these fungicides are occurring in the A. rabiei pathogen population.

Incidence of Myclobutanil- and Kresoxim-Methyl-Insensitive Isolates of Venturia inaequalis in Quebec Orchards

Sensitivity of baseline and exposed populations of Venturia inaequalis to myclobutanil and to kresoxim-methyl were evaluated in vitro. For myclobutanil, the population was constructed with 238 monoconidial isolates of V. inaequalis collected from 48 orchards. For kresoxim-methyl, the population was constructed with 251 monoconidial isolates collected from 49 orchards. Baseline populations were constructed with 34 and 29 monoconidial isolates collected from apple trees that had never been treated for myclobutanil and kresoxim-methyl, respectively. Sensitivity to fungicides was evaluated based on 50% effective dose (ED₅₀) values. The V. inaequalis population that was not exposed to myclobutanil had a baseline sensitivity (mean ED₅₀) of 0.064 μg/ml and showed a lognormal distribution. The V. inaequalis population constructed with isolates from commercial orchards had a mean ED₅₀ of 2.600 μg/ml, which was significantly higher than the baseline sensitivity. The distribution of ED₅₀ values did not follow a lognormal distribution. In response to declining levels of scab control with myclobutanil and other sterol demethylation inhibitor fungicides (DMIs), three orchards were more deeply investigated. The mean ED₅₀ values were 1.618 (n = 23), 3.079 (n = 29), and 1.500 μg/ml (n = 20) in orchards one, two, and three, respectively. Resistant isolates, according to criteria set by other studies, accounted for 39, 76, and 85% of the isolates tested. The V. inaequalis population that had never been exposed to kresoxim-methyl had a baseline sensitivity (mean ED₅₀) of 0.092 μg/ml and showed a lognormal distribution. The V. inaequalis population constructed with isolates from commercial orchards had a mean ED₅₀ of 6.093 μg/ml, which was significantly higher than the baseline sensitivity. The distribution of ED₅₀ values followed a lognormal distribution. However, when a subsample of isolates was retested for their sensitivity to kresoxim-methyl with the addition of salicylhydroxamic acid (an inhibitor of alternative oxidase) at 100 μg/ml to the growth medium, more than 98% inhibition was observed for all isolates. The results from in vitro tests showed a high level of resistance to myclobutanil and a low level of resistance to kresoxim-methyl, suggesting that the use of myclobutanil and DMIs should be discontinued or significantly reduced before practical resistance is reached.

Venturia inaequalis-inhibiting Diels-Alder adducts from Morus root bark

In organic apple orcharding there is a continuous need for natural fungicides effective against Venturia inaequalis (Cooke) Winter, the causal agent of apple scab. In this study an in vitro assay is presented for determining the germination inhibitory potential of extracts and pure compounds. From a screening of plant extracts, the methanol extract of Morus root bark revealed distinct V. inaequalis inhibiting qualities, which were subjected to a bioguided fractionation. Among the isolated metabolites [moracins M (1), O/P (2), kuwanon L (3), and sanggenons D (4), B (5), G (6), O (7), E (8), and C (9)] all the Diels-Alder adducts (3-9) showed an antifungal activity with IC50 values between 10 and 123 microM. The in vitro activity of the most active fraction (A5, IC50 39.0 +/- 4.2 microg/mL) was evaluated in vivo, confirming a distinct antifungal activity against V. inaequalis for the tested natural material.

Comparative Efficacy, Selection of Effective Partners, and Application Time of Strobilurin Fungicides for Control of Cercospora Leaf Spot of Sugar Beet

