Search

International audience; Plant pathogens often adapt to plant genetic resistance so characterization of the architecture underlying such an adaptation is required to understand the adaptive potential of pathogen populations. Erosion of banana quantitative resistance to a major leaf disease caused by polygenic adaptation of the causal agent, the fungus Pseudocercospora fijiensis, was recently identified in the northern Caribbean region. Genome scan and quantitative genetics approaches were combined to investigate the adaptive architecture underlying this adaptation. Thirty-two genomic regions showing host selection footprints were identified by pool sequencing of isolates collected from seven plantation pairs of two cultivars with different levels of quantitative resistance. Individual sequencing and phenotyping of isolates from one pair revealed significant and variable levels of correlation between haplotypes in 17 of these regions with a quantitative trait of pathogenicity (the diseased leaf area). The multilocus pattern of haplotypes detected in the 17 regions was found to be highly variable across all the population pairs studied. These results suggest complex adaptive architecture underlying plant pathogen adaptation to quantitative resistance with a polygenic basis, redundancy, and a low level of parallel evolution between pathogen populations. Candidate genes involved in quantitative pathogenicity and host adaptation of P. fijiensis were identified in genomic regions by combining annotation analysis with available biological data.

International audience; Understanding the mechanisms involved in pathogen adaptation to quantitative resistance in plants has a key role to play in establishing durable strategies for resistance deployment, especially in perennial crops. The erosion of quantitative resistance has been recently suspected in Cuba and the Dominican Republic for a major fungal pathogen of such a crop: Pseudocercospora fijiensis, causing black leaf streak disease on banana. This study set out to test whether such erosion has resulted from an adaptation of P. fijiensis populations, and to determine whether or not the adaptation is local. Almost 600 P. fijiensis isolates from Cuba and the Dominican Republic were sampled using a paired-population sampling design on resistant and susceptible banana varieties. A low genetic structure of the P. fijiensis populations was detected in each country using 16 microsatellite markers. Cross-inoculation experiments using isolates from susceptible and resistant cultivars were carried out, measuring a quantitative trait (the diseased leaf area) related to pathogen fitness on three varieties. A further analysis based on those data suggested the existence of a local pattern of adaptation to resistant cultivars in both of the study countries, due to the existence of specific (or genotype by genotype) host-pathogen interactions. However, neither cost nor benefit effects for adapted populations were found on the widely used "Cavendish" banana group. These results highlight the need to study specific host-pathogen interactions and pathogen adaptation on a wide range of quantitative resistance phenotypes in banana, in order to develop durable strategies for resistance deployment.

Black Leaf Streak Disease (BLSD) is the most severe foliar disease of banana and plantain. BLSD is caused by Pseudocercospora fijiensis, an ascomycete fungus which produces wind-borne spores responsible for its spatial dispersal. In order to evaluate the BLSD long-distance dispersal and to better understand the effect of environmental factors on its invasive spatial spread, a spatio-temporal study was set up during the recent BLSD invasion in the Martinique island (FWI). Disease detection was carried out from September2010 to May 2012 and sampling squares were defined from a regular spatial grid built over the island. In this paper, we consider a stochastic model of spatio-temporal propagation of BLSD in an heterogeneous landscape and we present mathematical and computational results for this continuous-time model. Statistical inference of parameters is carried out from presence-absence data using a Bayesian framework based on a data augmentation method with respect to square first colonization times. Parameter posterior distribution calculations made possible the evaluation of the BLSD long-distance dispersal and land-cover influence on the disease propagation. Our results enabled the re-enactment of the invasion.

