Grapevine Powdery Mildew Research Articles

Using Epidemiological Principles to Explain Fungicide Resistance Management Tactics: Why do Mixtures Outperform Alternations?

Whether fungicide resistance management is optimized by spraying chemicals with different modes of action as a mixture (i.e., simultaneously) or in alternation (i.e., sequentially) has been studied by experimenters and modelers for decades. However, results have been inconclusive. We use previously parameterized and validated mathematical models of wheat Septoria leaf blotch and grapevine powdery mildew to test which tactic provides better resistance management, using the total yield before resistance causes disease control to become economically ineffective ("lifetime yield") to measure effectiveness. We focus on tactics involving the combination of a low-risk and a high-risk fungicide, and the case in which resistance to the high-risk chemical is complete (i.e., in which there is no partial resistance). Lifetime yield is then optimized by spraying as much low-risk fungicide as is permitted, combined with slightly more high-risk fungicide than needed for acceptable initial disease control, applying these fungicides as a mixture. That mixture rather than alternation gives better performance is invariant to model parameterization and structure, as well as the pathosystem in question. However, if comparison focuses on other metrics, e.g., lifetime yield at full label dose, either mixture or alternation can be optimal. Our work shows how epidemiological principles can explain the evolution of fungicide resistance, and also highlights a theoretical framework to address the question of whether mixture or alternation provides better resistance management. It also demonstrates that precisely how spray tactics are compared must be given careful consideration. [Formula: see text] Copyright © 2018 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

Ectopic expression of Arabidopsis broad-spectrum resistance gene RPW8.2 improves the resistance to powdery mildew in grapevine (Vitis vinifera)

Powdery mildew is the most economically important disease of cultivated grapevines worldwide. Here, we report that the Arabidopsis broad-spectrum disease resistance gene RPW8.2 could improve resistance to powdery mildew in Vitis vinifera cv. Thompson Seedless. The RPW8.2-YFP fusion gene was stably expressed in grapevines from either the constitutive 35S promoter or the native promoter (NP) of RPW8.2. The grapevine shoots and plantlets transgenic for 35S::RPW8.2-YFP showed reduced rooting and reduced growth at later development stages in the absence of any pathogens. Infection tests with an adapted grapevine powdery mildew isolate En NAFU1 showed that hyphal growth and sporulation were significantly restricted in transgenic grapevines expressing either of the two constructs. The resistance appeared to be attributable to the ectopic expression of RPW8.2, and associated with the enhanced encasement of the haustorial complex (EHC) and onsite accumulation of H2O2. In addition, the RPW8.2-YFP fusion protein showed focal accumulation around the fungal penetration sites. Transcriptome analysis revealed that ectopic expression of RPW8.2 in grapevines not only significantly enhanced salicylic acid-dependent defense signaling, but also altered expression of other phytohormone-associated genes. Taken together, our results indicate that RPW8.2 could be utilized as a transgene for improving resistance against powdery mildew in grapevines.

An integrative AmpSeq platform for highly multiplexed marker-assisted pyramiding of grapevine powdery mildew resistance loci

Resistance breeding often requires the introgression and tracking of resistance loci from wild species into domesticated backgrounds, typically with the goal of pyramiding multiple resistance genes, to provide durable disease resistance to breeding selections and ultimately cultivars. While molecular markers are commonly used to facilitate these efforts, high genetic diversity and divergent marker technologies can complicate marker-assisted breeding strategies. Here, amplicon sequencing (AmpSeq) was used to integrate SNP markers with dominant presence/absence markers derived from genotyping-by-sequencing and other genotyping technologies, for the simultaneous tracking of five loci for resistance to grapevine powdery mildew. SNP haploblocks defined the loci for REN1, REN2 and REN3, which confer quantitative resistance phenotypes that are challenging to measure via field ratings of natural infections. Presence/absence markers for RUN1 and REN4 were validated to predict qualitative resistance phenotypes and corresponded with previous presence/absence fluorescent electrophoretic assays. Thus, 37 AmpSeq-derived markers were identified for the five loci, and markers for REN1, REN2, REN4 and RUN1 were used for multiplexed screening and selection within diverse breeding germplasm. Poor transferability of SNP markers indicated imperfect marker-trait association in some families. Together, AmpSeq SNP haploblocks and presence/absence markers provide a high-throughput, cost-effective tool to integrate divergent technologies for marker-assisted selection and genetic analysis of introgressed disease resistance loci in grapevine.