In this study, we attempt to optimize the use of strobilurin fungicides by testing the efficacy of azoxystrobin, kresoxim-methyl, pyraclostrobin, and trifloxystrobin under field conditions, by testing for the most efficient partners in fungicide mixtures, and by testing control efficacy of strobilurin fungicides applied at several application times to determine the better options for disease management. Results showed that trifloxystrobin was the most efficient strobilurin fungicide, followed by pyraclostrobin. Azoxystrobin provided a modest to poor control efficacy, whereas kresoxim-methyl provided only poor disease control efficacy. Mixtures of azoxystrobin and trifloxystrobin with either chlorothalonil or maneb and difenoconazole or flutriafol were tested for their efficacy in controlling the disease. The results showed that the azoxystrobin-containing mixtures provided significantly better control compared with that obtained by single applications of each mixture component. The mixtures of trifloxystrobin with maneb or with difenoconazole or flutriafol provided control efficacy similar to that obtained by single applications of trifloxystrobin, whereas the mixture of trifloxystrobin and chlorothalonil provided significantly lower control efficacy compared with the other trifloxystrobin-containing mixtures tested. For both strobilurin fungicides tested, the calculated ratio between the observed and the expected control efficacy ranged around the value of 1, suggesting additive interactions between the mixtures' components. To determine the most appropriate time for strobilurin fungicides application, trifloxystrobin was applied as the first two, the middle two, or the final two consecutive treatments of six fungicide applications. The remaining fungicide treatments in the spray schedules were carried out by applying the systemic fungicide difenoconazole. Results showed that a higher control efficacy was obtained when trifloxystrobin was applied in either of the earlier applications.

Investigating the role of ABC transporters in multifungicide insensitivity in Phytophthora infestans

SUMMARY Isolates of the oomycete Phytophthora infestans exhibit a wide range of intrinsic sensitivities to fungicides, which potentially influences the application rates of chemicals needed to control potato late blight. To help understand what determines such levels of sensitivity, a genetic approach was employed which followed the segregation of sensitivities to structurally diverse fungicides such as metalaxyl and trifloxystrobin. Progeny exhibited broad distributions of sensitivity phenotypes, consistent with the behaviour of a quantitative trait. Measurements of the inhibition of strains by seven fungicides revealed that basal sensitivities to metalaxyl and trifloxystrobin, and to cymoxanil and dimethomorph, correlated at the 95% confidence level. These compounds have distinct modes of action, suggesting the involvement of a multifungicide efflux phenomenon mediated by ABC transporters. To determine whether such proteins contribute to variation in sensitivity, 41 full transporters and 13 half transporters were identified from P. infestans and their mRNA levels compared in strains exhibiting higher or lower sensitivities to fungicides. No correlation was observed between the expression of any ABC transporter and fungicide sensitivity. Other genes, or variation in the activities of the transporters, may therefore explain the differences between strains. Five ABC transporters were induced by several fungicides in strains with both higher and lower sensitivities to fungicides, which probably reflects the existence of a network for protecting against natural and artificial toxins.

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.

Characterization of Colletotrichum graminicola Isolates Resistant to Strobilurin-Related QoI Fungicides

Isolates of Colletotrichum graminicola were collected from annual bluegrass or bent grass turf in Japan and the United States, and their sensitivities to QoI fungicides (QoIs) as well as their cytochrome b sequences were characterized. Five isolates sampled from turf treated repeatedly with azoxystrobin were highly QoI resistant under both in vivo and in vitro test conditions. The nucleotide sequences of a large cytochrome b gene segment involving the binding site of QoIs were fully homologous for all resistant isolates and contained the G143A target site mutation known to confer QoI resistance in other pathogens. QoI-sensitive isolates collected prior to treatments with QoIs were more diverse with regard to their cytochrome b gene sequences and their phenotype responses to QoIs. All wild-type isolates retained a glycine in position 143 of cytochrome b. Three of the four QoI-sensitive isolates were, in addition, distinguished by leucines in positions 95, 130, and 141, which were exchanged to threonine in all resistant but also in one of the sensitive isolates. In addition to a more pronounced divergence of cytochrome b sequences, the sensitive wild-type isolates also were diverse with regard to the induction of alternative respiration in response to QoI action, as indicated by comparisons of QoI sensitivities displayed in the absence or presence of the alternative oxidase inhibitor salicylhydroxamic acid. These different phenotype responses expressed under in vitro test conditions had no or only a slight impact on anthracnose control in protective applications of azoxystrobin. Isolate responses in vitro were very similar for trifloxystrobin, indicating cross-resistance among the class of QoIs. Our results imply that C. graminicola falls into the class of pathogens with a potential for rapid selection of highly QoI-resistant phenotypes. Frequent monitoring of population sensitivities will be required to determine the status of population responses toward practical QoI resistance.

