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Meloidogyne spp. are the most devastating plant‐parasitic nematodes affecting tomato worldwide. Although resistant cultivars and rootstocks are used, selection for virulence occurs in the pathogen. Consequently, using other resistance sources, such as Solanumtorvum, could improve resistance durability. Several experiments in microplots and plastic greenhouses were carried out to determine the potential use of S. torvum as a tomato rootstock to protect against M. incognita and M. javanica. In microplots, the relationship between nematode density at transplanting (Pi) and multiplication rate did not differ between Meloidogyne species in either ungrafted or grafted tomato. However, maximum multiplication rate and maximum density on grafted tomato were 1.27% and 2.93% those on ungrafted, respectively. The grafted tomato plants yielded between 2.9 and 7.5 more times than the ungrafted plants at Pi ≥ 100 eggs + J2s per 100 cm3 of soil, but no differences were observed in plastic greenhouse where a large amount of scion‐rooting occurred. In microplots, the quality of the tomato fruits of ungrafted and grafted plants was affected by the Pi. In parallel, some pot experiments were conducted on S. torvum and susceptible eggplant to determine the putative selection for nematode virulence to S. torvum and the nematode fitness cost. These showed that the nematode subpopulations infected and reproduced less on S. torvum than on eggplant. However, the female fertility was only reduced after development of three or four subpopulations on S. torvum. Finally, a histopathological study showed that nematode infection and development in S. torvum was delayed compared to eggplant. [ABSTRACT FROM AUTHOR]

Growing genetically resistant plants allows pathogen populations to be controlled and reduces the use of pesticides. However, pathogens can quickly overcome such resistance. In this context, how can we achieve sustainable crop protection? This crucial question has remained largely unanswered despite decades of intense debate and research effort. In this study, we used a bibliographic analysis to show that the research field of resistance durability has evolved into three subfields: (1) "plant breeding" (generating new genetic material), (2) "molecular interactions" (exploring the molecular dialogue governing plant–pathogen interactions) and (3) "epidemiology and evolution" (explaining and forecasting of pathogen population dynamics resulting from selection pressure[s] exerted by resistant plants). We argue that this triple split of the field impedes integrated research progress and ultimately compromises the sustainable management of genetic resistance. After identifying a gap among the three subfields, we argue that the theoretical framework of population genetics could bridge this gap. Indeed, population genetics formally explains the evolution of all heritable traits, and allows genetic changes to be tracked along with variation in population dynamics. This provides an integrated view of pathogen adaptation, in particular via evolutionary–epidemiological feedbacks. In this Opinion Note, we detail examples illustrating how such a framework can better inform best practices for developing and managing genetically resistant cultivars. [ABSTRACT FROM AUTHOR]

The dominant Pvr4 gene in pepper (Capsicum annuum) confers resistance to members of six potyvirus species, all of which belong to the Potato virus Y (PVY) phylogenetic group. The corresponding avirulence factor in the PVY genome is the NIb cistron (i.e., RNA-dependent RNA polymerase). Here, we describe a new source of potyvirus resistance in the Guatemalan accession C. annuum cv. PM949. PM949 is resistant to members of at least three potyvirus species, a subset of those controlled by Pvr4. The F1 progeny between PM949 and the susceptible cultivar Yolo Wonder was susceptible to PVY, indicating that the resistance is recessive. The segregation ratio between resistant and susceptible plants observed in the F2 progeny matched preferably with resistance being determined by two unlinked recessive genes independently conferring resistance to PVY. Inoculations by grafting resulted in the selection of PVY mutants breaking PM949 resistance and, less efficiently, Pvr4–mediated resistance. The codon substitution E472K in the NIb cistron of PVY, which was shown previously to be sufficient to break Pvr4 resistance, was also sufficient to break PM949 resistance, a rare example of cross-pathogenicity effect. In contrast, the other selected NIb mutants showed specific infectivity in PM949 or Pvr4 plants. Comparison of Pvr4 and PM949 resistance, which share the same target in PVY, provides interesting insights into the determinants of resistance durability. [ABSTRACT FROM AUTHOR]

Genetic resistance is a cornerstone for managing clubroot (Plasmodiophora brassicae). However, when used repeatedly, a clubroot resistance (CR) gene can be broken rapidly. In this study, canola inbred/hybrid lines carrying one or two CR genes (Rcr1/CRaM and Crr1rutb) were assessed against P. brassicae pathotype X by repeated exposure to the same inoculum source under a controlled environment. Lines carrying two CR genes, either Rcr1 + Crr1rutb or CRaM + Crr1rutb, showed partial resistance. Selected lines were inoculated with a field pathotype X population (L-G3) at 5 × 106 resting spores/g soil, and all clubs were returned to the soil they came from six weeks after inoculation. The planting was repeated for five cycles, with diseased roots being returned to the soil after each cycle. The soil inoculum was quantified using qPCR before each planting cycle. All lines with a single CR gene were consistently susceptible, maintaining high soil inoculum levels over time. The lines carrying two CR genes showed much lower clubroot severity, resulting in a 10-fold decline in soil inoculum. These results showed that the CR-gene stacking provided moderate resistance against P. brassicae pathotype X, which may also help reduce the pathogen inoculum buildup in soil. [ABSTRACT FROM AUTHOR]

Abstract We have modeled the evolutionary epidemiology of spore‐producing plant pathogens in heterogeneous environments sown with several cultivars carrying quantitative resistances. The model explicitly tracks the infection‐age structure and genetic composition of the pathogen population. Each strain is characterized by pathogenicity traits determining its infection efficiency and a time‐varying sporulation curve taking into account lesion aging. We first derived a general expression of the basic reproduction number R0 for fungal pathogens in heterogeneous environments. We show that the evolutionary attractors of the model coincide with local maxima of R0 only if the infection efficiency is the same on all host types. We then studied the contribution of three basic resistance characteristics (the pathogenicity trait targeted, resistance effectiveness, and adaptation cost), in interaction with the deployment strategy (proportion of fields sown with a resistant cultivar), to (i) pathogen diversification at equilibrium and (ii) the shaping of transient dynamics from evolutionary and epidemiological perspectives. We show that quantitative resistance affecting only the sporulation curve will always lead to a monomorphic population, whereas dimorphism (i.e., pathogen diversification) can occur if resistance alters infection efficiency, notably with high adaptation costs and proportions of the resistant cultivar. Accordingly, the choice of the quantitative resistance genes operated by plant breeders is a driver of pathogen diversification. From an evolutionary perspective, the time to emergence of the evolutionary attractor best adapted to the resistant cultivar tends to be shorter when resistance affects infection efficiency than when it affects sporulation. Conversely, from an epidemiological perspective, epidemiological control is always greater when the resistance affects infection efficiency. This highlights the difficulty of defining deployment strategies for quantitative resistance simultaneously maximizing epidemiological and evolutionary outcomes.