Mycosphaerella fijiensis Morelet (anamorph Pseudocercospora fijiensis Morelet), the causal agent of black Sigatoka disease of banana, is considered to be the greatest economical threat for export-banana cultivation throughout the world because most cultivars are highly susceptible. The disease has a worldwide distribution throughout the humid tropical regions, but was still absent in some Caribbean islands hitherto. In Martinique Island, an intensive survey has been conducted by the plant protection service and the Fédération Régionale de Défense Contre les Organismes Nuisibles (FREDON) since April 2008 to detect as early as possible any outbreak of infection by M. fijiensis. In September 2010, typical symptoms of black Sigatoka were observed in a plantain crop located in Ducos Municipality (14°35.702′N, 60°58.221′W) in the west-central area of the island. Typical early symptoms were 1- to 4-mm long brown streaks on the abaxial leaf surface. The presence of the disease was further confirmed by the in situ observation of microscopic features of the anamorphic form of the pathogen (3). Typical pale brown, straight or slightly geniculate conidiophores were observed occurring singly or in little groups without any stroma, with a thickened wall at the conidial scars. Conidia were hyaline to pale olive, straight or slightly curved, with one to eight septa, and a conspicuous scar at the basal cell. The diagnosis was confirmed by real-time PCR targeting M. fijiensis-specific regions within the β-tubulin gene (1). Positive results were consistently obtained with DNA extracted from infected banana tissue samples, and the identity of the amplicon was confirmed by sequencing (Accession No. HQ412771) and comparison with reference sequences deposited on GenBank. After this first finding, the survey was intensified and black Sigatoka symptoms were also observed in several other locations on the island, affecting a large range of susceptible cultivars (Grande Naine, French, and Figue Sucrée), and in plantations, backyards, and private gardens. The presence of the fungus in the samples was confirmed by PCR analysis of DNA extracted from symptomatic leaves with a M. fijiensis-specific ITS-based primer pair (2). The pathogen may have been introduced into Martinique by ascospores, from islands where black Sigatoka is present, that were blown by continuous southerly winds over a 2-week period in August 2010 that was immediately followed by heavy rains that favor disease development. To our knowledge, this is the first report of M. fijiensis on Martinique Island, showing that the disease is still spreading northward in this region of the Caribbean. References: (1) M. Arzanlou et al. Phytopathology 97:1112, 2007. (2) J. Henderson et al. Page 59 in: Mycosphaerella Leaf Spot Disease of Bananas: Present Status and Outlook. L. Jacome et al., eds. International Network for the Improvement of Banana and Plantain (INIBAP), Montpellier, France. 2003. (3) M. F. Zapater et al. Fruits 63:389, 2008.

International audience; Black leaf streak disease, or black Sigatoka, is caused by the fungus Pseudocercospora fijiensis, and has been identified as a major constraint to global production of banana and plantain. We fitted a climatic niche model (CLIMEX) for P. fijiensis to gain an understanding of the patterns of climate suitability, and hence hazard from this disease. We then calibrated the climate suitability patterns against the results of an expert elicitation of disease pressure patterns. We found a moderately strong non-linear relationship between modelled climate suitability for P. degrees fijiensis and the expert ratings for disease pressure. The strength of the relationship provides a cross-validation between the CLIMEX model and the expert elicitation process. The bulk of global banana production experiences high potential threat from P. fijiensis, and the higher yielding areas for banana and plantain production are at greatest threat. By explicitly considering the role of irrigation we have been able to identify how strategic irrigation could be used to support banana production in areas that are at low risk from P. fijiensis.

International audience; Among the emerging fungal diseases threatening food security, the Pseudocercospora fijiensis fungus causing black leaf streak disease of banana is one of the most marked examples of a recent worldwide pandemic on a major crop. We assessed how this pathogen spread throughout the latest invaded region, i.e. Central America and the Caribbean. We retraced its population history combining detailed monitoring information on disease outbreaks and population genetic analyses based on large-scale sampling of P. fijiensis isolates from 121 locations throughout the region. The results first suggested that sexual reproduction was not lost during the P. fijiensis expansion, even in the insular Caribbean context, and a high level of genotypic diversity was maintained in all the populations studied. The population genetic structure of P. fijiensis and historical data showed that two disease waves swept northward and southward in all banana-producing countries in the study area from an initial entry point in Honduras, probably mainly through gradual stepwise spore dispersal. Serial founder events accompanying the northern and southern waves led to the establishment of two different genetic groups. A different population structure was detected on the latest invaded islands (Martinique, Dominica and Guadeloupe), revealing multiple introductions and admixture events that may have been partly due to human activities. The results of this study highlight the need to step up surveillance to limit the spread of other known emerging diseases of banana spread mainly by humans, but also to curb gene flow between established pathogen populations which could increase their evolutionary potential.