Variations in the Sensitivity of Phytophthora infestans Isolates from Different Genetic Backgrounds to Dimethomorph

The sensitivities of 11 isolates of Phytophthora infestans to dimethomorph were examined at all stages of the asexual life cycle and when inoculated onto potato leaf discs. In vitro zoospore encystment and cyst germination were highly sensitive to dimethomorph with 50% reduction of mycelial growth and cyst germination (EC₅₀) values for most isolates <0.20 μg/ml, whereas direct sporangia germination and in vitro hyphal growth and sporulation were less sensitive (means of 0.45 and 0.22 μg/ml, respectively). Zoosporogenesis was not significantly inhibited at the maximum dimethomorph concentration examined, 10 μg/ml. Significant differences (Fisher's least significant difference, P = 0.05) in the EC₅₀ values were present between isolates for all stages of the asexual life cycle, except direct sporangia germination and zoosporogenesis. Sensitivity ratios between the least- and most-sensitive isolates were 6.11, 12.14, 12.36, and 10.56 for hyphal growth, in vitro sporulation, zoospore encystment, and cyst germination, respectively. Application of dimethomorph at 1,000 μg/ml to potato leaf discs at 24 or 48 h before inoculation completely inhibited symptom incidence for most isolates, whereas application after inoculation generally was not significantly different from the untreated control, regardless of concentration. Sporulation from leaf discs treated with dimethomorph at 24 or 48 h after inoculation was completely inhibited for all isolates with dimethomorph at 1,000 μg/ml, even when symptom incidence was not significantly reduced.

Cloning and expression analysis of the ATP-binding cassette transporter gene MFABC1 and the alternative oxidase gene MfAOX1 from Monilinia fructicola

Brown rot, caused by Moniliniafructicola (G Wint) Honey, is a serious disease of peach in all commercial peach production areas in the USA, including South Carolina where it has been primarily controlled by pre-harvest application of 14-alpha demethylation (DMI) fungicides for more than 15 years. Recently, the Qo fungicide azoxystrobin was registered for brown rot control and is currently being investigated for its potential as a DMI fungicide rotation partner because of its different mode of action. In an effort to investigate molecular mechanisms of DMI and Qo fungicide resistance in M fructicola, the ABC transporter gene MfABC1 and the alternative oxidase gene MfAOX1 were cloned to study their potential role in conferring fungicide resistance. The MfABC1 gene was 4380 bp in length and contained one intron of 71 bp. The gene revealed high amino acid homologies with atrB from Aspergillus nidulans (Eidam) Winter, an ABC transporter conferring resistance to many fungicides, including DMI fungicides. MfABC1 gene expression was induced after myclobutanil and propiconazole treatment in isolates with low sensitivity to the same fungicides, and in an isolate with high sensitivity to propiconazole. The results suggest that the MfABC1 gene may be a DMI fungicide resistance determinant in M fructicola. The alternative oxidase gene MfAOX1 from M fructicola was cloned and gene expression was analyzed. The MfAOX1 gene was 1077 bp in length and contained two introns of 54 and 67 bp. The amino acid sequence was 63.8, 63.8 and 57.7% identical to alternative oxidases from Venturia inaequalis (Cooke) Winter, Aspergillus niger van Teighem and A nidulans, respectively. MfAOX1 expression in some but not all M fructicola isolates was induced in mycelia treated with azoxystrobin. Azoxystrobin at 2 microg ml(-1) significantly induced MfAOX1 expression in isolates with low MfAOX1 constitutive expression levels.

