Competition and interactions among stripe rust pathotypes in wheat-cultivar mixtures

A series of experiments was conducted with wheat stripe rust to analyse competition between simple and complex pathotypes in host mixtures. Two different pathotype combinations were tested, with different host components. Each combination included a complex (able to infect two host components) and two simple pathotypes. For one of the combinations, induced resistance was tested in a separate experiment as a possible interaction among pathotypes. Disease severity and pathotype frequencies were measured three times during the epidemic, on each host component grown in pure stands and in mixtures. In one of the experiments, pathotype frequencies were also measured within secondary foci. One of the complex pathotypes appeared to have a low fitness on one of the host components and did not significantly increase in frequency in host mixtures relative to pure stands. The average frequency of the other complex pathotype increased during the first epidemic cycles, but remained stable afterwards, below expected values. The results suggest that the development of complex pathotypes in host mixtures may be influenced by differential aggressiveness on the host components, by induced resistance and by random effects resulting from the formation of disease foci, and depends on pathogen autoinfection rate and dispersal mechanisms.     

Influence of spatial structure on pathogen colonization and extinction: a test using an experimental metapopulation

The Linum marginale–Melampsora lini plant–pathogen interaction has been studied extensively with regard to its epidemiology and population genetic structure (host resistance and pathogen virulence) in a natural metapopulation. In this study, this system was used in an experimental metapopulation approach to investigate explicitly how the distance (degree of isolation) between local population patches influences disease dynamics within a growing season, as well as the genetic structure of pathogen populations through stochastic colonization and extinction processes. The experimental design centred on four replicate sets of populations, within which patches were spaced at increasingly greater distances apart. Each patch consisted of an identical set of host and pathogen genotypes, with each pathogen genotype having the ability to attack only one of four host-resistance types. Over the 2 years of the experiment, the results showed clear 'boom-and-bust' epidemic patterns, with the strongest determinant of disease dynamics within a growing season being the identity of particular host–pathogen genotypic combinations. However, there were also significant effects of spatial structure, in that more isolated patches tended to exhibit lower levels of disease during epidemic peaks than patches that were close together. Extinction of pathogen genotypes from individual populations was positively related to the severity of disease during preceding epidemic peaks, but negatively related to the level of disease present at the final census prior to overwintering. The probability of recolonization of pathotypes into populations during the second growing season was most strongly related to the distance to the nearest neighbouring source population in which a given pathotype was present. Overall, these results highlight the importance of spatial scale in influencing the numerical and genetical dynamics of pathogen populations.     

Analysis of diversity in populations of plant pathogens: the barley powdery mildew pathogen across Europe

Understanding population genetics and evolution within species requires recognition of variation within and between populations and the ability to distinguish between the potential causes of an observed distribution of variation. For this aim several established indices of diversity, and a novel one, were applied to population samples of the barley powdery mildew pathogen,Erysiphe graminis f. sp.hordei. Random spore samples were obtained from the air along transects through regions of interest across large parts of Europe in 1990. Significant geographical differences in diversity of virulence genotypes occurred among regional sub-samples. Diversity was highest in the samples from eastern Germany, Denmark and Austria, whereas the lowest values were found in the samples from Italy, southern France and parts of western Germany. Diversity in the pathogen population was generally related to the degree of diversification of host resistance in time and space, although there was considerable variation in ranking among different measures of diversity. Sensitivity to sample size proved to be the major problem with the use of several established indices of diversity. Working with very large sample sizes we used multiple random subsamples of various smaller sizes to determine how the mean index values changed with changing sample size. The Shannon index proved to be considerably affected by sample size, in contrast to the Simpson index that was therefore used as a global measure of diversity. Limits of confidence were estimated for the Simpson index using the bootstrap method of numerical resampling. The two aspects contributing to global diversity, richness and evenness, were considered separately to allow meaningful interpretation of the Simpson index. Random sub-sampling was used to reduce the influence of sample size differences for these measures. Dissimilarity, a novel measure of diversity for use in plant pathology, indicates the average number of major genes for host resistance against which pathotypes in a sample respond differently. It is thus able to account for the genetic relationship among pathotypes, which is not considered by any other index. The approaches developed in this study help to compare major forces driving evolution of large-scale populations of the barley mildew pathogen.     

Intrapathotype diversity for aggressiveness and pathogen evolution in cultivar mixtures

ABSTRACT A model was developed and used to study the consequences of diversity for aggressiveness within pathotypes on pathogen evolution in two-component and four-component cultivar mixtures. It was assumed that, within a pathotype, a proportion of the isolates would have higher or lower spore efficacy than the average on a given host genetic background. Two situations were examined in which the pathogen can have either independent or negatively correlated values for spore efficacy on different cultivars. In the latter case, a pathogen genotype more aggressive than the average on a host genotype was always less aggressive on other host genotypes. In the simulations, isolates with greater aggressiveness relative to a host genotype were selected for and increased in frequency. However, because simple pathotypes always reproduced on the same host genotype whereas complex pathotypes were able to grow on several hosts, selection was faster for simple pathotypes. Pathotypes with two different levels of diversity for aggressiveness were compared with nondiversified pathotypes. In order to make comparisons, the effect of a 5 and 10% cost of virulence on the development of complex pathotypes was simulated. In general, increased diversity within pathotypes reduced the rate of increase of complex pathotypes in host mixtures, and this effect was stronger with greater frequencies of autodeposition of pathogen spores.     

