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Fungicides are indispensable for high-quality crops, but the rapid emergence and evolution of fungicide resistance have become the most important issues in modern agriculture. Hence, the sustainability and profitability of agricultural production have been challenged due to the limited number of fungicide chemical classes. Resistance to site-specific fungicides has principally been linked to target and non-target site mechanisms. These mechanisms change the structure or expression level, affecting fungicide efficacy and resulting in different and varying resistance levels. This review provides background information about fungicide resistance mechanisms and their implications for developing anti-resistance strategies in plant pathogens. Here, our purpose was to review changes at the target and non-target sites of quinone outside inhibitor (QoI) fungicides, methyl-benzimidazole carbamate (MBC) fungicides, demethylation inhibitor (DMI) fungicides, and succinate dehydrogenase inhibitor (SDHI) fungicides and to evaluate if they may also be associated with a fitness cost on crop pathogen populations. The current knowledge suggests that understanding fungicide resistance mechanisms can facilitate resistance monitoring and assist in developing anti-resistance strategies and new fungicide molecules to help solve this issue. © 2023 Society of Chemical Industry.

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BACKGROUND: Quinone outside inhibitor (QoI) fungicides have not been effective in controlling the wheat blast disease [Pyricularia oryzae Triticum lineage (PoTl)] in Brazil. The first report of resistance of PoTl to QoIs in this country occurred in 2015. This study aimed to test hypotheses about the changes in fitness parameters and competitive advantage of the QoI-resistant (R) PoTl isolate group compared to the sensitive (S) isolate group. Mycelial growth on PDA medium and in vivo conidial production, incubation period and disease severity were analyzed as fitness parameters. The competitive ability was measured on wheat leaves and heads inoculated with mixtures of R:S isolates at the following proportions: 0S:100R, 20S:80R, 50S:50R, 80S:20R, 100S:0R, and 0S:0R. RESULTS: The QoI-R isolate group had significantly higher fitness than the sensitive isolate group, considering both in vitro and in vivo parameters. The highest in vivo conidial production on wheat leaves and the highest leaf and head disease severity were detected when resistant strains were predominant in the isolate's mixtures (20S:80R or 0S:100R proportions), in the absence of fungicide pressure. Conidia harvested from wheat blast lesions on leaves inoculated with 20S:80R and 0S:100R mixtures were resistant to QoIs in vitro assays based on discriminatory doses of the fungicide. CONCLUSION: Therefore, QoI resistance facilitated a higher fitness and a competitive advantage in PoTl, which contrasts with the evolutionary theory that associates a fitness cost to fungicide resistance. We discuss the evolutionary and ecological implications of the higher fitness as found in the fungicide-resistant adapted populations of the wheat blast pathogen. © 2022 Society of Chemical Industry.

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In recent years, there have been reductions in the efficacy of the fungicidal control of Phakopsora pachyrhizi, thereby hindering the management of soybean rust and compromising crop yield. This study evaluated the effects of incorporating hydrogen peroxide (H2O2) in commercial fungicide formulations on the control of soybean rust. We conducted two experiments, one of which was performed in a greenhouse environment and the other under field conditions. In both environments, we examined the following four control programs using commercial fungicide formulations: (I) azoxystrobin + cyproconazole (quinone outside inhibitor [QoI] + demethylation inhibitor [DMI]); (II) picoxystrobin + cyproconazole (QoI + DMI); (III) pyraclostrobin + epoxiconazole + fluxapyroxad (QoI + DMI + succinate dehydrogenase inhibitor); and (IV) water (H2O) (program without fungicide application), combined with the incorporation of (i) H2O2; (ii) mancozeb (positive control I); (iii) chlorothalonil (positive control II); or (iv) water (H2O) alone. Analyses of infected leaf area and grain yield revealed that the addition of H2O2 to the formulations of DMI and QoI fungicides led to a reduction in disease severity of between 33% and 44% relative to the effects of these products used alone, as well as an increase in yield and SPAD values. The use of H2O2 and multi-site fungicides alone failed to provide effective control of soybean rust. In addition to enhancing the efficacy of disease control, the use of H2O2 associated with commercial fungicide mixtures was shown to be a potential tool for the management of fungicide resistance and reduction in losses from Asian soybean rust.