A critical evaluation of the role of alternative oxidase in the performance of strobilurin and related fungicides acting at the Qo site of complex III

Mitochondrial respiration conserves energy by linking NADH oxidation and electron-coupled proton translocation with ATP synthesis, through a core pathway involving three large protein complexes. Strobilurin fungicides block electron flow through one of these complexes (III), and disrupt energy supply. Despite an essential need for ATP throughout fungal disease development, strobilurins are largely preventative; indeed some diseases are not controlled at all, and several pathogens have quickly developed resistance. Target-site variation is not the only cause of these performance difficulties. Alternative oxidase (AOX) is a strobilurin-insensitive terminal oxidase that allows electrons from ubiquinol to bypass Complex III. Its synthesis is constitutive in some fungi but in many others is induced by inhibition of the main pathway. AOX provides a strobilurin-insensitive pathway for oxidation of NADH. Protons are pumped as electrons flow through Complex I, but energy conservation is less efficient than for the full respiratory chain. Salicylhydroxamic acid (SHAM) is a characteristic inhibitor of AOX, and several studies have explored the potentiation of strobilurin activity by SHAM. We present a kinetic-based model which relates changes in the extent of potentiation during different phases of disease development to a changing importance of energy efficiency. The model provides a framework for understanding the varying efficacy of strobilurin fungicides. In many cases, AOX can limit strobilurin effectiveness once an infection is established, but is unable to interfere significantly with strobilurin action during germination. A less stringent demand for energy efficiency during early disease development could lead to insensitivity towards this class of fungicides. This is discussed in relation to Botrytis cinerea, which is often poorly controlled by strobilurins. Mutations with a similar effect may explain evidence implicating AOX in resistance development in normally well-controlled plant pathogens, such as Venturia inaequalis.

Factors Influencing the Efficacy of Myclobutanil and Azoxystrobin for Control of Grape Black Rot

We studied several factors influencing the efficacy of the demethylation inhibitor (DMI) fungicide myclobutanil and the strobilurin fungicide azoxystrobin for control of grape black rot, caused by the pathogen Guignardia bidwellii (anamorph Phyllosticta ampelicida). The distribution of sensitivities to myclobutanil among G. bidwellii isolates from an "organic" vineyard (no previous exposure to synthetic fungicides, n = 50) and from a commercial vineyard with a history of DMI applications (n = 60) was determined in vitro. There was little difference between the two populations, and the range of sensitivities was narrow; for the composite population of 110 isolates, the value of the mean effective dose for 50% inhibition (ED₅₀) was 0.04 mg/liter, and the most- and least-sensitive isolates were separated by a factor of 16. When applied from 2 to 6 days after inoculating grape seedlings with a suspension containing either 2 × 10⁴ or 1 × 10⁶ conidia per ml, myclobutanil (60 mg/liter) provided complete control of lesion development. When applied beyond 6 days after inoculation but prior to lesion appearance (9 to 11 days after inoculation, depending on temperature), it provided complete control of pycnidium production in those lesions that developed subsequently. In contrast, when applied 2 to 10 days after inoculation with 2 × 10⁴ conidia per ml, azoxystrobin (128 mg/liter) provided only 78 to 63% control of lesion formation and erratic control of pycnidium formation, although conidium production was reduced by 85 to 68% across this range of treatments. Relatively little control was provided by azoxystrobin treatments following inoculation with 1 × 10⁶ conidia per ml. On leaf disks treated with azoxystrobin at 20 mg/liter prior to inoculation, 8 to 43% of conidia from five G. bidwellii isolates germinated, and 4 to 19% formed appressoria. However, these processes were completely to near-completely inhibited when salicylhydroxamic acid (SHAM), which inhibits an alternative respiration pathway utilized to circumvent the activity of strobilurin fungicides, was added to the inoculum at 100 mg/liter. Thus, alternative respiration apparently allowed the conidia to germinate and form appressoria on azoxystrobin-treated leaves. When grape seedlings were sprayed with commercially formulated azoxystrobin at 200 mg/liter and inoculated the next day with G. bidwellii conidia, little or no disease was evident 4 weeks later. However, G. bidwellii pycnidia formed on up to 50% of the leaves from such plants when they were killed with paraquat 1 to 7 days after inoculation. These results suggest that latent infections became established on azoxystrobin-treated leaves and became active after the plants were killed with paraquat.