We describe a standard method for characterizing the virulence profile of Plasmopara viticola, the causal agent of grapevine downy mildew. We used 33 European strains to inoculate six grapevine varieties carrying the principal factors for resistance to downy mildew (Rpv1, Rpv3.1, Rpv3.2, Rpv5, Rpv6, Rpv10, and Rpv12) and the susceptible Vitis vinifera 'Chardonnay'. For each interaction, we characterized the level of sporulation by image analysis and the intensity of the grapevine hypersensitive response by visual score. We propose a definition for the breakdown of grapevine quantitative resistances combining these two traits. Among the 33 strains analyzed, 28 are virulent on at least one resistance factor. We identified five different pathotypes across the 33 strains analyzed: two pathotypes overcoming a single resistance factor (vir3.1 and vir3.2) and three complex pathotypes overcoming multiple resistance factors (vir3.1,3.2; vir3.2,12; vir3.1,3.2,10). Our findings confirm the widespread occurrence of P. viticola strains overcoming the Rpv3 haplotypes (28 strains). We also detected the first breakdown of resistance to the Rpv 10 by a strain from Germany and the breakdown of Rpv 12 factors by a strain from Hungary. The pathotyping method proposed here and the associated differential host range lay the groundwork for the early detection of resistance breakdown in grapevines. This approach will also facilitate the monitoring of the evolution of P. viticola populations at large spatial scales. This is an essential step forward to promoting durable management of the resistant grapevine varieties currently available. [ABSTRACT FROM AUTHOR]

Plant RNA viruses are one of the most destructive pathogens that cause a significant loss in crop production worldwide. They have evolved with high genetic diversity and adaptability due to the short replication cycle and high mutation rate during genome replication, which are characteristics of RNA viruses. Plant RNA viruses exist as quasispecies with high genetic diversity; thereby, a rapid population transition with new fitness can occur due to selective pressure resulting from environmental changes. Plant resistance can act as selective pressure and affect the fitness of the virus, which may lead to the emergence of resistance-breaking variants. In this paper, we introduced the evolutionary perspectives of plant RNA viruses and the driving forces in their evolution. Based on this, we discussed the mechanism of the emergence of variant viruses that overcome plant resistance. In addition, strategies for deploying plant resistance to viral diseases and improving resistance durability were discussed.

The dominant Pvr4 gene in pepper (Capsicum annuum) confers resistance to members of six potyvirus species, all of which belong to the Potato virus Y (PVY) phylogenetic group. The corresponding avirulence factor in the PVY genome is the NIb cistron (i.e., RNA-dependent RNA polymerase). Here, we describe a new source of potyvirus resistance in the Guatemalan accession C. annuum cv. PM949. PM949 is resistant to members of at least three potyvirus species, a subset of those controlled by Pvr4. The F1 progeny between PM949 and the susceptible cultivar Yolo Wonder was susceptible to PVY, indicating that the resistance is recessive. The segregation ratio between resistant and susceptible plants observed in the F2 progeny matched preferably with resistance being determined by two unlinked recessive genes independently conferring resistance to PVY. Inoculations by grafting resulted in the selection of PVY mutants breaking PM949 resistance and, less efficiently, Pvr4–mediated resistance. The codon substitution E472K in the NIb cistron of PVY, which was shown previously to be sufficient to break Pvr4 resistance, was also sufficient to break PM949 resistance, a rare example of cross-pathogenicity effect. In contrast, the other selected NIb mutants showed specific infectivity in PM949 or Pvr4 plants. Comparison of Pvr4 and PM949 resistance, which share the same target in PVY, provides interesting insights into the determinants of resistance durability.

We have modeled the evolutionary epidemiology of spore‐producing plant pathogens in heterogeneous environments sown with several cultivars carrying quantitative resistances. The model explicitly tracks the infection‐age structure and genetic composition of the pathogen population. Each strain is characterized by pathogenicity traits determining its infection efficiency and a time‐varying sporulation curve taking into account lesion aging. We first derived a general expression of the basic reproduction number R0 for fungal pathogens in heterogeneous environments. We show that the evolutionary attractors of the model coincide with local maxima of R0 only if the infection efficiency is the same on all host types. We then studied the contribution of three basic resistance characteristics (the pathogenicity trait targeted, resistance effectiveness, and adaptation cost), in interaction with the deployment strategy (proportion of fields sown with a resistant cultivar), to (i) pathogen diversification at equilibrium and (ii) the shaping of transient dynamics from evolutionary and epidemiological perspectives. We show that quantitative resistance affecting only the sporulation curve will always lead to a monomorphic population, whereas dimorphism (i.e., pathogen diversification) can occur if resistance alters infection efficiency, notably with high adaptation costs and proportions of the resistant cultivar. Accordingly, the choice of the quantitative resistance genes operated by plant breeders is a driver of pathogen diversification. From an evolutionary perspective, the time to emergence of the evolutionary attractor best adapted to the resistant cultivar tends to be shorter when resistance affects infection efficiency than when it affects sporulation. Conversely, from an epidemiological perspective, epidemiological control is always greater when the resistance affects infection efficiency. This highlights the difficulty of defining deployment strategies for quantitative resistance simultaneously maximizing epidemiological and evolutionary outcomes. [ABSTRACT FROM AUTHOR]

Root-knot nematode (RKN) species are predominant pests of crops, attacking stone fruit crops Prunus spp. under Mediterranean climate conditions worldwide. Natural resistance for rootstock breeding is a control method that is gaining interest as an alternative to the highly toxic nematicides. This review first reports an outline of the root-knot nematodes parasitizing stone fruit crops and the Prunus species and rootstocks. It then describes the main sources of resistance detected among the Prunus germplasm and focuses on the major resistance genes identified and their characteristics (spectrum, durability, histological mechanism, effect of temperature, interaction with other pests and diseases, etc.). In peach, besides the RMia reference gene, the new genes PkMi and Mf, also located on chromosome 2, need to be characterized regarding their spectrum and relationship. The two other Prunus reference genes, Ma from plum (complete spectrum) and RMja from almond (more restricted spectrum), are orthologs that belong to a TIR-NB-LRR (TNL) cluster on chromosome 7. The review finally summarizes the positional cloning of the Ma gene and the characterization of its unique TNL structure, encompassing a five-times repeated post-LRR domain. Deciphering how this structure is functionally involved in Ma’s remarkable biological properties is a real challenge for the future.