Nos últimos anos, a eficiência do controle de Phakopsora pachyrhizi por fungicidas tem sido reduzida, dificultando o manejo da ferrugem asiática da soja, o que ocasiona o comprometimento da produtividade. O objetivo deste estudo foi avaliar o efeito da adição do peróxido de hidrogênio (H2O2) em misturas comerciais fungicidas no controle da ferrugem da soja. Foram realizados dois ensaios, um em ambiente de casa de vegetação e outro em condições de campo. Em ambos ambientes foram estudados quatro programas de controle com formulações comerciais: I) azoxistrobina + ciproconazole (IQe + IDM); II) picoxistrobina + ciproconazole (IQe + IDM); III) piraclostrobina + epoxiconazole + fluxapiroxade (IQe + IDM + ISDH); IV) água - H2O (sem aplicação fungicida) associados à: i) peróxido de hidrogênio - H2O2, ii) mancozebe (controle positivo I), iii) clorotalonil (controle positivo II) e iv) água - H2O (sem associação). Análises de área foliar lesionada e de rendimento de grãos revelaram que a adição de peróxido de hidrogênio nas misturas de fungicidas IDMs e IQes proporcionou uma redução na severidade da doença entre 33 a 44% comparado aos produtos isolados, incremento na produtividade e maiores índices SPAD. O uso isolado do peróxido de hidrogênio e dos fungicidas multissítios não resultou em controle da ferrugem da soja. A utilização do H2O2 associado a misturas comerciais fungicidas, além de aprimorar a eficiência de controle, demonstra possibilidade de uso como ferramenta para manejo da resistência e redução dos prejuízos provenientes da ferrugem asiática da soja.

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Pyricularia oryzae is the etiological agent of rice blast, the most destructive disease in rice crops and chemical control based on fungicide is the main method used in its management. The aim of this study was characterize pathogenicity and identify P. oryzae isolates adapted to tricyclazole. P. oryzae monosporic isolates were collected in the state of Tocantins and inoculated in international differentiating series of rice cultivars for determination of pathotypes. After, the same isolates were inoculated in the rice cultivar IRGA 424 to evaluate resistance to fungicide Bim® 750 BR (Tricyclazole - 250 g/ha) that was applied 24 and 48 hours after pathogen inoculation (hai). Leaf blast severity and infection efficiency were evaluated 9 days after inoculation (dai), latency period (2 dai) and sporulation intensity (7 dai). Nine different pathotypes were identified, predominantly as IA group. The latent period of isolates occurred between from 48 to 120 h. The application of tricyclazole, 24 hai reduced disease severity with the exception of the isolate Py 7.1. The great variability of the pathogen allowed for adaptation to this molecule and can increase its aggressiveness and should be considered to guide the integrated management of the disease.