Black leaf streak disease (BLSD), caused by the fungal pathogen Mycosphaerella fijiensis, is considered as the most destructive foliar disease of banana. To advance our knowledge of the dynamics of the disease at plant scale as well as of the components of varietal resistance, we designed, calibrated and evaluated a mechanistic model to simulate the disease on a banana plant. The model runs in discrete time at plant scale and describes plant growth and pathogen dynamics under optimal epidemiological conditions. The model is divided into two modules: a deterministic plant sub-model that simulates the simplified architecture and growth of the banana, and a pathogen sub-model that simulates the detailed life cycle of the pathogen including infection, lesion growth, asexual and sexual sporulation, and the dispersal of spores on the plant. The three most influential epidemiological parameters identified by sensitivity analysis of the model (lesion growth rate, infection efficiency, and incubation period) were estimated in a Bayesian framework using Markov chain Monte Carlo methods and acquired data on the dynamics of leaf lesions under natural conditions. The posterior densities provided precise knowledge on pathogen life history traits. The evaluation of the model using an independent data set confirmed the good quality of the predictions. Simulations allowed us to evaluate the impact of host resistance components, auto-infection at plant scale, and of the leaf emergence rate, which is linked with cropping practices, on the severity of BLSD. This foliar disease simulation model will help design new methods of controlling BLSD. (Resume d'auteur)

Reconstructing and characterizing introduction routes is a key step towards understanding the ecological and evolutionary factors underlying successful invasions and disease emergence. Here, we aimed to decipher scenarios of introduction and stochastic demographic events associated with the global spread of an emerging disease of bananas caused by the destructive fungal pathogen Mycosphaerella fijiensis. We analysed the worldwide population structure of this fungus using 21 microsatellites and 8 sequence-based markers on 735 individuals from 37 countries. Our analyses designated South-East Asia as the source of the global invasion and supported the location of the centre of origin of M. fijiensis within this area. We confirmed the occurrence of bottlenecks upon introduction into other continents followed by widespread founder events within continents. Furthermore, this study suggested contrasting introduction scenarios of the pathogen between the African and American continents. While potential signatures of admixture resulting from multiple introductions were detected in America, all the African samples examined seem to descend from a single successful founder event. In combination with historical information, our study reveals an original and unprecedented global scenario of invasion for this recently emerging disease caused by a wind-dispersed pathogen.

Black Leaf Streak Disease is challenging the banana industry. Abstract — Introduction. Black Leaf Streak Disease (BLSD) is regarded as the most economically important threat that the banana industry has to face. Effectively, this foliar disease affects leaf pho- tosynthesis but, above all, reduces the greenlife of fruits, that cannot be exported in cases of severe infestation. Main characteristics of Black Leaf Streak Disease. More than 20 Mycosphaerella spe- cies have been described on bananas. Leaf spot diseases of bananas are caused by some species of this complex, of which M. fijiensis(BLSD) and M. musicola (Sigatoka disease) are the most important. M. fijiensisisaninvasivespeciesthathastotallyreplacedM. musicolainmostbanana-exportingcoun- tries,whichwasconducivetoincreasingdifficultiesinbananaleafspotcontrol.BLSDcausesincreas- ing difficulties for control. Since all banana cultivars grown in the banana industry are highly susceptible to BLSD, the control of this disease relies on aerial applications of fungicides according to either systematic frameworks (mostly contact fungicides) or forecasting strategies (mostly systemic fungicides). In a banana-exporting country where M. fijiensis has been reported, BLSD control becomes increasingly more difficult. This evolution is essentially due to the rapid emergence of fun- gicide resistance, and is conducive to a significant increase in the cost of disease control but, above all, to increasing negative environmental effects. Challenges for the banana industry. Because of the rapid adaptation of M. fijiensis, the banana industry must be prepared for significant evolution. We propose various parameters that should be monitored at different levels (disease assessment parameters, evaluation of chemical control efficiency, global evaluation of BLSD economic incidence) to rationalize such evolution. The modelization of BLSD effects on bunch mass and greenlife should enable defining acceptable disease thresholds and optimizing bunch weight and harvest stage accord- ing to agronomic practices. On the other hand, fungicide use is conducive to significant environmental impact and must be limited. Forecasting strategies should be used wherever systemic fungicides are still efficient. Finally, the predominance of a unique type of susceptible cultivar is unsustainable and the recourse to resistant varieties in an integrated strategy is undoubtedly the future of BLSD control. France / Musa / disease control / fungal diseases / Mycosphaerella fijiensis / resistance to chemicals

