Contrasting plant height can improve the control of rain-borne diseases in wheat cultivar mixture: modelling splash dispersal in 3-D canopies.

Background and Aims: Growing cultivars differing by their disease resistance level together (cultivar mixtures) can reduce the propagation of diseases. Although architectural characteristics of cultivars are little considered in mixture design, they could have an effect on disease, in particular through spore dispersal by rain splash, which occurs over short distances. The objective of this work was to assess the impact of plant height of wheat cultivars in mixtures on splash dispersal of Zymoseptoria tritici, which causes septoria tritici leaf blotch. Methods: We used a modelling approach involving an explicit description of canopy architecture and splash dispersal processes. The dispersal model computed raindrop interception by a virtual canopy as well as the production, transport and interception of splash droplets carrying inoculum. We designed 3-D virtual canopies composed of susceptible and resistant plants, according to field measurements at the flowering stage. In numerical experiments, we tested different heights of virtual cultivars making up binary mixtures to assess the influence of this architectural trait on dispersal patterns of spore-carrying droplets. Key Results: Inoculum interception decreased exponentially with the height relative to the main inoculum source (lower diseased leaves of susceptible plants), and little inoculum was intercepted further than 40 cm above the inoculum source. Consequently, tall plants intercepted less inoculum than smaller ones. Plants with twice the standard height intercepted 33 % less inoculum than standard height plants. In cases when the height of suscpeptible plants was doubled, inoculum interception by resistant leaves was 40 % higher. This physical barrier to spore-carrying droplet trajectories reduced inoculum interception by tall susceptible plants and was modulated by plant height differences between cultivars of a binary mixture. Conclusions: These results suggest that mixture effects on spore dispersal could be modulated by an adequate choice of architectural characteristics of cultivars. In particular, even small differences in plant height could reduce spore dispersal.

Worldwide population structure of the wheat rust fungus Puccinia striiformis in the past.

Puccinia striiformis is a basidiomycete causing yellow rust on wheat. The availability of historic samples of this pathogen from the 'Stubbs collection' enabled us to investigate past population structure and temporal dynamics on a global scale. A set of 212 single genotype urediniospore isolates, representing samples collected from five continents between 1958 and 1991, were genotyped using 19 polymorphic microsatellite markers. The population genetic analyses revealed the existence of seven genetic groups in the past worldwide P. striiformis population. This genetic grouping generally corresponded with geographical sample origin except for the Middle East, where six of the seven genetic groups were represented. The presence of many genetic groups in the Middle Eastern population reflected a low differentiation from the populations in East Africa (FST=0.052) and in South Asia (FST=0.064). A high diversity and recombinant population structure was observed in China and South Asia, while a clonal population structure was observed in NW Europe, East Africa and the Mediterranean region. The high genetic diversity in the Himalayan region supported recent studies suggesting a putative center of diversity for P. striiformis in this area. Four of the 89 multilocus genotypes detected were resampled in different geographical regions suggesting long-distance migration in the past. Comparison of the past populations with more recent ones, represented by 309 isolates mainly collected between 2001 and 2009, revealed temporal divergence for all populations except for Northwest Europe. Overall, we observed a clear subdivision within the worldwide population structure of P. striiformis and migration in the past.

Using virtual 3-D plant architecture to assess fungal pathogen splash dispersal in heterogeneous canopies: a case study with cultivar mixtures and a non-specialized disease causal agent.

BACKGROUND AND AIMS: Recent developments in plant disease management have led to a growing interest in alternative strategies, such as increasing host diversity and decreasing the use of pesticides. Use of cultivar mixtures is one option, allowing the spread of plant epidemics to be slowed down. As dispersal of fungal foliar pathogens over short distances by rain-splash droplets is a major contibutor to the spread of disease, this study focused on modelling the physical mechanisms involved in dispersal of a non-specialized pathogen within heterogeneous canopies of cultivar mixtures, with the aim of optimizing host diversification at the intra-field level. METHODS: Virtual 3-D wheat-like plants (Triticum aestivum) were used to consider interactions between plant architecture and disease progression in heterogeneous canopies. A combined mechanistic and stochastic model, taking into account splash droplet dispersal and host quantitative resistance within a 3-D heterogeneous canopy, was developed. It consists of four sub-models that describe the spatial patterns of two cultivars within a complex canopy, the pathway of rain-splash droplets within this canopy, the proportion of leaf surface area impacted by dispersal via the droplets and the progression of disease severity after each dispersal event. KEY RESULTS: Different spatial organization, proportions and resistance levels of the cultivars of two-component mixtures were investigated. For the eight spatial patterns tested, the protective effect against disease was found to vary by almost 2-fold, with the greatest effect being obtained with the smallest genotype unit area, i.e. the ground area occupied by an independent unit of the host population that is genetically homogeneous. Increasing both the difference between resistance levels and the proportion of the most resistant cultivar often resulted in a greater protective effect; however, this was not observed for situations in which the most resistant of the two cultivars in the mixture had a relatively low level of resistance. CONCLUSIONS: The results show agreement with previous data obtained using experimental approaches. They demonstrate that in order to maximize the potential mixture efficiency against a splash-dispersed pathogen, optimal susceptible/resistant cultivar proportions (ranging from 1/9 to 5/5) have to be established based on host resistance levels. The results also show that taking into account dispersal processes in explicit 3-D plant canopies can be a key tool for investigating disease progression in heterogeneous canopies such as cultivar mixtures.