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BACKGROUND: Fungicides of the succinate dehydrogenase inhibitors (SDHIs) group have been used in soybean to control Asian soybean rust (ASR) caused by Phakopsora pachyrhizi. Fungal populations with less sensitivity to SDHI fungicides have been reported since 2015. RESULTS: In this study, fungal sensitivity to benzovindiflupyr (BZV) and fluxapyroxad (FXD) was assessed using a total of 770 P. pachyrhizi populations sampled over four soybean growing seasons. Cross-resistance, intrinsic activity, and frequency of SDHC-I86F mutation were also analyzed. The average effective concentration to inhibit 50% (EC50 ) and SDHC-I86F frequency increased over the 2015/2016, 2016/2017, 2017/2018 and 2018/2019 soybean-seasons. Fourteen P. pachyrhizi populations had the EC50 value above 10 mg L-1 for both carboxamides. No difference was found in intrinsic active to BZV and FXD fungicides for sensitive P. pachyrhizi populations. For P. pachyrhizi classified as less sensitive BZV showed the highest fungitoxicity effect. High frequency of the C-I86F mutation was observed in samples collected in volunteer soybean plants. The maximum frequency of SDHC-I86F mutation in the population was 50% and resulting in ASR populations with low sensitivity to SDHIs. A low correlation between bioassay and SDHC-I86F mutation was observed possible due to the dikaryotic nature of rust fungi or other mutations in the other succinate dehydrogenase subunits. CONCLUSION: The present work provides an overview of a large sampling size of P. pachyrhizi populations and their performance over the four crop seasons. The high frequency of SDHC-I86F mutation and low sensitivity to SDHIs are widely distributed in the main soybean growing regions in Brazil and present in volunteer plants in the soybean-free period. Further detailed studies are needed to identify novel point mutations affecting the effectiveness of SDHIs. © 2021 Society of Chemical Industry.

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Asian grapevine leaf rust, caused by Neophysopella meliosmae-myrianthae and N. tropicalis, is often controlled by quinone outside inhibitor (QoI) and demethylation inhibitor (DMI) fungicides in Brazil. Here, we evaluated the sensitivity of 55 Neophysopella spp. isolates to pyraclostrobin (QoI) and tebuconazole (DMI). To elucidate the resistance mechanisms, we analyzed the sequences of the cytochrome b (CYTB) and cytochrome P450 sterol 14α-demethylase (CYP51) target proteins of QoI and DMI fungicides, respectively. The CYP51 expression levels were also determined in a selection of isolates. In leaf disc assays, the mean 50% effective concentration (EC50) value for pyraclostrobin was about 0.040 µg/ml for both species. CYTB sequences were identical among all 55 isolates, which did not contain an intron immediately after codon 143. No amino acid substitution was identified at codons 129, 137, and 143. The mean EC50 value for tebuconazole was 0.62 µg/ml for N. tropicalis and 0.46 µg/ml for N. meliosmae-myrianthae, and no CYP51 sequence variation was identified among isolates of the same species. However, five N. meliosmae-myrianthae isolates grew on leaf discs treated at 10 µg/ml tebuconazole, and these were further exposed to tebuconazole selection pressure. Tebuconazole-adapted laboratory isolates of N. meliosmae-myrianthae showed an eight- to 25-fold increase in resistance after four rounds of selection that was not associated with CYP51 target alterations. In comparison with sensitive isolates, CYP51 expression was induced in the presence of tebuconazole in three out of four tebuconazole-adapted isolates tested. These results suggest a potential risk for QoI and DMI resistance development in Neophysopella spp.