Dispersal processes of fungal plant pathogens can be inferred from analysis of spatial genetic structures resulting from recent range expansion. The relative importance of longdistance dispersal (LDD) events vs. gradual dispersal in shaping population structures depends on the geographical scale considered. The fungus Mycosphaerella fijiensis, pathogenic on banana, is an example of a recent worldwide epidemic. Founder effects in this species were detected at both global and continental scale, suggesting stochastic spread of the disease through LDD events. In this study, we analysed the structure of M. fijiensis populations in two recently (_1979-1980) colonized areas in Costa Rica and Cameroon. Isolates collected in 10-15 sites distributed along a _250- to 300- km-long transect in each country were analysed using 19 microsatellite markers. We detected lowto- moderate genetic differentiation among populations in both countries and isolation by distance in Cameroon. Combined with historical data, these observations suggest continuous range expansion at the scale of banana-production area through gradual dispersal of spores. However, both countries displayed specific additional signatures of colonization: a sharp discontinuity in gene frequencies was observed along the Cameroon transect, while the Costa Rican populations seemed not yet to have reached genetic equilibrium. These differences in the genetic characteristics of M. fijiensis populations in two recently colonized areas are discussed in the light of historical data on disease spread and ecological data on landscape features. (Resume d'auteur)

The aim of the current work was to harmonise protocols between three laboratories by performing independent isolations and cultures of human hepatocytes and to assess their responses to prototypical cytochrome P450 (CYP) enzyme inducers, beta-naphthoflavone (BNF), rifampicin (RIF) or phenobarbital (PB). The magnitudes of the induction responses were CYP and donor-dependent but there was a good reproducibility between laboratories. CYP1A2 activity was evident in all cultures treated with BNF but not RIF or PB. Likewise, CYP3A4/5 activity was induced to the same extent by RIF and PB, while BNF did not affect this CYP in any of the cultures tested. All three compounds caused a concentration-dependent increase in CYP2B6 in cultures from 2 of the 3 laboratories and the response to PB was at least twice that of the other two inducers. In conclusion, the harmonised protocols used to study the response of primary cultures of human hepatocytes to prototypical inducers are transferable, reproducible within a given laboratory and between laboratories. The results obtained will support setting up a definitive validation study of the harmonised protocols.

We have compared induction responses of human hepatocytes to known inducers of CYP1A2, CYP2B6, CYP2C and CYP3A4/5 to determine whether the culture format, treatment regimen and/or substrate incubation conditions affected the outcome. CYP induction responses to prototypical inducers were equivalent regardless of pre-culture time (24h or 48h), plate format (60mm or 24-well plates) used or whether CYP activities were measured in microsomes or whole cell monolayers. Fold-induction of CYP3A4/5 by 1000muM PB and 10microM RIF were equivalent. In contrast, the fold-induction of CYP2B6 by PB was 3-fold higher that by 10microM RIF. In addition to inducing CYP1A2, 50microM OME also induced CYP3A4/5 in 50% of the donors tested. CYP2B6 was induced in 14 out of 21 donors by BNF; however CYP3A4/5 was unaffected by BNF in these donors. In order to confirm that donor-to-donor variation was not due to inter-laboratory differences, the induction responses of 5 different batches of cryopreserved human hepatocytes were compared in two different laboratories. The induction of CYP1A2, CYP2B6 and CYP3A4 measured in our laboratory were equivalent to those obtained by the commercial companies, proving good between-laboratory reproducibility. In conclusion, there is some flexibility in the treatment and incubation protocols for classical CYP induction assays on human hepatocytes. Both RIF and PB are suitable positive control inducers of CYP3A4/5 but PB may be more appropriate for CYP2B6 induction. BNF may be more appropriate for CYP1A2 induction than OME since, in contrast to the latter, it does not induce CYP3A4. Induction responses using hepatocytes from the same donor but in different labs can be expected to be similar. The good reproducibility of induction responses between laboratories using cryopreserved hepatocytes underlines the usefulness of these cells for these types of studies.

The effects of fibrates on cytochrome P450 4A (CYP4A) expression have not been clearly evaluated in human hepatocytes, human being reported as a non-responsive species. We have evaluated the effects of clofibrate, bezafibrate (BEZA), WY-14643, nafenopin and ciprofibrate at the concentration of 250 microM on CYP4A expression in primary cultures of rat and human hepatocytes. BEZA greatly induced mRNA expression in both species. Eight out of 10 human cultures responded to BEZA 250 microM. CYP4A-dependent activity was increased in rat, but not in human hepatocytes. The antioxidant N-acetylcysteine (Nac) enhanced the inducing effect of BEZA on mRNA expression, this potentialization being higher in human compared to rat hepatocytes. By contrast, Nac decreased the inducing effect of BEZA on CYP4A-dependent activity in rat and had either no effect or decreased the activity in BEZA-treated human hepatocytes. In conclusion, the cellular environment appears as an important parameter to take into account when studying CYP4A induction and could partly explain interspecies differences in the complex regulation of CYP4A expression.