Investigations on the role of cuticular wax in resistance to powdery mildew in grapevine

Cuticular wax on the plant epidermis inhibits or enhances prepenetration events of powdery mildew (Erysiphe necator Schwein). We examined the role of cuticular leaf and berry waxes as a resistance mechanism in four grapevine genotypes (Italia × Mercan-174, Gurcu, Isabella, Ozer Karasi) resistant to powdery mildew after natural infection and inoculation. To understand cuticular wax properties, we determined the amount of wax and antifungal effects of thin layer chromatography (TLC) fractions from cuticular leaf and berry waxes, then assessed the chemical composition of fractions with different antifungal activities using gas chromatography/mass spectrometry (GC/MS). Susceptible genotypes Cabernet Sauvignon and Italia were used for comparison. Resistant and sensitive genotypes did not differ significantly in the total amount of wax on leaves and berries; however, 27 fatty acids, 26 alkanes, 6 terpenes, 4 indole derivatives and 4 ketones, and 3 amides, 3 phenols and 3 steroids were detected in fractions with high antifungal activity (≥75% inhibition of germination) in leaf and/or berry cuticular waxes of resistant genotypes only. These compounds may be evaluated as markers for powdery mildew resistance during genotype selection in a grapevine breeding program.

Overwintering form of Erysiphe necator, the causal agent of grapevine powdery mildew in southern Syria

AbstractThe main aims of this study were to determine the overwintering form and the primary inoculum ofErysiphe necator,the causal agent of grapevine powdery mildew in southern Syria. Eleven vineyards located at five different geographical sites were visited every week from March to November in 2014 and 2015. The results of field and histopathological studies showed thatE. necatorsurvived as mycelium in dormant grapevine buds during the winter season. The first flag shoots were observed shortly after bud break in spring. The number of flag shoots varied greatly according to vineyards, cultivars and years, being present on 27.4 to 61.9% of the grapevines in 2014, and on 5.2 to 40% of the grapevines in 2015. The percentage of flag shoots on the same grapevine also varied according to the year, cultivar and location. It was between 4.3 to 9.4% in 2014, and 2.1 to 3.6% in 2015. The disease was observed only on Balady and Black cultivars. Conidia were released from the second week of May to early September. The first conidia were trapped around mid-May, and the first secondary symptoms were observed on leaves from mid-May to early June according to the site. Chasmothecia were observed on leaves in 45.5% of the studied vineyards. The first observation of chasmothecia on leaves was in July, and their numbers varied greatly between vineyards and years. Chasmothecia were not detected on bark, nor were ascospores trapped at the beginning of the season. These results confirmed that the ascospores did not have any role in the initiation of spring infection. To the best of our knowledge, this is the first report of the overwintering form ofE. necatorin Syria.

Cleistothecia and Flag Shoots: Sources of Primary Inoculum for Grape Powdery Mildew in the Western Cape Province, South Africa

Little is known about the mode of survival and sources of primary inoculum of Uncinula necator, the causal pathogen of grapevine powdery mildew, in vineyards in the Western Cape province. A study was therefore undertaken to determine whether cleistothecia and flag shoots are formed on vines in local vineyards. Flag shoots were found shortly after budbreak in September 1997 in a Carignane vineyard near Somerset West. Cleistothecia were first observed during April to May 1996 on severely infected leaves from three vineyards in the main grapegrowing areas of Stellenbosch. This was the first report of cleistothecia and flag shoot formation in vineyards in the Western Cape province. Cleistothecia occurred in small numbers on leaves (1 - 10 per leaf) and all were immature. Cleistothecia were dispersed by late summer and autumn rains from leaves to bark of grapevines, where they overwinter. No conclusion could be made regarding the viability of cleistothecia. However, the characteristics of the first symptoms that developed on leaves, namely separate, individual lesions formed at random on first-formed leaves growing in close proximity to the bark, provided circumstantial evidence that cleistothecia are dispersed to the bark. Weather conditions suitable for release of ascospores from overwintered cleistothecia occurred frequently between budbreak and bloom in all the areas.