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BACKGROUND: Reliance on fungicides to manage disease creates selection pressure for the evolution of resistance in fungal and oomycete pathogens. Rust fungi (Pucciniales) are major pathogens of cereals and other crops and have been classified as low-risk for developing resistance to fungicides; no case of field failure of fungicides in a cereal rust disease has yet been recorded. Recently, the Asian soybean rust pathogen, Phakopsora pachyrhizi evolved resistance to several fungicide classes, prompting us to screen a large sample of the globally widespread wheat yellow rust pathogen, Puccinia striiformis f. sp. tritici (Pst), for mutations associated with fungicide resistance. RESULTS: We evaluated 363 Pst isolates from Europe, the USA, Ethiopia, Chile, China and New Zealand for mutations in the target genes of demethylase inhibitor (DMI; Cyp51) and succinate dehydrogenase inhibitor (SDHI; SdhB, SdhC and SdhD) fungicides. A high proportion of Pst isolates carrying a Y134F DMI resistance-associated substitution in the Cyp51 gene was found among those from China and New Zealand. A set of geographically diverse Pst isolates was also found to display a substitution in SdhC (I85V) that is homologous to that reported recently in P. pachyrhizi and linked to SDHI resistance. CONCLUSION: The identification of resistance-associated alleles confirms that cereal rusts are not immune to fungicide resistance and that selection for resistance evolution is operating at high levels in certain locations. It highlights the need to adopt fungicide resistance management practices and to monitor cereal rust species for development of resistance. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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The long-term dynamics of fungicide resistance of the rice blast fungus Pyricularia oryzae was monitored by examining the reaction of the fungal field isolates, collected over a period of 26 years, to the active ingredients of commercially relevant fungicides. The in vitro sensitivity of all isolates was measured against quinone outside inhibitors (QoI), melanin biosynthesis inhibitors, and sterol demethylation inhibitor (DMI) fungicides, namely azoxystrobin (as a QoI), tricyclazole (as a melanin biosynthesis inhibitor), tebuconazole (as a DMI), and trifloxystrobin + tebuconazole (QoI + DMI). Over the 26-year collection period, a gradual rise in the EC50 estimates for mycelial growth sensitivity was observed for all fungicides, but most strikingly for azoxystrobin. A rise in conidial germination and appressorium formation was also noted, most markedly for azoxystrobin. Consistently, the earlier isolates were much more sensitive to the active ingredients than the more contemporary isolates. The sequencing of the amplified cyt b fragment distinguished two haplotypes, H1 and H2. Haplotype H1 (six isolates) contained the G to C transversion at codon 143 (resulting in change G143A), linked to the resistant phenotype QoI-R. Haplotype H2 (40 isolates), gathered the isolates sensitive to QoI. This work documents the gradual rise in the frequency of fungicide-resistant isolates in P. oryzae rice populations on a long-term basis.

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Postbloom fruit drop (PFD) of citrus is caused by the Colletotrichum acutatum and C. gloeosporioides species complexes. The disease is important when frequent rainfall occurs during the flowering period of citrus trees. In Brazil, until 2012, PFD was mainly controlled by preventive applications of the methyl-benzimidazole carbamate (MBC) carbendazim and demethylation-inhibitor (DMI) fungicides such as difenoconazole. Since then, mixtures containing the DMI tebuconazole and the quinone-outside inhibitor (QoI) trifloxystrobin have been commonly used. Fungicides are often applied preventively, sometimes even when conditions are not conducive for PFD development. Excessive fungicide applications may favor the selection of resistant populations of Colletotrichum spp. In this study, we assessed the fungicide sensitivity of C. acutatum isolates collected during the two distinct periods of PFD management in Brazil: before and after the trifloxystrobin and tebuconazole mixture became widely employed. The sensitivity of 254 C. acutatum isolates to carbendazim and difenoconazole and of 164 isolates to tebuconazole and trifloxystrobin was assessed. Mycelial growth inhibition of these isolates was evaluated for all the fungicides using either serial dilution of fungicide rates or the spiral gradient dilution method. In addition, inhibition of conidial germination was also assessed for trifloxystrobin. Analysis of partial ß-tub, cytb, and cyp51b gene sequences did not reveal any mutations related to resistance to MBCs, QoIs, and DMIs, respectively. In mycelial growth assays, mean EC50 values were 0.14, 0.11, and 0.21 µg/ml for difenoconazole, tebuconazole, and trifloxystrobin, respectively. The conidial germination inhibition by trifloxystrobin was similar among the tested isolates, and the mean EC50 value was 0.002 µg/ml. All isolates had similar mean mycelial growth inhibition for carbendazim, regardless of the fungicide concentrations. Therefore, based on similar EC50 values and molecular analyses, no shift in the sensitivity of isolates has been observed to the fungicides commonly used in different citrus-producing areas in Brazil.