The eggplant Solanum melongena cv. Cristal, either ungrafted or grafted onto the Solanum torvum rootstock cv. Brutus, was cultivated for two consecutive years in the same plots in a plastic greenhouse to assess the level of resistance to Meloidogyne incognita and crop yield. At the end of the second crop, the putative selection for virulence of the nematode subpopulations coming from infected ungrafted and grafted eggplant was assessed in the eggplant and in S. torvum in a pot experiment. Nematode population densities at transplantation in 2017 ranged from 2 to 378 per 100 cm3 of soil and did not differ between ungrafted and grafted eggplant. At the end of each crop, a higher galling index and number of nematodes in soil and in roots was registered in ungrafted compared to grafted eggplant. The grafted eggplant was categorized as resistant in 2017 and as highly resistant in 2018. Eggplant yield did not differ irrespective of grafting in 2017 after being cultivated for 135 days, but it differed after 251 days of cultivation in 2018. In the pot experiment, S. torvum was categorized as resistant to both M. incognita subpopulations. However, the M. incognita subpopulation obtained from roots of S. torvum produced 49.4% fewer egg masses and 56% fewer eggs per plant in the eggplant than the nematode subpopulation obtained from roots of the eggplant cv. Cristal. The results of this study revealed that the infective and reproductive fitness of the nematode decreased without having been selected for virulence. [ABSTRACT FROM AUTHOR]

BACKGROUND: Susceptible tomato cv. Durinta, ungrafted or grafted onto cv. Aligator resistant rootstock, both followed by the susceptible melon cv. Paloma, ungrafted or grafted onto Cucumis metuliferus BGV11135, and in the reverse order, were cultivated from 2015 to 2017 in the same plots in a plastic greenhouse, infested or not with Meloidogyne incognita. For each crop, soil nematode densities, galling index, number of eggs per plant and crop yield were determined. Virulence selection was evaluated in pot experiments. RESULTS: In the tomato–melon rotation, nematode densities increased progressively for the grafted tomato, being higher than for ungrafted plants at the end of the study; this was not the case in the melon–tomato rotation. Grafted crops yielded more than ungrafted crops in the infested plots. Virulence against the Mi1.2 gene was detected, but not against C. metuliferus. Reproduction of M. incognita on the resistant tomato was ∼ 120% that on the susceptible cultivar after the first grafted tomato crop, but this decreased to just 25% at the end of the experiment. CONCLUSION: Alternating different resistant plant species suppresses nematode population growth rate and yield losses. Although this strategy does not prevent virulence selection, the level was reduced. © 2018 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Plant qualitative resistances to viruses are natural exhaustible resources that can be impaired by the emergence of resistance-breaking (RB) virus variants. Mathematical modelling can help determine optimal strategies for resistance durability by a rational deployment of resistance in agroecosystems. Here, we propose an innovative approach, built up from our previous empirical studies, based on plant cultivars combining qualitative resistance with quantitative resistance narrowing population bottlenecks exerted on viruses during host-to-host transmission and/or within-host infection. Narrow bottlenecks are expected to slow down virus adaptation to plant qualitative resistance. To study the effect of bottleneck size on yield, we developed a stochastic epidemic model with mixtures of susceptible and resistant plants, relying on continuous-time Markov chain processes. Overall, narrow bottlenecks are beneficial when the fitness cost of RB virus variants in susceptible plants is intermediate. In such cases, they could provide up to 95 additional percentage points of yield compared with deploying a qualitative resistance alone. As we have shown in previous works that virus population bottlenecks are at least partly heritable plant traits, our results suggest that breeding and deploying plant varieties exposing virus populations to narrowed bottlenecks will increase yield and delay the emergence of RB variants. This article is part of the theme issue 'Modelling infectious disease outbreaks in humans, animals and plants: approaches and important themes'. This issue is linked with the subsequent theme issue 'Modelling infectious disease outbreaks in humans, animals and plants: epidemic forecasting and control'. [ABSTRACT FROM AUTHOR]

Pyramiding resistance genes is predicted to increase the durability of resistant rice varieties against phloem-feeding herbivores. We examined responses by the green leafhopper, Nephotettix virescens (Hemiptera: Cicadellidae), to near-isogenic rice lines with zero, one and two resistance genes. The recurrent parent (T65) and monogenic lines ( GRH2- NIL and GRH4- NIL) with genes for resistance to the green rice leafhopper, Nephotettix cincticeps (Hemiptera: Cicadellidae), were susceptible to the green leafhopper, but the pyramided line ( GRH2/GRH4- PYL) was highly resistant to the green leafhopper. We selected green leafhoppers, N. virescens , from five sites in the Philippines for over 20 generations on each of the four lines. Populations selected on GRH2/GRH4- PYL gained partial virulence (feeding and development equal to that on T65) to the pyramided line within 10 generations and complete virulence (egg-laying equal to that on T65) within 20 generations. After 20 generations of rearing on the susceptible monogenic lines, green leafhoppers were also capable of developing and laying eggs on GRH2/GRH4- PYL. Furthermore, green leafhoppers reared on the susceptible GRH4- NIL for 20 generations showed equal preferences for T65 and GRH2/GRH4- PYL in choice bioassays. Our results indicate that previous long-term exposure to ineffective genes (including unperceived resistance genes) could dramatically reduce the durability of pyramided resistance. We suggest that informed crop management and deployment strategies should be developed to accompany rice lines with pyramided resistance and avoid the build-up of virulent herbivore populations. [ABSTRACT FROM AUTHOR]