Grapevine powdery mildew (Uncinula necator (Schw.) Burr.) – a permanent issue concerning the health status of grapes cenosis in Bulgaria

Grapes powdery mildew appeared permanently in the grapes growing regions of Bulgaria. The disease was reported in our country by Kostov in 1900. More than a century the powdery mildew disease has been causing less or greater yield losses depending on the climate. In the past when the vineyards were small and scattered, the disease was not an economic problem. Later, during the second half of the 20th century, the grapes growing areas consolidated and enlarged, and the concentration and intensification of production took place. The pathogen spread permanently to epidemic in the vineyards and the disease became destructive and economically important. During that period commercial varieties were grown susceptible to the causal agent of the powdery mildew, and organic fungicides were applied in the disease control system. Mistakes in the grapes growing technology are observed and the disease management strategies applied are not always scientifically proved. The statements that in Bulgaria there exist conditions suitable for the appearance and development of grapes powdery mildew only in the Black sea region are disproved. A new research is necessary to be done to answer the questions about: the sources of primary inoculum, the influence of the ecological conditions on the appearance and spread of the powdery mildew, and the timing for the disease control. In the survey the overwintering of the pathogen, the appearance of the first symptoms and the dynamics of the disease spread have been discussed. As for the ecological conditions in Bulgaria, it is considered that the fungus mainly survives as mycelia in the buds and on the shoots of the vines serving as a source of a permanent infection background. The studies carried through during 1994–2002 proved that the pathogen influenced by the ecological conditions could also form cleistothecia and they could be the source of the initial infection. The effect of the leaves removal around the clusters on the spread of the grapes powdery mildew and the field resistance of some varieties were studied. Critical stages of the disease appearance and spread are defined based on the research data. Management strategies are proposed including alternative approach practices.

Initiation and development of Erysiphe necator chasmothecia and their role in the epidemiology of grapevine powdery mildew in southern Syria

Powdery mildew caused by <em>Erysiphe necator</em> is the most important fungal disease of grapevine in southern Syria. The purpose of this study was to determine the development of chasmothecia and their role as a primary inoculum in spring. Leaves and/or branches were examined by a stereo binocular from July to December 2014 and 2015. The number of chasmothecia was estimated on both surfaces of the leaves, and their viability was estimated by microscopic examination. During 2 years of survey chasmothecia were detected in 45.5% of vineyards. The initial development of chasmothecia on infected leaves was observed in the second half of July. Their numbers increased from July to October, and the sudden reduction at the beginning of November was noted. Chasmothecia were formed on 38.7% of infected leaves, with 12.5%, 18.4%, and 7.5% on the upper, under and on both surfaces of infected leaves respectively. Chasmothecia were more frequent on the leaf under side (0.6 / leaf) than on the leaf upper side (0.4 / leaf), but their occurrence on both sides together was relatively low (0.2 / leaf), and their numbers were highly variable between vineyards and years. Microscopic examination showed that chasmothecia contained 1–5 (usually three) asci with 1–4 (usually three) ascospores in each asci, and 65.6% of chasmothecia were empty. Their viability decreased between December and February, with an average viability of 1.2% and 0.2% in March and April, respectively. Chasmothecia were not detected on bark and ascospores were not trapped at the beginning of the season. These results indicate that the ascospores have no or little role in the initiation of spring infection. To the best of our knowledge, this is the first report of <em>E. necator</em> chasmothecia development and their role in the initiating infection on grapevine in Syria.