Le protocole vise a diagnostiquer, en laboratoire, les principales especes de Mycosphaerella pathogenes des bananiers et plantains. Les trois agents pathogenes Mycosphaerella fijiensis (anamorphe de Paracercospora fijiensis), M. musicola (anamorphe de Pseudocercospora musae) et M. eumusae (anamorphe de Pseudocercospora eumusae) sont respectivement responsables de la maladie des raies noires, de la maladie de Sigatoka et de la cercosporiose due a eumusae. Le principe, les principaux avantages, le materiel vegetal et le temps necessaires a la methode sont presentes. Materiel et methodes. Le materiel de laboratoire necessaire et le detail des treize etapes de realisation du protocole (prelevement des tissus et observations microscopiques in situ, isolement sur milieu artificiel et clonage d'isolats monospores, sporulation in vitro et observations microscopiques des conidies, conservation a long terme des isolats) sont decrits. Resultats. Le diagnostic est base sur les observations des anamorphes (conidiophores et conidies) qui peuvent etre observes directement a partir des feuilles de bananes ou apres sporulation des isolats mis en culture s'il n'y a pas de lesions sporulantes sur les echantillons de bananiers. (Resume d'auteur)

International audience

Isolated primary human hepatocytes are a well accepted system for evaluating pharmacological and toxicological effects in humans. However, questions remain regarding how culturing affects the liver-specific functions of the hepatocytes. In addition, cryopreservation could also potentially affect the differentiation state of the hepatocytes. The first aim of the present study was to compare gene expression in freshly isolated primary hepatocytes to that of the liver of origin and to evaluate the expression changes occurring after cryopreservation/thawing, both when maintained in suspension and after plating. The second aim of the present study was to evaluate gene expression in hepatocytes after cold storage of suspensions up to 24 h compared with freshly isolated hepatocytes in suspension. Our results show that the gene expression in freshly isolated human hepatocytes in suspension after isolation is similar to that of the liver of origin. Furthermore, gene expression in primary human hepatocytes in suspension is not affected by hepatocyte cold storage and cryopreservation. However, the gene expression is profoundly affected in monolayer cultures after plating. Specifically, gene expression changes were observed in cultured relative to suspensions of human hepatocytes that are involved in cellular processes such as phase I/II metabolism, basolateral and canalicular transport systems, fatty acid and lipid metabolism, apoptosis, and proteasomal protein recycling. An oxidative stress response may be partially involved in these changes in gene expression. Taken together, these results may aid in the interpretation of data collected from human hepatocyte experiments and suggest additional utility for cold storage and cryopreservation of hepatocytes.

Many varieties of banana (Musa acuminata) lack resistance to biotic stresses. An EST collection was developed, including transcripts expressed in banana-Mycosphaerella fijiensis interactions. Developed polymorphic gene-derived SSR markers are applicable for genetic mapping, diversity characterization and marker assisted breeding., Background and aims Banana (Musa acuminata) is a crop contributing to global food security. Many varieties lack resistance to biotic stresses, due to sterility and narrow genetic background. The objective of this study was to develop an expressed sequence tag (EST) database of transcripts expressed during compatible and incompatible banana–Mycosphaerella fijiensis (Mf) interactions. Black leaf streak disease (BLSD), caused by Mf, is a destructive disease of banana. Microsatellite markers were developed as a resource for crop improvement. Methodology cDNA libraries were constructed from in vitro-infected leaves from BLSD-resistant M. acuminata ssp. burmaniccoides Calcutta 4 (MAC4) and susceptible M. acuminata cv. Cavendish Grande Naine (MACV). Clones were 5′-end Sanger sequenced, ESTs assembled with TGICL and unigenes annotated using BLAST, Blast2GO and InterProScan. Mreps was used to screen for simple sequence repeats (SSRs), with markers evaluated for polymorphism using 20 diploid (AA) M. acuminata accessions contrasting in resistance to Mycosphaerella leaf spot diseases. Principal results A total of 9333 high-quality ESTs were obtained for MAC4 and 3964 for MACV, which assembled into 3995 unigenes. Of these, 2592 displayed homology to genes encoding proteins with known or putative function, and 266 to genes encoding proteins with unknown function. Gene ontology (GO) classification identified 543 GO terms, 2300 unigenes were assigned to EuKaryotic orthologous group categories and 312 mapped to Kyoto Encyclopedia of Genes and Genomes pathways. A total of 624 SSR loci were identified, with trinucleotide repeat motifs the most abundant in MAC4 (54.1 %) and MACV (57.6 %). Polymorphism across M. acuminata accessions was observed with 75 markers. Alleles per polymorphic locus ranged from 2 to 8, totalling 289. The polymorphism information content ranged from 0.08 to 0.81. Conclusions This EST collection offers a resource for studying functional genes, including transcripts expressed in banana–Mf interactions. Markers are applicable for genetic mapping, diversity characterization and marker-assisted breeding.