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Pseudocercospora musae, causal agent of Sigatoka leaf spot, or yellow Sigatoka disease, is considered a major pathogen of banana (Musa spp.). Widely disseminated in Brazil, this study explored the genetic diversity in field populations of the pathogen from production areas in the Distrito Federal and the States of Bahia, Minas Gerais, and Rio Grande do Norte. Resistance to demethylation inhibitor (DMI) fungicides was also examined. For 162 isolates from 10 banana growing regions, analysis of mating type idiomorph frequency was conducted, together with estimation of genetic diversity at 15 microsatellite loci. A total of 149 haplotypes were identified across the examined populations, with an average genetic diversity of 4.06. In general, populations displayed 1:1 proportions of idiomorphs MAT1-1 and MAT1-2, providing evidence for sexual recombination. Multilocus linkage disequilibrium also indicated asexual reproduction contributing to the genetic structure of certain populations. AMOVA revealed that 86.3% of the genetic differentiation of the pathogen occurred among isolates within populations. Discriminant Analysis of Principal Components (DAPC) identified six most probable genetic groups, with no population structure associated with geographic origin or collection site. Although genetic similarity was observed among certain populations from different states, data revealed increasing genetic differentiation with increasing geographic distance, as validated by Mantel's test (r = 0.19, P < 0.001). On the basis of DMI fungicide sensitivity testing and CYP51 gene sequence polymorphism, isolates from the Distrito Federal separated into two main groups, one with generally higher EC50 values against eight DMI fungicides. A clear phenotype-to-genotype relationship was observed for isolates carrying the CYP51 alteration Y461N. Conventionally adopted fungicides for control of Sigatoka leaf spot are likely to be overcome by combined sexual and asexual reproduction mechanisms in P. musae driving genetic variability. Continued analysis of pathogen genetic diversity and monitoring of DMI sensitivity profiles of Brazilian field populations is essential for the development of integrated control strategies based on host resistance breeding and rational design of fungicide regimes.

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BACKGROUND: Stem-end rot caused by Lasiodiplodia theobromae is one of the most important diseases of papaya in northeastern Brazil. It can be controlled effectively by demethylation inhibitor (DMI) fungicides, but the occurrence of DMI resistance may decrease fungicide efficacy. RESULTS: Detached fruit studies revealed that isolates with EC50 values of 6.07 and 6.28 µg mL-1 were not controlled effectively, but reduced virulence and ability to grow at temperatures ranging from 12 to 32 °C suggesting fitness penalties were observed. Cross-resistance was observed only between difenoconazole and propiconazole. The entire cytochrome P450 sterol 14α-demethylase (LtCYP51) gene and its flanking regions were cloned. The gene was 1746 bp in length and contained three introns. The predicted protein contained 525 amino acids. Phylogenetic tree analysis showed that the LtCYP51 belongs to the CYP51B clade. No amino acid variation was found between sensitive and resistant isolates; however, the gene was constitutively more highly expressed in resistant isolates. CONCLUSION: Resistance to DMI fungicides in L. theobromae is based on LtCYP51 gene overexpression and fitness penalties may be present in difenoconazole-resistant isolates. © 2019 Society of Chemical Industry.

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Stem-end rot caused by Lasiodiplodia theobromae is one of the most devastating diseases of papaya in northeastern Brazil. It is most effectively controlled by applications of fungicides, including site-specific fungicides at risk for resistance development. This study investigated the molecular mechanisms of reduced sensitivity to the QoI fungicide azoxystrobin and resistance to the MBC fungicide thiophanate-methyl in L. theobromae from Brazilian orchards. The EC50 values for azoxystrobin in sixty-four isolates ranged from 0.36 µg/ml to 364.24 µg/ml and the frequency distribution of EC50 values formed a multimodal curve, indicating reduced sensitivity to azoxystrobin. In detached fruit assays reduced sensitive isolates were not controlled as effectively as sensitive isolates at lowest label rate. Partial fragments were obtained from target genes ß-tubulin (751 bp) and Cytb (687 bp) of isolates resistant to thiophanate-methyl and reduced sensitivity to azoxystrobin. Sequence analysis of the ß-tubulin fragment revealed a mutation corresponding to E198K in all thiophanate-methyl-resistant isolates, while reduced sensitivity to axoxystrobin was not attributable to Cytb gene alterations. The target gene-based mechanism conferring resistance to thiophanate-methyl will likely be stable even if selection pressure subsides. However, the mechanism conferring reduced sensitivity to azoxystrobin is not based on target gene modifications and thus may not be as stable as other genotypes with mutations in Cytb gene.