Durable resistance to stripe rust is due to three specific resistance genes in French bread wheat cultivar Apache.

Quantitative resistance is postulated to be more durable than qualitative (R-gene mediated) resistance, which is usually quickly overcome by the pathogen population. Despite its wide use for nearly 10 years in France, the French bread wheat cultivar Apache remains resistant to stripe rust. Here, we investigated the genetic architecture of cv. Apache resistance to examine whether its durability could be explained by quantitative characteristics. We identified quantitative trait loci (QTL) by composite interval mapping of disease progress data recorded throughout 4 years of field assays. These assays included inoculation with three different pathotypes on a segregating population originating from a cross between cv. Apache and cv. Taldor, a French cultivar susceptible to stripe rust. Three QTLs derived from Apache, QYr.inra-2AS, QYr.inra-2BL and QYr.inra-4B, were detected. Each of these QTLs contributed between approximately 15 and 69 % of the phenotypic variance and corresponds to a race-specific resistance gene. We showed that QYr.inra-2AS and QYr.inra-2BS map to the positions of Yr17 and Yr7, respectively, whereas QYr.inra-4B corresponds to an adult plant resistance gene. Our results demonstrate that a combination of two or more race-specific resistance genes can confer durable resistance provided that it is properly managed at a continental level. Race-specific resistance genes should not be removed from breeding programs provided that they are properly managed.

Puccinia striiformis f.sp. tritici presents high diversity and recombination in the over-summering zone of Gansu, China.

Puccinia striiformis f.sp. tritici (PST), a basidiomycota responsible for wheat yellow rust, has a strict clonal behavior and a low genetic diversity in European and Australian populations. On the other hand high diversity has been reported in Chinese populations. Moreover it is thought that in China yellow rust epidemics start recurrently from the western highlands where over-summering occurs. To compare PST genetic diversity in this area to the one described in France seven AFLP primer combinations were used to analyze a sample of 160 isolates collected in 2001 in five counties of Gansu Province. The AFLP data revealed 40 polymorphic bands, discriminating 139 AFLP genotypes. Linkage disequilibrium and phylogeographic analyses support the hypothesis of a reproductive mode that is not strictly clonal. In this regard Chinese isolates from Gansu strongly contrast with the European studies using the same markers. Genetic diversity of this 1 y sampling in Gansu is found to be seven times higher than the one observed in France over 20 y and exhibits lower linkage disequilibrium. The effective population size of the French sample was estimated to be 1000 times smaller than the Gansu population. These results support the hypothesis of large population size as well as the occurrence of genetic recombination, while the importance of Gansu as a main over-summering area requires assessment through larger scale studies.

Geographic limits of a clonal population of wheat yellow rust in the Mediterranean region.

Most plant pathogens present complex life cycles, in which the clonal reproduction may impede the delimitation of population entities. By studying the genetic structure of the wheat yellow rust caused by Puccinia striiformis f.sp. tritici (PST), we highlighted difficulties impeding population delimitation in highly clonal species. Despite the high dispersal potential of PST, southern France isolates were shown to be divergent from a northwestern European population. A 2-year survey was performed in the Mediterranean region to assess the geographic distribution of southern isolates: 453 isolates collected in 11 countries were genotyped using 15 simple sequence repeat markers. A subsample was analysed for virulence against 23 resistance genes. The dominant strain in the western Mediterranean region was further studied with amplified fragment length polymorphism markers to test for a geographic substructure. Both 'individual'- and 'population'-centred analyses of polymorphism markers revealed two south-specific groups: a predominant group, with a broad variability and a wide distribution in both western and eastern Mediterranean countries, and a minor group in the western Mediterranean. The east-west gradient of genetic diversity suggested gene flow from the Middle East with subsequent founder effects and genetic divergence, and demonstrated the local survival of a western Mediterranean population. The high frequency of the resistance gene Yr8 observed in cultivars from Tunisia and Algeria may contribute to maintain the north/south structure observed in France. In addition to migration and local adaptation, the dynamics of clonal lineage diversification and replacement should be considered to define population entities in strongly clonal species.