Studying phenotypic and genomic modifications associated with pathogen adaptation to resistance is a crucial step to better understand and anticipate resistance breakdown. This short review summarizes recent results obtained using experimentally evolved populations of the potato cyst nematode Globodera pallida. In a first step, the variability of resistance durability was explored in four different potato genotypes carrying the resistance quantitative trait loci (QTL) GpaVvrn originating from Solanum vernei but differing by their genetic background. The consequences of the adaptation to resistance in terms of local adaptation, cross-virulence and virulence cost were then investigated. Finally, a genome scan approach was performed in order to identify the genomic regions involved in this adaptation. Results showed that nematode populations were able to adapt to the QTL GpaVvrn, and that the plant genetic background has a strong impact on resistance durability. A trade-off between the adaptations to different resistant potato genotypes was detected, and we also showed that adaptation to the resistance QTL GpaVvrn from S. vernei did not allow adaptation to the colinear locus from S. sparsipilum (GpaVspl). Unexpectedly, the adaptation to resistance led to an increase of virulent individual’s fitness on a susceptible host. Moreover, the genome scan approach allowed the highlighting of candidate genomic regions involved in adaptation to host plant resistance. This review shows that experimental evolution is an interesting tool to anticipate the adaptation of pathogen populations and could be very useful for identifying durable strategies for resistance deployment. [ABSTRACT FROM AUTHOR]

With the banning of most chemical nematicides, the control of root-knot nematodes (RKNs) in vegetable crops is now based essentially on the deployment of single, major resistance genes (R-genes). However, these genes are rare and their efficacy is threatened by the capacity of RKNs to adapt. In pepper, several dominant R-genes are effective against RKNs, and their efficacy and durability have been shown to be greater in a partially resistant genetic background. However, the genetic determinants of this partial resistance were unknown. Here, a QTL analysis was performed on the F2:3 population from the cross between Yolo Wonder, an accession considered partially resistant or resistant, depending on the RKN species, and Doux Long des Landes, a susceptible cultivar. A genetic linkage map was constructed from 130 F2 individuals, and the 130 F3 families were tested for resistance to the three main RKN species, M. incognita, M. arenaria and M. javanica. For the first time in the pepper-RKN pathosystem, four major QTLs were identified and mapped to two clusters. The cluster on chromosome P1 includes three tightly linked QTLs with specific effects against individual RKN species. The fourth QTL, providing specific resistance to M. javanica, mapped to pepper chromosome P9, which is known to carry multiple NBS-LRR repeats, together with major R-genes for resistance to nematodes and other pathogens. The newly discovered cluster on chromosome P1 has a broad spectrum of action with major additive effects on resistance. These data highlight the role of host QTLs involved in plant-RKN interactions and provide innovative potential for the breeding of new pepper cultivars or rootstocks combining quantitative resistance and major R-genes, to increase both the efficacy and durability of RKN control by resistance genes.

'Gene-for-gene' theory predicts that gain of virulence by an avirulent pathogen on plants expressing resistance ( R) genes is associated with fitness loss in susceptible hosts. However, the validity of this prediction has been studied in only a few plant viral pathosystems. In this study, the Soybean mosaic virus (SMV)- Rsv4 pathosystem was exploited to test this prediction. In Rsv4-genotype soybeans, P3 of avirulent SMV strains provokes an as yet uncharacterized resistance mechanism that restricts the invading virus to the inoculated leaves. A single amino acid substitution in P3 functionally converts an avirulent to a virulent strain, suggesting that the genetic composition of P3 plays a crucial role in virulence on Rsv4-genotype soybeans. In this study, we examined the impact of gain of virulence mutation(s) on the fitness of virulent variants derived from three avirulent SMV strains in a soybean genotype lacking the Rsv4 gene. Our data demonstrate that gain of virulence mutation(s) by all avirulent viruses on Rsv4-genotype soybean is associated with a relative fitness loss in a susceptible host. The implications of this finding on the durable deployment of the Rsv4 gene in soybean are discussed. [ABSTRACT FROM AUTHOR]

With the banning of most chemical nematicides, the control of root-knot nematodes (RKNs) in vegetable crops is now based essentially on the deployment of single, major resistance genes (R-genes). However, these genes are rare and their efficacy is threatened by the capacity of RKNs to adapt. In pepper, several dominant R-genes are effective against RKNs, and their efficacy and durability have been shown to be greater in a partially resistant genetic background. However, the genetic determinants of this partial resistance were unknown. Here, a quantitative trait loci (QTL) analysis was performed on the F2:3 population from the cross between Yolo Wonder, an accession V considered partially resistant or resistant, depending on the RKN species, and Doux Long des Landes, a susceptible cultivar. A genetic linkage map was constructed from 130 F2 individuals, and the 130 F3 families were tested for resistance to the three main RKN species, Meloidogyne incognita, M. arenaria, and M. javanica. For the first time in the pepper-RKN pathosystem, four major QTLs were identified and mapped to two clusters. The cluster on chromosome P1 includes three tightly linked QTLs with specific effects against individual RKN species. The fourth QTL, providing specific resistance to M. javanica, mapped to pepper chromosome P9, which is known to carry multiple NBS-LRR repeats, together with major R-genes for resistance to nematodes and other pathogens. The newly discovered cluster on chromosome P1 has a broad spectrum of action with major additive effects on resistance. These data highlight the role of host QTLs involved in plant-RKN interactions and provide innovative potential for the breeding of new pepper cultivars or rootstocks combining quantitative resistance and major R-genes, to increase both the efficacy and durability of RKN control by resistance genes. [ABSTRACT FROM AUTHOR]

Root-knot nematodes ( RKNs), Meloidogyne spp., are a major disease problem in solanaceous crops worldwide, including pepper ( Capsicum spp.). Genetic control provides an economically and environmentally sustainable protection alternative to soil fumigants. In pepper, resistance to the main RKN species ( M. incognita, M. javanica and M. arenaria) is conferred by the major genes ( R genes) Me1, Me3 and N. However, RKNs are able to develop virulence, thus endangering the efficiency of R genes. Quantitative resistance ( QR) against Meloidogyne spp. is expected to provide an alternative to R genes, or to be combined with R genes, to increase the resistance efficiency and durability in pepper. In order to explore the ability of QR to protect pepper against RKNs, five pepper inbred lines, differing in their QR level, were tested directly, or after combination with the Me1 and Me3 genes, for their resistance to a panel of M. arenaria, M. javanica and M. incognita isolates. The M. arenaria and M. javanica isolates showed low pathogenicity to pepper, unlike the M. incognita isolates. The QR, controlled by the pepper genetic background, displayed a high resistance level with a broad spectrum of action, protecting pepper against Me3-virulent as well as avirulent M. incognita isolates. The QR was also expressed when combined with the Me1 and Me3 genes, but presented additive genetic effects so that heterozygous F1 hybrids proved less resistant than homozygous inbred lines. The discovery of this QR is expected to provide promising applications for preserving the efficiency and durability of nematode resistance. [ABSTRACT FROM AUTHOR]