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Soybean rust (Phakopsora pachyrhizi) in Brazil is mainly controlled with applications of fungicides, including demethylation inhibitors (DMI) and quinone outside inhibitors (QoI). Isolates with less sensitivity to DMI and QoI have been reported, and these have been found to have mutations in the CYP51 and CYTB genes, respectively. There have been no reports of fitness costs in isolates with mutations in CYP51 and CYTB, and the aim of this work was to compare the competitive ability of isolates with lower DMI or QoI sensitivities with that of sensitive (wild-type) isolates. Urediniospores of sensitive wild-type isolates and isolates with different CYP51 or CYTB alleles were mixed and inoculated on detached soybean leaves. After 3 weeks, urediniospores were harvested and used as inoculum for the next disease cycle. Frequencies of relevant target site mutations were monitored using the pyrosequencing method over four disease cycles. Isolates with lower DMI sensitivity and different CYP51 alleles had competitive disadvantages compared with a DMI-sensitive, wild-type CYP51 isolate. In contrast, the isolate with the F129L mutation in the CYTB gene competed equally well with a QoI-sensitive, wild-type CYTB isolate under the conditions of this experiment. The CYP51 and CYTB alleles were stable in all isolates over four disease cycles when cultivated alone.

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BACKGROUND: Asian soybean rust, caused by Phakopsora pachyrhizi, is mostly controlled by demethylation inhibitor (DMI) and quinone outside inhibitor (QoI) fungicides. Mutations in the cytochrome b (CYTB) gene can lead to pathogen resistance to QoIs. The occurrence of the mutations in codons 129, 137 and 143 in the CYTB gene was investigated, and a pyrosequencing assay was developed for rapid and quantitative detection of the F129L mutation. RESULTS: Molecular analysis of the CYTB gene showed the presence of the F129L mutation in field samples and monouredinial isolates, while other mutations (G143A and G137R) were not found. The pyrosequencing was an effective method for quantitative detection of the F129L mutation, and many of the P. pachyrhizi samples showed high frequency of F129L. CONCLUSION: This is the first report of the occurrence of the F129L mutation in P. pachyrhizi. The practical relevance of this mutation for field efficacy of QoIs needs further investigation. © 2015 Society of Chemical Industry.

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Objetivou-se avaliar o uso de óleos essenciais sobre isolados de Botrytis cinerea, causador do mofo cinzento em morangueiro. Foram testados óleos essenciais de capim-limão, palmarosa, citronela, cravo, canela, menta, lavanda, tangerina, eucalipto, melaleuca, alecrim e laranja, todos estes analisados em cromatógrafo a gás acoplado a detector de massas, para identificação dos principais componentes dos óleos. Foram avaliados o crescimento micelial, produção e germinação de conídios de B. cinerea, com a incorporação do óleo no meio de cultura. Realizou-se ainda uma avaliação de voláteis e a eficiência de óleos em isolado resistente a fungicida. Para cada teste, diferentes óleos apresentaram eficiência, contudo capim limão, palmarosa, canela e menta demonstraram os melhores efeitos em todos os testes realizados. Todos os tratamentos a base de óleos demonstraram efeito semelhante a um fungicida recomendado para a cultura, a base de tiofanato metílico. Dois tratamentos mostraram-se efetivos no caso de isolado resistente (óleo de capim limão e de canela). Óleos essenciais mostram-se como opção promissora para o desenvolvimento de possíveis produtos fitossanitários para o manejo de doenças em plantas.