The worldwide destructive epidemic of the fungus Mycosphaerella fijiensis on banana started recently, spreading from South-East Asia. The founder effects detected in the global population structure of M. fijiensis reflected rare migration events among continents through movements of infected plant material. The main objective of this work was to infer gene flow and dispersal processes of M. fijiensis at the continental scale from population structure analysis in recently invaded regions. Samples of isolates were collected from banana plantations in 13 countries in Latin America and the Caribbean and in Africa. The isolates were analysed using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and microsatellite molecular markers. The results indicate that a high level of genetic diversity was maintained at the plantation and the plant scales. The loci were at gametic equilibrium in most of the samples analysed, supporting the hypothesis of the existence of random-mating populations of M. fijiensis, even at the plant scale. A low level of gene diversity was observed in some populations from the Africa and Latin America-Caribbean regions. Nearly half the populations analysed showed a significant deviation from mutation-drift equilibrium with gene diversity excess. Finally, a high level of genetic differentiation was detected between populations from Africa (FST = 0.19) and from the Latin America-Caribbean region (FST = 0.30). These results show that founder effects accompanied the recent invasion of M. fijiensis in both regions, suggesting stochastic spread of the disease at the continental scale. This spread might be caused by either the limited dispersal of ascospores or by movements of infected plant material.

International audience; Les bananes sont généralement récoltées vertes, en phase pré-climactérique, avant la mise en marché et la commercialisation. Le temps de transport nécessaire entre la zone de production et les chambres où sont mûris des fruits est appelé durée de vie verte ou green life (GL), qui est défini par le nombre de jours entre la récolte et l'initiation naturelle des processus de maturation. Il a été montré que la Sigatoka Disease (SD) a une influence sur le comportement post récolte des fruits. Ce travail a pour objectif de déterminer s'il existe une relation entre le niveau de sévérité de la SD et des paramètres de qualité de la banane caractérisés par des caractéristiques pomologiques, notamment la couleur de la pulpe. Dans une expérimentation conduite avec des fruits récoltés à âge physiologique constant, nos résultats montrent que la SD a un effet très marqué sur le pourcentage de fruits mûrs au champ et sur le poids des fruits à la récolte, mais pas sur le calibre. Nous avons également montré que la couleur de la pulpe était fortement affectée par la SD ; quand l'intensité de la maladie augmente, les paramètres définissant la couleur de la pulpe sont modifiés - la valeur de L* (clarté) décroit ; les valeurs de a* (axe rouge - vert) et b* (axe jaune-bleu) augmentent, donnant un teint grisâtre à la couleur de la pulpe, avec un peu moins de teinte verte et plus de jaune et de rouge. Ces résultats nous permettent de proposer une méthode de contrôle de la qualité des fruits, par mesure au champ de la couleur de la pulpe, dans le but de réduire les pertes après-récoltes. (Résumé d'auteur)

Our aim was to assess the level of suppressiveness to #Fusarium# wilt of a palm grove soil from Ivory Coast in relation to the cultivation of a commonly used leguminous cover-plant, #Pueraria javanica#. The cultivation of #P. javanica# resulted after 49 and 230 d in a significant increase in the amount of suppressiveness. This increase was correlated with an increase in the densities of the populations of fungi, #Fusarium# spp. and #Fusarium oxysporum#, respectively. Using a PCR-RFLP based method, 16 IGS types were identified among 180 isolates of #F. oxysporum# recovered from uncultivated, cultivated and rhizospheric soils, but the distributions of the isolates among the 16 IGS types were similar in both the cultivated soils and the uncultivated soils. Thus, the changes in densities of the #F. oxysporum# populations were not associated with any change in the structure of these populations. Therefore, the increased degree of soil suppressiveness induced by the cultivation of the cover-plant could he attributed to quantitative but not to qualitative changes affecting the populations of #F. oxysporum#.