The management of genes conferring resistance to plant-pathogens should make it possible to control epidemics (epidemiological perspective) and preserve resistance durability (evolutionary perspective). Resistant and susceptible cultivars must be strategically associated according to the principles of cultivar mixture (within a season) and rotation (between seasons). We explored these questions by modeling the evolutionary and epidemiological processes shaping the dynamics of a pathogen population in a landscape composed of a seasonal cultivated compartment and a reservoir compartment hosting pathogen year-round. Optimal deployment strategies depended mostly on the molecular basis of plant-pathogen interactions and on the agro-ecological context before resistance deployment, particularly epidemic intensity and landscape connectivity. Mixtures were much more efficient in landscapes in which between-field infections and infections originating from the reservoir were more prevalent than within-field infections. Resistance genes requiring two mutations of the pathogen avirulence gene to be broken down, rather than one, were particularly useful when infections from the reservoir predominated. Combining mixture and rotation principles were better than the use of the same mixture each season as (i) they controlled epidemics more effectively in situations in which within-field infections or infections from the reservoir were frequent and (ii) they fulfilled the epidemiological and evolutionary perspectives. [ABSTRACT FROM AUTHOR]

Stem rust and leaf rust are important diseases affecting durum wheat production in India. Inheritance and extent of diversity in resistance were studied in five durum genotypes, viz. ʽB 662', ʽ ED 2398-A', ʽ HG 110', ʽ IWP 5019' and ʽLine 1172ʼ using Pgt pathotypes 40A and 117-6, and Pt pathotypes 12-2 and 104-2. F2 and F3 analyses showed that resistance was conferred by one or two genes in each line. In all, four genes for resistance to 40A, and eight each for pathotypes 117-6 and 12-2 were identified among the five genotypes, and three for resistance to 104-2 among B 662, ED 2398-A and IWP 5019 were indicated by tests of allelism. Although the gene identities are not known, at least some of them should be different from Sr2, Sr7b, Sr8a, Sr8b, Sr9e, Sr9g, Sr11, Sr12, Sr13, Sr14, Sr17, Sr23 and Sr28; and from Lr3, Lr14a, Lr23 and Lr27 + Lr31. These genotypes enrich the diversity of resistance to stem rust and leaf rust for durum wheat improvement. [ABSTRACT FROM AUTHOR]

Quantitative plant disease resistance is believed to be more durable than qualitative resistance, since it exerts less selective pressure on the pathogens. However, the process of progressive pathogen adaptation to quantitative resistance is poorly understood, which makes it difficult to predict its durability or to derive principles for its sustainable deployment. Here, we study the dynamics of pathogen adaptation in response to quantitative plant resistance affecting pathogen reproduction rate and its colonizing capacity., We developed a stochastic model for the continuous evolution of a pathogen population within a quantitatively resistant host. We assumed that pathogen can adapt to a host by the progressive restoration of reproduction rate or of colonizing capacity, or of both., Our model suggests that a combination of quantitative trait loci ( QTLs) affecting distinct pathogen traits was more durable if the evolution of repressed traits was antagonistic. Otherwise, quantitative resistance that depressed only pathogen reproduction was more durable., In order to decelerate the progressive pathogen adaptation, QTLs that decrease the pathogen's maximum capacity to colonize must be combined with QTLs that decrease the spore production per lesion or the infection efficiency or that increase the latent period. Our theoretical framework can help breeders to develop principles for sustainable deployment of QTLs. [ABSTRACT FROM AUTHOR]

Rhizomania, one of the most devastating diseases in sugar beet production, is caused by Beet necrotic yellow vein virus ( BNYVV) and transmitted by Polymyxa betae. Previously, disease control was possible by cultivation of sugar beet hybrids carrying a major resistance gene Rz1, which restricts virus accumulation in taproots and suppresses symptom development. Over the last few years, BNYVV strains with four RNA components have arisen, which are able to overcome Rz1-mediated resistance. All strains described so far possess an A67V amino acid exchange within the RNA3-encoded P25 pathogenicity factor. In this study, BNYVV was isolated from Rz1 plants, collected in the United Kingdom, the Netherlands and Germany, displaying patches of strong rhizomania symptoms. Sequencing of the coat protein and P25 gene of three isolates showed 100% nucleotide sequence identity and detected AYPR as the P25 tetrad amino acid composition. The ability of this strain to accumulate to higher levels in young plants of Rz1 resistant but not in Rz1 + Rz2 resistant genotypes was initially demonstrated in a greenhouse assay in natural field soil from the Netherlands. This strain was loaded into a virus-free P. betae population and compared to reference strains. The AYPR strain retained its resistance-breaking ability in the Rz1 genotypes and displayed replication at a higher rate compared to the Rz1-resistance-breaking P type. The strain origin is unclear and it remains speculative whether the occurrence at different geographic locations is the result of independent selection or displacement of infested soil. [ABSTRACT FROM AUTHOR]

Quantitative resistance mediated by multiple genetic factors has been shown to increase the potential for durability of major resistance genes. This was demonstrated in the Leptosphaeria maculans / Brassica napus pathosystem in a 5 year recurrent selection field experiment on lines harboring the qualitative resistance gene Rlm6 combined or not with quantitative resistance. The quantitative resistance limited the size of the virulent isolate population. In this study we continued this recurrent selection experiment in the same way to examine whether the pathogen population could adapt and render the major gene ineffective in the longer term. The cultivars Eurol, with a susceptible background, and Darmor, with quantitative resistance, were used. We confirmed that the combination of qualitative and quantitative resistance is an effective approach for controlling the pathogen epidemics over time. This combination did not prevent isolates virulent against the major gene from amplifying in the long term but the quantitative resistance significantly delayed for 5 years the loss of effectiveness of the qualitative resistance and disease severity was maintained at a low level on the genotype with both types of resistance after the fungus population had adapted to the major gene. We also showed that diversity of AvrLm6 virulence alleles was comparable in isolates recovered after the recurrent selection on lines carrying either the major gene alone or in combination with quantitative resistance: a single repeat-induced point mutation and deletion events were observed in both situations. Breeding varieties which combine qualitative and quantitative resistance can effectively contribute to disease control by increasing the potential for durability of major resistance genes. [ABSTRACT FROM AUTHOR]