A non-Mendelian inheritance of resistance to strobilurin fungicides in Ustilago maydis

Mutants of Ustilago maydis (DC) Corda with high resistance to azoxystrobin (RF 164 to 4714, based on EC50 values), an inhibitor of mitochondrial electron transport at the cytochrome bc1 complex, were isolated in a mutation frequency of 2.3 x 10(-7) after nitrosoguanidine mutagenesis and selection on media containing 1 microgram ml-1 azoxystrobin in addition to 0.5 mM salicylhydroxamate (SHAM), a specific inhibitor of cyanide-resistant (alternative) respiration. Oxygen uptake in whole cells was strongly inhibited in the wild-type strains by azoxystrobin (1.5 micrograms ml-1) in addition to SHAM (1 mM), but not in the mutant isolates. Genetic analysis with nine such mutant isolates resulted in progeny phenotypes which did not follow Mendelian segregation, but satisfied the criteria of non-Mendelian (cytoplasmic) heredity. In crosses between three mutant isolates with the compatible wild-type strains, the sensitivity was inherited by progeny maternally from the wild-type parent strain (criterion of uniparental inheritance). In crosses between wild-type strains and remaining mutant isolates, a continuous distribution of sensitivity in the progeny was found (criterion of vegetative segregation). The third criterion of cytoplasmic resistance (criterion of intracellular selection) was fulfilled by experiments on the stability of resistance phenotypes. With two exceptions, a reduction of resistance was observed in the mutant strains when they were grown on inhibitor-free medium. Recovery of the high resistance level was observed after they were returned to the selection medium. Cross-resistance studies with other fungicides, which also inhibit electron transport through complex III of respiratory chain, showed that mutations for resistance to azoxystrobin were also responsible for reduced sensitivity to kresoxim-methyl (RF 18 to 1199) and to antimycin-A (RF 20 to 305), which act at the Qo and Qi sites of the cytochrome bc1 complex, respectively. Studies of the fitness of azoxystrobin-resistant isolates showed that these mutations appeared to be pleiotropic, having significant adverse effects on growth in liquid culture and pathogenicity on young corn plants.

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.

Resistance to QoI (Strobilurin-like) Fungicides in Isolates of Pyricularia grisea from Perennial Ryegrass

In August 2000, azoxystrobin was ineffective in controlling gray leaf spot of perennial ryegrass at a golf course in Lexington, KY and at two golf courses in Illinois. Isolates suspected of being fungicide-resistant ("suspect isolates") were compared to "baseline" isolates obtained from sites with no known use of quinol-oxidizing inhibitor (Q_oI) fungicides. Conidial germination of Pyricularia grisea was tested in vitro with 100 μg of salicylhydroxamic acid per ml. For baseline isolates, 50% effective concentration (EC₅₀) values for azoxystrobin and trifloxystrobin were 0.015 to 0.064 and 0.013 to 0.078 μg/ml, respectively; EC₅₀ values for suspect isolates were 2.39 to 44.8 and 0.31 to 111, respectively. All suspect isolates exhibited significantly (P = 0.05) lower sensitivity to Q_oI fungicides than all baseline isolates. The mean EC₅₀ values for suspect isolates for azoxystrobin and trifloxystrobin were 690 and 827 times higher, respectively, than the means for baseline isolates. In the laboratory, azoxystrobin and trifloxystrobin provided essentially complete control of disease induced by nine baseline isolates in vivo. Azoxystrobin and trifloxystrobin provided poor to no control of disease induced by six of eight suspect isolates; control of disease induced by the remaining two isolates was partial for azoxystrobin and complete for trifloxystrobin. We conclude that one or more biotypes of perennial ryegrass-infecting strains of P. grisea with resistance to Q_oI fungicides have emerged. This is the first report of resistance to Q_oI fungicides in P. grisea. Furthermore, this is one of two Q_oIresistant fungal pathogens collected in the United States during the 2000 growing season, the first instances reported for North America.