Identification and Characterization of Erysiphe necator-Responsive MicroRNAs in Chinese Wild Vitis pseudoreticulata by High-Throughput Sequencing.

Grapevine powdery mildew is one of the most damaging fungal diseases. Therefore, a precise understanding of the grapevine disease resistance system becomes a subject of significant importance. Plant microRNAs(miRNAs) have been implicated to play regulatory roles in plant biotic stress responses. In this study, high-throughput sequencing and miRDeep-P were employed to identify miRNAs in Chinese wild Vitis pseudoreticulata leaves following inoculation with Erysiphe necator. Altogether, 126 previously identified microRNAs and 124 novel candidates of miRNA genes were detected. Among them, 43 conserved miRNAs belong to 20 families and 23 non-conserved but previously-known miRNAs belong to 15 families. Following E. necator inoculation, 119 miRNAs were down-regulated and 131 were up-regulated. Furthermore, the expression changes occurring in 32 miRNAs were significant. The expression patterns of some miRNAs were validated by semi-quantitative RT-PCR and qRT-PCR. A total of 485 target genes were predicted and categorized by Gene Ontology (GO). In addition, 14 vvi-miRNAs were screened with 36 targets which may be involved in powdery mildew resistance in grape. Highly accumulated vvi-NewmiR2118 was detected from accession “Baihe-35-1,” whose targets were mostly NBS-LRR resistance genes. It was down-regulated rapidly and strongly in “Baihe-35-1” leaves after inoculated with E. necator, indicating its involvement in grape powdery mildew resistance. Finally, the study verified interaction between vvi-NewmiR2118 and RPP13 by histochemical staining and GUS fluorescence quantitative assay.

Knockdown of MLO genes reduces susceptibility to powdery mildew in grapevine.

Erysiphe necator is the causal agent of powdery mildew (PM), one of the most destructive diseases of grapevine. PM is controlled by sulfur-based and synthetic fungicides, which every year are dispersed into the environment. This is why PM-resistant varieties should become a priority for sustainable grapevine and wine production. PM resistance can be achieved in other crops by knocking out susceptibility S-genes, such as those residing at genetic loci known as MLO (Mildew Locus O). All MLO S-genes of dicots belong to the phylogenetic clade V, including grapevine genes VvMLO7, 11 and 13, which are upregulated during PM infection, and VvMLO6, which is not upregulated. Before adopting a gene-editing approach to knockout candidate S-genes, the evidence that loss of function of MLO genes can reduce PM susceptibility is necessary. This paper reports the knockdown through RNA interference of VvMLO6, 7, 11 and 13. The knockdown of VvMLO6, 11 and 13 did not decrease PM severity, whereas the knockdown of VvMLO7 in combination with VvMLO6 and VvMLO11 reduced PM severity up to 77%. The knockdown of VvMLO7 and VvMLO6 seemed to be important for PM resistance, whereas a role for VvMLO11 does not seem likely. Cell wall appositions (papillae) were present in both resistant and susceptible lines in response to PM attack. Thirteen genes involved in defense were less upregulated in infected mlo plants, highlighting the early mlo-dependent disruption of PM invasion.

Co-Occurrence of Two Allelic Variants of CYP51 in Erysiphe necator and Their Correlation with Over-Expression for DMI Resistance.

Demethylation inhibitors (DMIs) have been an important tool in the management of grapevine powdery mildew caused by Erysiphe necator. Long-term, intensive use of DMIs has resulted in reduced sensitivity in field populations. To further characterize DMI resistance and understand resistance mechanisms in this pathogen, we investigated the cyp51 sequence of 24 single-spored isolates from Virginia and surrounding states and analyzed gene expression in isolates representing a wide range of sensitivity. Two cyp51 alleles were found with respect to the 136th codon of the predicted EnCYP51 sequence: the wild-type (TAT) and the mutant (TTT), which results in the known Y136F amino acid change. Some isolates possessed both alleles, demonstrating gene duplication or increased gene copy number and possibly a requirement for at least one mutant copy of CYP51 for resistance. Cyp51 was over-expressed 1.4- to 19-fold in Y136F-mutant isolates. However, the Y136F mutation was absent in one isolate with moderate to high resistance factor. Two additional synonymous mutations were detected as well, one of which, A1119C was present only in isolates with high cyp51 expression. Overall, our results indicate that at least two mechanisms, cyp51 over-expression and the known target-site mutation in CYP51, contribute to resistance in E. necator, and may be working in conjunction with each other.