The green leafhopper, Nephotettix virescens ( Distant) ( Hemiptera: Cicadellidae), occasionally damages rice in Asia either directly, by feeding on the host phloem, or indirectly by transmitting tungro virus. We assessed the nature of resistance against the leafhopper in monogenic and pyramided near-isogenic rice lines containing the resistance genes Grh2 and Grh4. Only the pyramided line was resistant to leafhopper damage. Leafhopper nymphs and adults had high mortality and low weight gain when feeding on the pyramided line and adults laid few eggs. In contrast, although there was some minor resistance in 45-day-old plants that possessed either Grh2 or Grh4 genes, the monogenic lines were generally as susceptible to the leafhopper as the recurrent parent line Taichung65 (T65). Resistance in the pyramided line was stable as the plant aged and under high nitrogen, and affected each of five Philippine leafhopper populations equally. Furthermore, in a selection study, leafhoppers failed to adapt fully to the pyramided resistant line: nymph and adult survival did improve during the first five generations of selection and attained similar levels as on T65, but egg-laying failed to improve over 10 generations. Our preliminary results suggested that resistance was associated with physiological costs to the plants in some experiments. The results of this study demonstrate the success of pyramiding resistance genes through marker-assisted breeding, to achieve a strong and potentially durable resistance. We discuss the utility of gene pyramiding and the development of near-isogenic lines for leafhopper management. [ABSTRACT FROM AUTHOR]

The variability of resistance durability in different potato genotypes harbouring the same resistance QTL but differing by their genetic background was explored. The indirect consequences of the resistance adaptation in terms of local (i.e. genotype-specific) adaptation and cross-virulence was also investigated. Following the virulence of the potato cyst nematode Globodera pallida in a long-term experimental evolution protocol, the results showed that nematode populations were able to adapt to the resistance of four potato genotypes carrying the QTL GpaV from Solanum vernei, and that the plant genetic background has an impact upon the durability of resistance. The pattern of local adaptation observed here indicates that divergent selection has occurred during the experimental evolution performed from the same initial nematode population, and revealed a trade-off between the adaptation to a resistant potato genotype and the adaptation to another resistant genotype differing in its genetic background. In terms of cross-virulence between potato genotypes derived from different resistance sources ( S. sparsipilum and S. spegazzinii), this study shows that the adaptation to resistance QTL GpaV vrn does not necessarily allow the adaptation to collinear GpaV loci. The results presented here could be useful for predicting evolution of nematode populations in natural agro-ecosystems and identifying durable strategies for resistance deployment. [ABSTRACT FROM AUTHOR]

Adaptation of populations to new environments is frequently costly due to trade-offs between life history traits, and consequently, parasites are expected to be locally adapted to sympatric hosts. Also, during adaptation to the host, an increase in parasite fitness could have direct consequences on its aggressiveness (i.e. the quantity of damages caused to the host by the virus). These two phenomena have been observed in the context of pathogen adaptation to host's qualitative and monogenic resistances. However, the ability of pathogens to adapt to quantitative polygenic plant resistances and the consequences of these potential adaptations on other pathogen life history traits remain to be evaluated. Potato virus Y and two pepper genotypes (one susceptible and one with quantitative resistance) were used, and experimental evolutions showed that adaptation to a quantitative resistance was possible and resulted in resistance breakdown. This adaptation was associated to a fitness cost on the susceptible cultivar, but had no consequence either in terms of aggressiveness, which could be explained by a high tolerance level, or in terms of aphid transmission efficiency. We concluded that quantitative resistances are not necessarily durable but management strategies mixing susceptible and resistant cultivars in space and/or in time should be useful to preserve their efficiency. [ABSTRACT FROM AUTHOR]

The pathogenic properties of Potato virus Y (PVY; genus Potyvirus, family Potyviridae) isolates collected in naturally infected pepper ( Capsicum annuum) fields in Tunisia were evaluated against recessive resistance alleles at the pvr2 locus of pepper. Two pathotypes were observed. Pathotype (0,1,3) isolates were able to infect plants carrying the susceptibility allele pvr2+, together with pvr2 1/ pvr2 1 and pvr2 3/ pvr2 3 plants, but not pvr2 2/ pvr2 2 plants. Pathotype (0) isolates were only able to infect pvr2 +/ pvr2 + plants. On the other hand, sequence data and phylogenetic analyses revealed three major groups of isolates, each characterized by particular amino acid residues in the central part of the VPg, the pathogenicity factor towards pvr2. Correspondence between pathogenic properties and phylogeny suggested a single evolutionary step for pathogenicity towards the pvr2 1 and pvr2 3 resistances, possibly under the selective pressure of pvr2 1. Indeed, 23% of the pepper plants in this area were shown to carry the pvr2 1 resistance, while pvr2 3 was not detected. The data suggested that pathogenicity towards pvr2 1 and pvr2 3 were not costly for PVY to infect susceptible pepper genotypes and supported the matching allele model for pepper-PVY interactions. [ABSTRACT FROM AUTHOR]

Abstract: To durably control phoma stem canker (Leptosphaeria maculans/Leptosphaeria biglobosa) in oilseed rape by preserving the efficacy of specific resistances, the design of cropping systems is required, combining different aspects of crop management over time and at the regional scale. Modelling is a practical means to do so. To this end, the structure of SIPPOM, a Simulator for Integrated Pathogen POpulation Management, is presented. By linking epidemiological, population and crop model approaches, it simulates both the quantitative evolution (size) and qualitative evolution (genetic structure) of L. maculans population under the influence of various cropping systems. It involves five sub-models: (i) primary inoculum production; (ii) ascospore dispersal; (iii) crop development; (iv) plant infection, disease severity, and yield loss; and (v) changes in the genetic structure of pathogen populations over time. The input variables are weather data, soil characteristics, the description of cropping systems (crop sequence and winter oilseed rape crop management, including cultivars, sowing, fungicide, and tillage) and their spatial distribution (the model is spatially explicit), plus the initial size and genetic structure of pathogen populations. The genetic evolutionary forces taken into account are migration, selection, and recombination. The model simulates disease severity and the genetic structure of the pathogen population, in addition to the attainable yield less yield loss caused by the disease. The economic and environmental performances of the simulated strategies are computed using technical operation costs, crop prices, and yields. The underlying hypotheses are discussed in light of advantages and possible improvements of the model. Possible uses of SIPPOM are proposed with respect to the design of collective strategies for the durable control of phoma stem canker on winter oilseed rape at a regional scale. [Copyright &y& Elsevier]