Sensitivity of Mycosphaerella fijiensis from Banana to Trifloxystrobin

An ascospore germination method was developed and validated to assess the sensitivity of bulk samples of Mycosphaerella fijiensis to trifloxystrobin. Using this method, the sensitivity of 142 ascospore samples from banana plantations not treated with strobilurins was analyzed to establish a baseline of pathogen sensitivity. A bulk method was utilized for monitoring purposes because it avoids potential complications due to the isolation and propagation of single-spore isolates and enables the testing of larger samples. Following intensive use of strobilurins (6 to 11 applications per year) over 4 years, under conditions of high disease pressure and the absence of sanitary measures at a development site in Costa Rica, bulk samples with 50% effective concentration (EC₅₀) resistance factors (RFs) in excess of 500 compared with the mean baseline sensitivity were detected. Single-ascospore isolates derived from spores germinating at the discriminatory dose of 3 μg/ml were also resistant, suggesting that the frequency of resistant individuals in bulk samples could be estimated from the relative numbers of ascospores growing at this dose. The resistance of selected isolates was confirmed in planta. In vitro tests with four resistant and two sensitive single-ascospore isolates collected from different locations and times indicated possible cross-resistance of trifloxystrobin to azoxystrobin, famoxadone, and fenamidone, but not to propiconazole.

Sensitivity of mitochondrial respiration to different inhibitors in Venturia inaequalis

The sensitivity of Venturia inaequalis field isolates to inhibitors of the cytochrome bc1 complex at the Qo site (QoIs) was characterised at the molecular, biochemical and physiological level, and compared to other respiration inhibitors. Comparison of a sensitive and a QoI-resistant isolate revealed very high resistance factors both in mycelium growth and spore germination assays. Cross-resistance was observed among QoIs such as trifloxystrobin, azoxystrobin, famoxadone, strobilurin B and myxothiazol. In the mycelium growth assay, antimycin A, an inhibitor of the cytochrome bc1 complex at the Qi site, was less active against the QoI-resistant than against the sensitive isolate. The mixture of QoIs with salicylhydroxamic acid (SHAM), an inhibitor of the alternative oxidase, exerted synergistic effects in the spore germination but not in the mycelium growth assay. Thus, the cytochrome and the alternative respiration pathways are assumed to play different roles, depending on the developmental stage of the fungus. Induction of alternative oxidase (AOX) by trifloxystrobin was observed in mycelium cells at the molecular level for the sensitive but not the resistant isolate. Following QoI treatment, respiration parameters such as oxygen consumption, ATP level, membrane potential and succinate dehydrogenase activity were only slightly reduced in Qo-resistant mycelium cells, and remained at much higher levels than in sensitive cells. In contrast, no difference was observed between sensitive and resistant isolates when NADH consumption was measured. Comparison of the cytochrome b (cyt b) gene of the sensitive and resistant isolates did not reveal any point mutations as is known to occur in resistant isolates of other plant pathogens. It is assumed that QoI resistance in V inaequalis may be based on a compensation of the energy deficiency following QoI application upstream of the NADH dehydrogenase of the respiratory chain.

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.

Mitochondrial electron transfer in the wheat pathogenic fungus Septoria tritici: on the role of alternative respiratory enzymes in fungicide resistance

Certain phytopathogenic fungi are able to express alternative NADH- and quinol-oxidising enzymes that are insensitive to inhibitors of the mitochondrial respiratory Complexes I and III. To assess the extent to which such enzymes confer tolerance to respiration-targeted fungicides, an understanding of mitochondrial electron transfer in these species is required. An isolation procedure has been developed which results in intact, active and coupled mitochondria from the wheat pathogen Septoria tritici, as evidenced by morphological and kinetic data. Exogenous NADH, succinate and malate/glutamate are readily oxidised, the latter activity being only partly (approx. 70%) sensitive to rotenone. Of particular importance was the finding that azoxystrobin (a strobilurin fungicide) potently inhibits fungal respiration at the level of Complex III. In some S. tritici strains investigated, a small but significant part of the respiratory activity (approx. 10%) is insensitive to antimycin A and azoxystrobin. Such resistant activity is sensitive to octyl gallate, a specific inhibitor of the plant alternative oxidase. This enzyme, however, could not be detected immunologically. On the basis of the above findings, a conceptual mitochondrial electron transfer chain is presented. Data are discussed in terms of developmental and environmental regulation of the composition of this chain.

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.