Evidence of genetic recombination in wheat yellow rust populations of a Chinese oversummering area.

Wheat yellow rust (Puccinia striiformis f.sp. tritici) (PST) has been described as a strongly clonal species in both European and Australian populations, with very limited molecular diversity but rapidly evolving virulences. Contrastingly, marked genetic diversity has been reported in Chinese PST populations. To test whether such variability could originate from oversummering areas, we assessed the diversity of virulence and molecular markers (AFLP and SSR) using 412 PST isolates from the highlands of Tianshui county in Gansu province. Very marked phenotypic and genotypic diversity (38% and 89%, respectively) was found. No genetic structure dependent on the sites sampled (Fst=0.004) or altitude distribution (Fst=0.0098) was detected, indicating important gene flow at the county scale. This study also revealed genetic recombination between molecular markers and thus strongly suggests the existence of a sexual or parasexual cycle in PST in Tianshui county. The observations of higher rates of sexual spore production in genotypes originating from Tianshui are the very first elements suggestive of the existence of a sexual cycle in this species.

Genetic evidence of local adaptation of wheat yellow rust (Puccinia striiformis f. sp. tritici) within France.

Puccinia striiformis f. sp. tritici (PST), a clonal basidiomycete causing yellow rust disease on wheat, has a long record of 'overcoming' cultivar resistance introduced by breeders. Despite the long dispersal capacity of its spores, the French population of PST presents a strong geographical structure, with the presence of a specific pathotype (array of avirulence genes) at high frequencies in the south of France. The genetic diversity underlying this differentiation was analysed by microsatellite and AFLP markers. A total of 213 French isolates belonging to 10 pathotypes collected over a 15-year period were investigated. For each of the 12 microsatellites used, polymorphism resulted from a unique allelic variant associated to the south-specific pathotype. This pathotype was characterized by 40 specific markers over the total of 63 polymorphins detected using 15 AFLP primer combinations. Phylogeographical analysis indicated a strictly clonal structure of the population, and a strong genomic divergence between the northern population and a south-specific clone. Both virulence and molecular data show that the northern French population belongs to the northwestern European population, whereas the southern clone is most likely related to a Mediterranean population, the two subpopulations resulting from the ancient divergence of two clonal lineages. While the virulence complexity in the northern population may be explained by the successive introduction of corresponding resistance genes in cultivars, the maintenance of a simple virulence type in southern France, despite gene flow between the two populations, may be explained in terms of host cultivars repartition and local adaptation to specific host or climatic conditions.

Preinoculation Effects of Light Quantity on Infection Efficiency of Puccinia striiformis and P. triticina on Wheat Seedlings.

ABSTRACT In a previous study under controlled conditions, a model was developed to predict the infection efficiency for the wheat leaf and stripe rust fungi based on temperature and dew period during the 24 h after inoculation. The two pathogens differed in their maximum infection efficiency under controlled conditions for temperature and dew period, the infection efficiency was 12 times greater for Puccinia triticina than for P. striiformis. In the present study, the model was validated by field results to predict P. triticina infection efficiency as a function of temperature and dew period only. However, this model failed to predict infection efficiency caused by P. striiformis in the field. The model was adapted to include the effects of light quantity on infection efficiency. Wheat seedlings, grown in climate-controlled rooms and exposed to various regimes of light duration and intensity for 24 h in either field or controlled conditions, were inoculated and incubated in climate-controlled rooms under optimal dew and temperature conditions. Quantity of natural or artificial light (light intensity x duration) received by the plants prior to inoculation enhanced infection efficiency of wheat seedlings inoculated by P. striiformis. Infection efficiency increased from 0.4 to 36% depending on the light quantity according to a Richards' function. For stripe rust, three environmental variables, preinoculation light quantity received by the plants, postinoculation temperature, and postinoculation dew period, were used for fitting a model for infection efficiency measured in the field.