Abstract: Host plant resistance is an efficient and environmentally friendly means of controlling insects, including aphids, but resistant-breaking biotypes have occurred in several plant–aphid systems. Our review of the genetic and molecular bases of aphid resistance in crop species emphasizes the limited number of aphid resistance genes and alleles. Inheritance of aphid resistance may be monogenic (dominant or recessive genes) or polygenic. Two dominant, aphid resistance genes have been isolated to date. They both encode NBS-LRR proteins involved in the specific recognition of aphids. Strategies to ensure aphid resistance effectiveness and durability are discussed. Innovative research activities are proposed. [Copyright &y& Elsevier]

The effect of RUS, a major quantitative resistance (QR) factor inherited from Populus trichocarpa, was assessed in two Populus deltoides × P. trichocarpa F1 progenies against eight isolates of Melampsora larici-populina, the causal agent of Eurasian poplar rust. Six isolates were identified on which RUS had no significant effect. The first RUS-defeating isolate identified suggested a pre-existing potential to overcome RUS in the pathogen’s populations. The P. deltoides genetic background made no difference either to the ability of a given isolate to overcome RUS or to the relative and absolute aggressiveness of the isolates. This study illustrates how extreme the isolate-specificity of QR can be. It also yields insights into the relationship between size of uredinia and sporulation rate, discussing the epidemiological significance of spore production per mm2 uredinium. [ABSTRACT FROM AUTHOR]

• The breakdown of plant resistance by pathogen populations is a limit to the genetic control of crop disease. Polygenic resistance is postulated as a durable alternative to defeated major resistance genes. Here, we tested this postulate in the pepper– Potato virus Y interaction. • The virus was selected for virulence towards monogenic and polygenic host resistance, using serial inoculations in laboratory and in natural epidemic conditions. The frequency of resistance breakdown and the genetic changes in the virus avirulence gene were analysed. • The monogenic resistance provided by the pvr23 gene was defeated at high frequency when introgressed in a susceptible genetic background whereas it was not when combined to partial resistance quantitative trait loci. The suppression of emergence of virulent mutants because of the genetic background resulted both from a differential selection effect and the necessity for the virus to generate multiple mutations. The virus adaptation to the polygenic resistance required a step-by-step selection with a primary selection for virulence towards the major gene, followed by selection for adaptation to the genetic background. • Polygenic resistance proved more durable than monogenic resistance, but breeding strategies giving priority to major resistance factors may jeopardize the progress in durability expected from polygenic resistance. [ABSTRACT FROM AUTHOR]

Quantitative resistance is considered more durable than qualitative resistance as it does not involve major resistance genes that can be easily overcome by pathogen populations, but rather a combination of genes with a lower individual effect. This durability means that quantitative resistance could be an interesting tool for breeding crops that would not systematically require phytosanitary products. Quantitative resistance has yet to reveal all of its intricacies. Here, we delve into the case of the wheat/Septoria tritici blotch (STB) pathosystem. Using a population resulting from a cross between French cultivar Renan, generally resistant to STB, and Chinese Spring, a cultivar susceptible to the disease, we built an ultra-dense genetic map that carries 148,820 single nucleotide polymorphism (SNP) markers. Phenotyping the interaction was done with two different Zymoseptoria tritici strains with contrasted pathogenicities on Renan. A linkage analysis led to the detection of three quantitative trait loci (QTL) related to resistance in Renan. These QTL, on chromosomes 7B, 1D, and 5D, present with an interesting diversity as that on 7B was detected with both fungal strains, while those on 1D and 5D were strain-specific. The resistance on 7B was located in the region of Stb8 and the resistance on 1D colocalized with Stb19. However, the resistance on 5D was new, so further designated Stb20q. Several wall-associated kinases (WAK), nucleotide-binding and leucine-rich repeats (NB-LRR) type, and kinase domain carrying genes were present in the QTL regions, and some of them were expressed during the infection. These results advocate for a role of Stb genes in quantitative resistance and for resistance in the wheat/STB pathosystem being as a whole quantitative and polygenic. [ABSTRACT FROM AUTHOR]

Abstract: Plant resistance based on a gene-for-gene relationship tends not to be durable because virulent strains can emerge through mutation and colonize formerly resistant cultivars over large areas. Our objective was to determine the origin and colonization history of newly virulent strains of the fungus Venturia inaequalis. Four hundred and eighty-one strains from seven multi-cultivar orchards within a 300km wide area were collected from a cultivar carrying the overcome Vf-gene (Vfcv populations) and from non-Vf cultivars (nVfcv populations). Using 9 microsatellite loci, we showed that Vfcv populations were closely related though highly differentiated, indicating they have been recently founded following rare long distance dispersal events from a common origin. Assignment tests revealed that the Vfcv populations had not emerged from any of the nVfcv populations. Vfcv populations had a low diversity and were strongly differentiated from nVfcv populations several years after the first reported breakdown, suggesting reproductive isolation between the two populations. [Copyright &y& Elsevier]

Genetic resistance in sugar beet (Beta vulgaris) to Beet necrotic yellow vein virus (BNYVV), which causes the disease rhizomania, is conferred by the single dominant gene Rzl. However, since 2002, Rzl cultivars grown in the Imperial Valley of California have been increasingly damaged by a new strain of BNYVV. Viral RNA 3 was extracted from asymptomatic and symptomatic sugar beets and, after amplification and sequencing of a region including the p25 cistron, two polymorphic sites, A67V and D135E, associated with the capability of the virus to overcome resistance were identified. Using the real-time reverse transcription-polymerase chain reaction allelic discrimination technique, TaqMan probes designed to detect the responsible nucleotide substitutions permitted the differentiation between wild type (WT) and resistance-breaking (RB) isolates. This method also allowed easy detection of mixed infections by giving a heterozygous call, which was verified by DNA sequencing of individual clones. The capability of this technology to typify numerous isolates facilitated the analysis of the spatial distribution of virus haplotypes in the field. Thus, RB variants were mostly baited from yellow strips with high incidence of rhizomania, whereas WT variants predominated in the surrounding green areas. Mixed infections were found mainly in green areas and transitional zones. The predominance of the RB isolates in yellow strips suggests that they have gained fitness in Rzl cultivars and will eventually become the dominant haplotype. [ABSTRACT FROM AUTHOR]