International audience; Banana fruit are typically harvested at a green preclimacteric stage prior to sale. The green life (GL), which is defined as the number of days between harvesting and initiation of the natural ripening process, is important for harvesting, shipping and marketing. Sigatoka disease (SD) has previously been shown to influence the postharvest quality of banana fruit. The present study was conducted to determine (1) whether there is a relationship between the level of SD severity and the banana quality as characterized by several pomological traits and fruit green life, and (2) whether there is a level of SD severity that is acceptable for fruit exporting. In an experiment conducted with fruit harvested at a constant physiological age, our results showed that the disease decreases fruit GL. Based on the relationship established between SD severity and GL, it is possible to define an acceptable disease level threshold corresponding to a specified GL. Additionally, we found that high SD infections caused a reduction in fruit weight but did not affect fruit size. Possible explanations are discussed. (Résumé d'auteur)

Mycosphaerella leaf spot diseases of banana caused by Mycosphaerella musicola (Sigatoka leaf spot, SLS) and M. fijiensis (black leaf streak, BLS) are very destructive. The cultivation of resistant banana clones appears to be the most appropriate means to combat disease because genetic resistance is usually more durable than chemical control. Since pathogen populations maintain a high level of genetic diversity and single gene resistances, such as is suspected in total or hypersensitive resistance situations, are easily overcome, the phenomenon of "partial resistance", which may be multigenic and hence more durable, is used in CIRAD breeding programmes. New hybrids of banana developed by CIRAD are routinely selected for resistance against Mycosphaerella leaf spot diseases. There are four steps in the evaluation process. Firstly, the level of resistance to M. musicola is assessed under field conditions in Guadeloupe. Secondly, the most resistant hybrids are selected for detached-leaf resistance tests to M. musicola and M. fijiensis under controlled conditions. Thirdly, the efficacy of partial resistance of selected hybrids against M. musicola is confirmed in the field in the French West Indies and against M. fijiensis in the field in Mayotte. Finally, the durability of the hybrids' partial resistance is assessed by measuring disease severity in experimental plots for several years. This complete 4-steps process allows, through experimental approaches, to select banana cultivars with durable resistance against Mycosphaerella leaf spot diseases. (Resume d'auteur)

Sigatoka disease (SD) of bananas is caused by the pathogenic fungus Mycosphaerella musicola Leach. This disease provokes necrotic lesions on leaves and serious infestations can lead to a substantial reduction in the leaf area of infected plants and thus to yield losses. In addition to these effects on yield, SD was found to have an impact on fruit quality, especially because exported bananas ripen prematurely. In the present work, a plantation survey and experiments have been conducted in Guadeloupe (FWI) to assess the effect of this disease on the greenlife of bananas harvested at a constant physiological age, as measured in degree-days (dd). Our results revealed that bananas harvested at 900 dd from plants with high Sigatoka disease severity had normal diameter growth, but a shorter greenlife (GL) than bananas harvested from uninfected plants. These results indicate that SD is directly responsible for the reduction of banana greenlife since the reduction of GL could not be attributed to the harvest of fruits at a more advanced physiological age (dd). Furthermore, a correlation was noted between SD severity and GL. The potential physiological mechanisms involved are also discussed. (Resume d'auteur)

Leaf spot diseases of banana caused by Mycosphaerella musicola or Mycosphaerella fijiensis are the main constraints in commercial plantations in most banana-producing countries. In humid, high-rainfall conditions, plantation production of dessert bananas for export can only be achieved through intensive chemical control. The development of chemical control strategies in Belize, Cameroon and the French West Indies (FWI) over the past 20 years is described. In all of these countries, which are representative of many banana-growing regions, there have been difficulties in terms of the efficacy, cost and environmental impact of chemical control. This is the result of three major factors: (i) the tightening of legislation governing aerial spraying, which is becoming more and more restrictive, especially in the FWI; (ii) the small number of fungicides now approved for leaf spot control; and (iii) the development of resistance to systemic fungicides. In Belize, the intensive use of systemic fungicides over the past 20 years (35-40 applications/year) has led to M. fijiensis developing fungicide resistance. Regular applications (40-45 applications/year) of contact fungicides (mancozeb, chlorothalonil) are now used to control black leaf streak. Although in Cameroon systemic fungicides (12-14 applications/year) were originally used in combination with a forecasting strategy to control black leaf streak, fungicide resistance has developed in commercial plantations, and this has led to regular application of contact fungicides (40-45 applications/year). In Guadeloupe and Martinique, M. musicola has been controlled efficiently through the use of systemic fungicides (5-7 applications/year) in a forecasting strategy for more than 30 years. Recent French legislation limiting aerial spraying and the kind of fungicides that can be used has resulted in the development of fungicide-resistant strains of the pathogen. As a consequence, the number of treatments/year has recently doubled (10-14 applications/year). New alternatives for control are needed. The introduction of fungicides that have less negative environmental effects may be a short-term solution. Organic fungicides or biofungicides applied in mixtures with contact fungicides may allow producers to reduce the amount of active ingredient applied. The possibility of reversing fungicide resistance in order to reintroduce a more sustainable forecasting strategy may be a mid-term solution. The introduction of cultivars bred for resistance is a long-term goal. (Resume d'auteur)