Sensitivity of grapevine powdery mildew (<i>Erysiphe necator</i>) to demethylation inhibitor and quinone outside inhibitor fungicides in New Zealand

Isolates of Erysiphe necator from Hawkes Bay and Marlborough vineyards were tested for sensitivity to three demethylation inhibitor (DMI) fungicides (myclobutanil penconazole and cyproconazole) and one quinone outside inhibitor (QoI) fungicide (trifloxystrobin) Mean colony diameter was determined in a detached grape leaf assay for approximately 20 isolates per vineyard at 1 and 10 mg/litre of each fungicide Resistance to trifloxystrobin was prevalent in Hawkes Bay and Marlborough Mycobutanil resistance was found in both regions but particularly in Marlborough There was some loss of sensitivity to penconazole particularly in Marlborough Cyproconazole showed greater efficacy against E necator than the other two DMIs tested There was no explanation for the high trifloxystrobin resistance in sampled vineyards with few reports of trifloxystrobin use For DMIs there was a strong relationship between number of DMI fungicide applications over 10 years and DMI resistance across all three DMI active ingredients in both regions

Efficacy of Biopesticides and Leaf Removal in Grapevine Powdery Mildew Management

Integrating biological-based fungicides into conventional spray programs may help with fungicide resistance management. However, little is known about how to best integrate these products while still maintaining maximum disease control. Programs with as few as one synthetic fungicide or as many as three synthetic fungicides added to a biopesticide-based rotation during the bloom period of Vitis vinifera had significantly better disease control than a biopesticide-only-based program. When integrated with different timings of fruit-zone leaf removal, specific combinations of biopesticide programs and fruit-zone leaf removal enhanced the efficacy of that program to be on par with disease control seen in a program entirely based on synthetic fungicides. This suggests that when designing a fungicide program using biopesticides as a base, the addition of a synthetic fungicide during the window of ontogenic susceptibility in clusters and the adoption of cultural practices such as leaf removal can significantly improve the efficacy of that program. Accepted for publication 11 April 2016. Published 20 April 2016.

Sporulation rate in culture and mycoparasitic activity, but not mycohost specificity, are the key factors for selecting Ampelomyces strains for biocontrol of grapevine powdery mildew (Erysiphe necator)

To develop a new biofungicide product against grapevine powdery mildew, caused by Erysiphe necator, cultural characteristics and mycoparasitic activities of pre-selected strains of Ampelomyces spp. were compared in laboratory tests to the commercial strain AQ10. Then, a 2-year experiment was performed in five vineyards with a selected strain, RS1-a, and the AQ10 strain. This consisted of autumn sprays in vineyards as the goal was to reduce the number of chasmothecia of E. necator, and, thus, the amount of overwintering inocula, instead of targeting the conidial stage of the pathogen during spring and summer. This is a yet little explored strategy to manage E. necator in vineyards. Laboratory tests compared the growth and sporulation of colonies of a total of 33 strains in culture; among these, eight strains with superior characteristics were compared to the commercial product AQ10 Biofungicide® in terms of their intra-hyphal spread, pycnidial production, and reduction of both asexual and sexual reproduction in E. necator colonies. Mycoparasitic activities of the eight strains isolated from six different powdery mildew species, including E. necator, did not depend on their mycohost species of origin. Strain RS1-a, isolated from rose powdery mildew, showed, together with three strains from E. necator, the highest rate of parasitism of E. necator chasmothecia. In field experiments, each strain, AQ10 and RS1-a, applied twice in autumn, significantly delayed and reduced early-season development of grapevine powdery mildew in the next year. Therefore, instead of mycohost specificity of Ampelomyces presumed in some works, but not confirmed by this study, the high sporulation rate in culture and the mycoparasitic patterns became the key factors for proposing strain RS1-a for further development as a biocontrol agent of E. necator.