An appreciation of the risks caused by emergent plant viruses is critical in tropical areas that rely heavily on agriculture for subsistence and rural livelihood. Molecular ecology, within 10 years, has unraveled the factors responsible for the emergence of several of the economically most important tropical plant viruses: Rice yellow mottle virus (RYMV), Cassava mosaic geminiviruses (CMGs), Maize streak virus (MSV), and Banana streak virus (BSV). A large range of mechanisms--most unsuspected until recently--were involved: recombination and synergism between virus species, new vector biotypes, genome integration of the virus, host adaptation, and long-distance dispersal. A complex chain of molecular and ecological events resulted in novel virus-vector-plant-environment interactions that led to virus emergence. It invariably involved a major agricultural change: crop introduction, cultural intensification, germplasm movement, and new genotypes. A current challenge is now to complement the analysis of the causes by an assessment of the risks of emergence. Recent attempts to assess the risks of emergence of virulent villas strains are described. [ABSTRACT FROM AUTHOR]

Barley yellow dwarf disease (BYDD) is one of the main viral diseases of small grain cereals. This disease, reported on numerous plant species of the Poaceae family, is caused by a complex of viral species including the species Barley yellow dwarf virus-PAV (BYDV-PAV, family Luteoviridae, genus Luteovirus), frequently found in western Europe. Resistance sources towards BYDD are scarce. Indeed, breeding-resistant genotypes is a long and expensive process. Thus, estimating the durability of the resistance genes before the achievement of selection would be an asset for breeders. One isolate of BYDV-PAV has been serially passaged on two hosts, ‘Zhong ZH’ and ‘TC14’, carrying a gene for partial resistance. The resulting viral population showed an increase of the speed of development of the infection in controlled conditions. In this study, these viral populations were evaluated in a 3-year field trial, including a susceptible host, ‘Rendezvous’, and a host carrying the resistance gene of ‘TC14’ in a ‘Rendezvous’ background, to assess the effect of serial passages in field conditions. Results indicate that isolates issued from serial passages on hosts carrying a gene for partial resistance induced increased damage in field conditions when compared with the initial isolate. Yield losses are mainly due to a decrease of the number of kernels per square metre. The interest on using partial resistance gene to control BYDD is discussed. [ABSTRACT FROM AUTHOR]

Many race-specific resistance genes to potato late blight are overcome in France, but the disease appears later on genotypes carrying the R2 gene. This study examined whether R2 could contribute to durable late-blight control in France, and analysed the conditions of its performance. Plants grown from tubers of different physiological ages showed no difference in R2 expression in field and climate-chamber experiments, demonstrating that the delay in epidemic onset provided by R2 was not the result of gene inactivation in old plants. Among isolates collected at one site, those virulent on R2 were classified into three AFLP profiles. AFLP-VII comprised exclusively isolates virulent to R2, whereas AFLP-IV and AFLP-V included both virulent and avirulent isolates. No significant aggressiveness differences were observed between virulent and avirulent isolates from AFLP-V; however, isolates from AFLP-VII were significantly less aggressive than virulent isolates from AFLP-V. These results indicate that: (i) the delayed onset of epidemics on R2 cultivars is the result of the breakdown of R2 by virulent isolates; (ii) aggressiveness of isolates virulent to R2 depends primarily on the genetic background of the pathogen where the mutation to virulence occurs; and (iii) this mutation does not lead per se to lower pathogenic fitness. It is suggested that R2 is unlikely to make a lasting contribution to late-blight control in France, and that diversification strategies such as cultivar mixtures might not considerably increase its durability. [ABSTRACT FROM AUTHOR]

Powdery mildew, caused by Sphaerotheca pannosa var. persicae is one of the most important diseases in European peach orchards. Quantitative trait loci controlling powdery mildew resistance were detected using three related F1, F2 and BC2 populations derived from the cross between the resistant parent P. davidiana clone P1908 and the susceptible peach cultivar Summergrand. Powdery mildew resistance of each population was evaluated under natural exposure, in several locations and over several years. Thirteen QTLs were detected. For nine of them, the favourable allele came from the resistant parent. Five QTLs were consistently detected across the three populations. The F1 hybrid used to produce F2 and BC2 populations had not inherited the favourable allele from P1908 for QTL detected on LG3 and LG8 in F1 population. QTLs were not detected in the corresponding regions in F2 and BC2 populations. In two other genomic areas, significant substitution effects between P1908 alleles were evidenced in the F1 population, but the favourable allele came from Summergrand in the F2 and BC2 populations. Analysis of phenotypic data suggested an important qualitative change in the distribution of powdery mildew resistance after 1996, confirmed by QTL analysis. Indeed, a dramatic decrease of the effect of the major QTL previously detected on LG6 was observed after 1996, while the QTL on LG8 was increasingly involved in the control of powdery mildew resistance. Consequences for peach breeding strategies to improve powdery mildew resistance are discussed. [ABSTRACT FROM AUTHOR]

Two sources of resistance to Rice yellow mottle virus were challenged in host passage experiments. Pronounced changes in pathogenicity occurred over serial passages of virus isolates inoculated to partially or highly resistant cultivars. The changes encompassed the known existing pathogenic variability of field isolates. Ultimately, the high resistance of the Oryza indica cv. Gigante was overcome and the partial resistance of the O. sativa japonica cv. Azucena broke down. The effect was resistance-specific as different isolates overcame partial and high resistance, and may also be allele-specific as different isolates overcame the resistance of cultivars carrying the same resistance gene. The ability of isolates to break resistance was not linked to a high initial pathogenicity of the isolates and did not result in higher virus content in the infected plants. Implications for resistance breeding and deployment are discussed. [ABSTRACT FROM AUTHOR]

Root-knot nematode (RKN) species are predominant pests of crops, attacking stone fruit crops Prunus spp. under Mediterranean climate conditions worldwide. Natural resistance for rootstock breeding is a control method that is gaining interest as an alternative to the highly toxic nematicides. This review first reports an outline of the root-knot nematodes parasitizing stone fruit crops and the Prunus species and rootstocks. It then describes the main sources of resistance detected among the Prunus germplasm and focuses on the major resistance genes identified and their characteristics (spectrum, durability, histological mechanism, effect of temperature, interaction with other pests and diseases, etc.). In peach, besides the RMia reference gene, the new genes PkMi and Mf, also located on chromosome 2, need to be characterized regarding their spectrum and relationship. The two other Prunus reference genes, Ma from plum (complete spectrum) and RMja from almond (more restricted spectrum), are orthologs that belong to a TIR-NB-LRR (TNL) cluster on chromosome 7. The review finally summarizes the positional cloning of the Ma gene and the characterization of its unique TNL structure, encompassing a five-times repeated post-LRR domain. Deciphering how this structure is functionally involved in Ma's remarkable biological properties is a real challenge for the future. [ABSTRACT FROM AUTHOR]