Phenylahistin is a fungal diketopiperazine derived from isoprenylated (Phe-DeltaHis) cyclodipeptide. The (-)-enantiomer is a cell cycle inhibitor, which can be potentially used as an antitumor agent. By contrast, the (+)-enantiomer exhibits no antimicrotubule activity. To better understand the differences that could arise from a difference of bioavailability, we investigated the interaction and metabolism of both enantiomers with mammalian cytochromes P450 (P450s). We found that both enantiomers were metabolized by various isoforms of mammal P450 with a noticeable activity for the (+)-enantiomer. P450 3A isoforms were mainly responsible for this metabolism, the bioactive (-)-enantiomer being 1.5 to 8 times less metabolized than the (+)-enantiomer. Spectral analysis of the interaction with P450s revealed that (-)-phenylahistin led to a hydrophobic type I signature, whereas the (+)-isomer yielded a Fe-N type II one. Structural analysis of metabolites by liquid chromatography-tandem mass spectrometry allowed us to characterize two major metabolites (P1 and P3) for both enantiomers. In human liver microsomal preparations, P1 was predominant in the (-)-phenylahistin metabolic profile. In contrast, (+)-phenylahistin mainly produced P3 in human microsomes and CYP3A human expressed P450s. (-)-Phenylahistin proved to be less toxic on P450-rich hepatocytes than on P450-deprived KB lines. The slower metabolism of this enantiomer could account for its higher toxicity. This is strengthened by the fact that isolated metabolites of (-)-phenylahistin showed no toxic effects toward KB lines. Finally, differences of metabolism and interaction mode between both phenylahistin enantiomers and CYP3A4 were supported by in silico molecular docking calculations.

Ce protocole vise (i) a analyser la nature des interactions hotes-parasites (resistance ou sensibilite), (ii) a etudier la variabilite de la pathogenicite de differentes populations pathogenes (intra/interspecifiques) quant a leur virulence et leur agressivite mesurees a partir d''inoculation artificielle avec Mycosphaerella fijiensis ou Mycosphaerella musicola sur des plants et des morceaux de feuille de bananier. Le principe, les principaux avantages, le materiel vegetal de depart et le temps necessaire de la methode sont presentes. Materiel et methodes. Le materiel de laboratoire necessaire et le detail des seize etapes de realisation du protocole (preparation de l'inoculum conidial, inoculation artificielle sur des plants de bananier, et inoculation artificielle sur des morceaux de feuille de bananier) sont decrits. Des problemes potentiels sont evoques. Resultats. Des symptomes-types de la maladie sont obtenus apres inoculation artificielle (Resume d'auteur)

Sixty strains of #Fusarium oxysporum# were characterized by (i) in vitro growth parameter estimates on four carbon sources and (ii) vegetative compatibility grouping. Growth was assessed by measuring optical density changes of fungal cultures in microtiter plates. Principal component analyses of growth parameters estimates permitted a good discrimination of the isolates on the four carbon sources and revealed a high level of diversity within populations of #F. oxysporum#. The 60 strains were assigned to 40 vegetative compatibility groups. Strains grouped in a given vegetative compatibility group had similar growth parameters. Trophic characterization and vegetative compatibility groups were compared to a genotypic characterization previously performed on this collection. Trophic characterization and vegetative compatibility grouping were more discriminating than the genotypic characterization. The three characterization methods suggested that vegetative compatibility groups could be considered the population unit among natural #F. oxysporum# populations.