Foreword: Special issue on fungal grapevine diseases

An impressively large proportion of fungicides applied in European, North American and Australian agriculture has been used to manage grapevine powdery mildew (Erysiphe necator), grapevine downy mildew (Plasmopara viticola), and botrytis bunch rot (Botrytis cinerea). These fungal and oomycetous plant pathogens remain amongst the most important plant pathological problems worldwide, despite extensive research on pathogenesis, epidemiology and management.

Verticillium lecanii als Hyperparasit des Rebmehltaus (Uncinula necator)

Verticillium lecanii as a hyperparasite of grapevine powdery mildew ( Uncinula necator ) Oidiospores of Uncinula necator germinating on leaf discs of Vitis vinifera cvs or on ultrathin metacrylate membranes were infected by a hyperparasite which was identified as Verticillium lecanii . Oidiospores were penetrated by hyphae of this fungus either directly or by means of appressoria. Infected spores eventually shrunk and died.

Cold Stress-Induced Disease Resistance (SIDR): indirect effects of low temperatures on host-pathogen interactions and disease progress in the grapevine powdery mildew pathosystem

Erysiphe necator is an obligate biotroph capable of infecting three genera within the Vitaceae (Vitis, Parthenocissus, and Ampelopsis). The pathogen inhabits a niche unique to most powdery mildews, i.e., wholly external mycelial growth supported by haustoria within the subtending host epidermal cells. This growth habit coupled with its biotrophic reliance on the host makes E. necator sensitive to both direct effects of abiotic stresses on the pathogen and indirect abiotic effects via the host responses. Development of the pathogen during acute cold events (e.g., 1 h at 4 °C) results in death of hyphal segments and a prolonged latency, an effect further increased by the development of ontogenic resistance as epidermal tissues of leaves and berries age. Acute cold events can also stress the host prior to the arrival of the pathogen, and thereby reduce susceptibility to infection via cold Stress-Induced Disease Resistance (SIDR), a recently described phenomenon. This effect requires approximately 24 h post-cold before maximal resistance effect occurs, and is also transient in that the effect diminishes to a basal level within 48 h after exposure. Although the phenotypic responses to cold SIDR may be similar to those observed on ontogenically-resistant leaves, the effects of cold SIDR and ontogenic resistance are additive. Sufficient tools are now available for investigating the mechanistic basis of cold SIDR. While pathosystems involving obligate biotrophs complicate research on direct and indirect environmental effects on the pathogen, this requisite interaction also creates interesting systems to understand how the condition of the host may influence subsequent disease development. At the population level, the effects of repeated cold events have profound effects on the nature of epidemic progress and implications for management of grape powdery mildew. The objective in this review is to summarize our current knowledge regarding the indirect and consequential effects of low temperature on the development of grapevine powdery mildew.

Strategies for RUN1 Deployment Using RUN2 and REN2 to Manage Grapevine Powdery Mildew Informed by Studies of Race Specificity.