This study aimed evaluates essential oils in Botrytis cinerea isolates growth, which causes gray mould on strawberry. Were tested essential oils of lemon grass, palmrose, citronella, clove, cinnamon, mint, lavender, tangerine, eucalyptus, tea tree, rosemary and orange. The oils were analyzed in gas chromatograph attached to mass detector for identifying the mainly oils components. Were evaluated mycelial growth, conidia production and conidia germination of B. cinerea, with oil incorporation in culture medium. Were conducted an evaluation of oils volatile components and the efficiency of oils in fungicide-resistant isolate. For each test, different oils showed efficiency, however lemongrass, palmarosa, cinnamon and mint showed the best effects in all tests. All treatments demonstrated similar effects to recommended fungicide for the culture, which had methyl thiophanate. Two treatments (lemon grass and cinnamon oils), were effective against resistant isolate. Essential oils are shown as promising option for development of possible product for plant disease management.

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In this review article, we show that occurrence of fungicide resistance is one of the most important issues in modern agriculture. Fungicide resistance may be due to mutations of genes encoding fungicide targets (qualitative fungicide resistance) or to different mechanisms that are induced by sub-lethal fungicide stress. These mechanisms result in different and varying levels of resistance (quantitative fungicide resistance). We discuss whether or not extensive use of fungicides in agricultural environments is related to the occurrence of fungicide resistance in clinical environments. Furthermore, we provide recommendations of how development of fungicide resistant pathogen populations may be prevented or delayed.

A ocorrência de resistência a fungicidas é uma das mais importantes conseqüências da agricultura moderna. Este fato pode ser resultado de mutações em genes codificadores de resistência a fungicidas (resistência quantitativa) ou a diferentes mecanismos que são induzidos por stresse devido a doses subletais dos produtos utilizados. Estes mecanismos produzem diferentes e variados níveis de resistência (resistência quantitativa). Também é discutido se o uso extensivo de fungicidas em ambientes agricultáveis é relacionado ou não com a ocorrência de resistência em ambientes clínicos. Além disso, também são fornecidas recomendações de como prevenir ou mesmo retardar o desenvolvimento de resistência a fungicidas em patógenos.

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Despite the wide adoption of mefenoxam (Ridomil Gold EC) for vegetables in North Carolina, the incidence of Phytophthora blight on pepper (Capsicum annuum) and squash (Cucurbita pepo) is high. Seventy-five isolates of Phytophthora capsici were collected in five pepper and one squash field in order to assess mefenoxam sensitivity. The relative fitness of resistant and sensitive isolates was contrasted in vitro by their respective rates of colony growth and their ability to produce sporangia in unamended V8 juice agar medium. In in vivo experiments, the aggressiveness of isolates on pepper was evaluated. The frequency of resistant isolates in North Carolina populations was 63%, considerably higher than resistance levels in areas where mefenoxam is not widely adopted. Resistant isolates grew on amended media at rates >80 to 90% and >100% of the nonamended control at 100 µg ml-1 and 5 µg ml-1, respectively. Sensitive isolates did not growth at 5 or 100 µg ml-1. All isolates from three fields, including two pepper and a squash field, were resistant to mefenoxam. Populations from other fields were composed of either mixes of sensitive and resistant isolates or only sensitive isolates. Response to mefenoxam remained stable during the course of in vitro and in planta experiments. Occurrence of a mefenoxam-resistant population of P. capsici on squash is reported here for the first time in North Carolina. When measured by rate of colony growth, sporulation in vitro, or aggressiveness in planta, fitness of resistant isolates was not reduced. Mefenoxam-resistant isolates from squash were as aggressive on pepper as sensitive or resistant pepper isolates. These results suggest that mefenoxam-resistant populations of P. capsici are as virulent and fit as sensitive populations.

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In this review article, we show that occurrence of fungicide resistance is one of the most important issues in modern agriculture. Fungicide resistance may be due to mutations of genes encoding fungicide targets (qualitative fungicide resistance) or to different mechanisms that are induced by sub-lethal fungicide stress. These mechanisms result in different and varying levels of resistance (quantitative fungicide resistance). We discuss whether or not extensive use of fungicides in agricultural environments is related to the occurrence of fungicide resistance in clinical environments. Furthermore, we provide recommendations of how development of fungicide resistant pathogen populations may be prevented or delayed.

RESUMO

In this review article, we show that occurrence of fungicide resistance is one of the most important issues in modern agriculture. Fungicide resistance may be due to mutations of genes encoding fungicide targets (qualitative fungicide resistance) or to different mechanisms that are induced by sub-lethal fungicide stress. These mechanisms result in different and varying levels of resistance (quantitative fungicide resistance). We discuss whether or not extensive use of fungicides in agricultural environments is related to the occurrence of fungicide resistance in clinical environments. Furthermore, we provide recommendations of how development of fungicide resistant pathogen populations may be prevented or delayed.

A ocorrência de resistência a fungicidas é uma das mais importantes conseqüências da agricultura moderna. Este fato pode ser resultado de mutações em genes codificadores de resistência a fungicidas (resistência quantitativa) ou a diferentes mecanismos que são induzidos por stresse devido a doses subletais dos produtos utilizados. Estes mecanismos produzem diferentes e variados níveis de resistência (resistência quantitativa). Também é discutido se o uso extensivo de fungicidas em ambientes agricultáveis é relacionado ou não com a ocorrência de resistência em ambientes clínicos. Além disso, também são fornecidas recomendações de como prevenir ou mesmo retardar o desenvolvimento de resistência a fungicidas em patógenos.

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Isolates of the US-1 and BR-1 clonal lineages of Phytophthora infestans, collected from tomato and potato fields of two main producing regions of Brazil (south and southeast), were tested for sensitivity to the systemic fungicide metalaxyl, plus the three protectant fungicides mancozeb, chlorothalonil, and cymoxanil. For metalaxyl, one agar test and two leaf-disc tests were carried out. For all tests, metalaxyl insensitive (I), intermediately insensitive (II), and sensitive (S) isolates were detected. There was no association among metalaxyl sensitivity and region, lineage, or host. In the agar test, 35.0% of 210 isolates were classified as I, 36.0% as II, and 29.0% as S. US-1 isolates were classified as 36.1% I, 30.6% II, and 33.3% S. BR-1 isolates were 33.3% I, 23.8% II, and 42.9% S. In leaf-disc test 1 (240 isolates tested), 24.3% were classified as I, 35.0% as II, and 40.7% as S. Isolates of US-1 were 21.0% I, 39.8% II, and 39.2% S, whereas BR-1 isolates were 36.0% I, 20.0% II, and 44.0% S. In leaf-disc test 2 (96 isolates tested), which was based on an effective dose for 50% sporulation inhibition (ED50), most isolates were either I (44.8%) or II (51.0%), and only three (4.2%) were S. The US-1 isolates were 46.0% I, 51.0% II, and 3.0% S. Isolates of the BR-1 lineage were 45.0% I, 52.0% II, and 3.0% S. For mancozeb, the ED50 for 53 of 59 isolates was below 1.0 µg/ml. No isolate grew on medium amended with more than 100 µg of chlorothalonil/ml and the ED50 for 38 of 50 isolates was below 1.0 µg/ml. For cymoxanil, the ED50 for all 47 isolates tested was below 1 µg/ml. There is no evidence of resistance of resistance of P. infestans to protectant fungicides commonly used in Brazil.