The Toll/interleukin-1 receptor nucleotide-binding site leucine-rich repeat gene, "resistance to Uncinula necator 1" (RUN1), from Vitis rotundifolia was recently identified and confirmed to confer resistance to the grapevine powdery mildew fungus Erysiphe necator (syn. U. necator) in transgenic V. vinifera cultivars. However, sporulating powdery mildew colonies and cleistothecia of the heterothallic pathogen have been found on introgression lines containing the RUN1 locus growing in New York (NY). Two E. necator isolates collected from RUN1 vines were designated NY1-131 and NY1-137 and were used in this study to inform a strategy for durable RUN1 deployment. In order to achieve this, fitness parameters of NY1-131 and NY1-137 were quantified relative to powdery mildew isolates collected from V. rotundifolia and V. vinifera on vines containing alleles of the powdery mildew resistance genes RUN1, RUN2, or REN2. The results clearly demonstrate the race specificity of RUN1, RUN2, and REN2 resistance alleles, all of which exhibit programmed cell death (PCD)-mediated resistance. The NY1 isolates investigated were found to have an intermediate virulence on RUN1 vines, although this may be allele specific, while the Musc4 isolate collected from V. rotundifolia was virulent on all RUN1 vines. Another powdery mildew resistance locus, RUN2, was previously mapped in different V. rotundifolia genotypes, and two alleles (RUN2.1 and RUN2.2) were identified. The RUN2.1 allele was found to provide PCD-mediated resistance to both an NY1 isolate and Musc4. Importantly, REN2 vines were resistant to the NY1 isolates and RUN1REN2 vines combining both genes displayed additional resistance. Based on these results, RUN1-mediated resistance in grapevine may be enhanced by pyramiding with RUN2.1 or REN2; however, naturally occurring isolates in North America display some virulence on vines with these resistance genes. The characterization of additional resistance sources is needed to identify resistance gene combinations that will further enhance durability. For the resistance gene combinations currently available, we recommend using complementary management strategies, including fungicide application, to reduce populations of virulent isolates.

Metrafenone resistance in a population of Erysiphe necator in northern Italy.

BACKGROUNDMetrafenone has been used in Europe in integrated pest management programmes since 2006 to control powdery mildews, including Erysiphe necator. Its exact mode of action is not known, but it is unique among fungicide classes used in powdery mildew management. Recently, resistance to metrafenone was reported in Blumeria graminis f. sp. tritici. In this study we investigated metrafenone resistance in Erysiphe necator in northern Italy.RESULTSMetrafenone efficacy to control grapevine powdery mildew was monitored in three consecutive years in the field, and its reduced activity was observed in 2013. Out of 13 monoconidial isolates, two sensitive strains were identified, which did not grow at the fungicide concentration recommended for field application. The remaining strains showed variable response to metrafenone, and five of them grew and sporulated similarly to the control, even at 1250 mg L−1 of metrafenone. Moreover, the resistant strains showed cross‐resistance to pyriofenone, which belongs to the same FRAC group as metrafenone.CONCLUSIONThe results indicate the emergence of metrafenone resistance in an Italian population of Erysiphe necator. Further studies are needed to gain insight into the metrafenone's mode of action and to understand the impact of resistance on changes in the pathogen population structure, fitness and spread of resistant strains, which will be indicative for designing appropriate antiresistance measures. © 2015 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

BREEDING FOR DURABLE RESISTANCE TO DOWNY AND POWDERY MILDEW IN GRAPEVINE

The current strategy to control grapevine downy and powdery mildew relies on chemical treatments. An alternative solution to the use of chemicals is the development of varieties resistant to pathogens. Therefore a program to create new grapevine varieties with a durable resistance to downy and powdery mildew and with a berry quality suitable for high quality wine production was launched in 2000. The situation seemed very favourable to start up such a program: plenty of resistance sources were available, and several resistance QTLs with various effects had already been discovered. Nevertheless a first case of downy mildew resistance breakdown was described in the cultivar ?Bianca?. Even if resistance durability is a retrospective judgment, some principles can be considered to monitor the risk of resistance break-down: pyramiding resistance genes, taking into account the evolutionary potential of the pathogen or assessing the cost of virulence, from a practical point of view, a durable management of resistance meaning to reduce the selection pressure applied by the resistance genes on the pathogen population. Therefore we chose to use multiple sources of resistance as genitors. The genetic determinism of most of them was analysed and the molecular markers derived from these studies were quickly integrated to assist with selection. Concerning the shortening of the breeding process, we developed a greenhouse growing method allowing to go from seed to seed in one year and a resistance phenotyping platform to get the precise plant phenotype as early as possible in the process. Moreover all the field trials have been performed through an experimental network, which allows us to save time and to take into account the differences in environment conditions. The overall process of selection is organized into three steps and takes 16 years. We expect to propose our first varieties for registration in 2016.