The population genetic structure of the rust fungus Melampsora lini as revealed by pathogenicity, isozyme and RFLP markers

Isozyme and RFLP markers were used to investigate genetic variation underlying the racial structure of populations of Melampsora lini occurring on the Australian endemic host species, Linum marginale . Previous studies of the pathogenicity of isolates occurring in a series of discrete but interconnected populations detected an overall structure characterized by linkage disequilibrium—indicative of a pathogen population reproducing asexually as a number of clonal lines (Burdon & Jarosz, 1992), Using samples from that study, we detected two isozyme multi-locus phenotypes. One of these was restricted to two closely related races which appear to be examples of recent migrants into the area. Variation was also detected among and between races of the pathogen using two RFLP markers. Differences detected by one of these markers suggested the occurrence of a previous migrational event. Variation in the other marker detected two phenotypes within three common, widespread races. However, not all possible phenotypes were recovered, suggesting that the variation detected was not generated by recent sexual recombination.     

Cultivar preference exhibited by two sympatric and genetically distinct populations of the soybean fungal pathogen Phialophora gregata f.sp. sojae

Phialophora gregata f.sp. sojae , a soilborne vascular pathogen causing brown stem rot of soybean, has been divided into A and B populations based on variation in the intergenic spacer region of nuclear rDNA (rDNA marker). The A and B populations correlate with defoliating and nondefoliating pathotypes, respectively. In this study, eight additional polymorphic anonymous marker loci (five inter simple sequence repeat loci and three long-primer random amplified polymorphic DNA loci) were identified and applied to a total of 189 isolates. Alleles of these eight loci were invariant within, but different between the A and B populations, providing further evidence that the rDNA marker identifies genetically distinct populations. The two populations were sympatric, residing not only in the same field, but also in the same plants under field conditions. Representative strains of the two populations, when used individually in inoculations, infected both resistant cv. Bell and susceptible cv. Sturdy. However, when the same representatives of the two populations were mixed in a 1 : 1 ratio and used as a mixed inoculum in a competitive bioassay, differential cultivar preference was revealed using PCR detection of populations in infected plants. Population A was detected significantly more often (18 out of 24 plants) in the susceptible cv. Sturdy, whereas population B was detected significantly more often (17 out of 24 plants) in the resistant cv. Bell, corroborating earlier field studies. This is the first controlled experiment to demonstrate a differential host preference of P. gregata f.sp. sojae toward different cultivars of the same host species. Unification of terminologies used in P. gregata f.sp. sojae is discussed.     

Virulence phenotypes in powdery mildew (Blumeria graminis) populations and resistance genes in triticale (x Triticosecale)

Triticale (xTriticosecale) is a new host for powdery mildew (Blumeria graminis) being not infected by this pathogen in Germany before 2001. To evaluate population structure of the pathogen and race-specific resistances in the host, 694 isolates were collected in 12 states of Germany in the years 2007 to 2009 on triticale. They were tested by a newly developed initial differential set of 20 triticale cultivars. Corresponding virulences were found for all differentials except for cultivar Grenado. In total, 272 different virulence phenotypes (=pathotypes) were detected. The virulence complexity of the isolates ranged from 6 to 19 of 20 possible virulences with a mean of 15. In all years, a high level of diversity of the powdery mildew populations was observed with Simpson indices in the range 0.95 to 0.97. The distribution of the pathotypes was even across Germany with an Evenness index in the range 0.82 to 0.88. A set of 19 isolates with different virulence pathotypes and 10 cultivars were selected to be used to identify race-specific resistances of triticale cultivars and breeding lines. Some cultivars susceptible to most of the isolates in seedling stage were moderately resistant in adult-plant stage. The high diversity and complexity of the pathotypes found in German powdery mildew populations as well as an increasing acreage of only a few dominating triticale cultivars accelerate the adaptation of the pathogen to race-specific host resistances suggesting restricted durability only. More durable resistance might be achieved by combining new effective race-specific (qualitative) resistance genes with race-nonspecific (quantitative) resistances effective in the adult-plant stage that are already available.     

鲁豫皖交界地区四个小麦禾谷孢囊线虫群体致病型鉴定

为了明确鲁豫皖交界地区小麦禾谷孢囊线虫的致病类型,利用23个国际鉴别寄主品种和1个当地小麦品种温麦19对采自山东菏泽、安徽颍上、河南商丘和淮阳的4个小麦禾谷孢囊线虫群体致病型进行了鉴定。4个群体的致病型不同于国际上已命名的16个小麦禾谷孢囊线虫致病型。13个A组鉴别寄主对山东菏泽与安徽颍上群体具有相同的抗性反应,这2个线虫群体属于同一致病型;河南商丘群体与上述2个群体的毒性相似,但大麦寄主KVL191对山东菏泽群体和安徽颍上群体表现为抗病,而对河南商丘群体表现感病;河南淮阳群体的致病谱较宽,明显不同于其它3个群体。对照小麦品种温麦19对4个小麦禾谷孢囊线虫群体均表现感病。     

Diversity of Pathotypes and DNA Fingerprint Haplotypes in Populations of Magnaporthe grisea in Korea over Two Decades

ABSTRACT Using isolates collected over 2 decades, we determined the population structure and dynamics of the rice blast fungus, Magnaporthe grisea, in Korea at both the genotypic and phenotypic levels. Pathotype analysis on 6,315 isolates collected from 328 rice cultivars from 1981 to 2000 revealed the presence of a total of 91 pathotypes. Among these 91 patho-types, nine dominated, comprising 76.5% of the isolates. The expected number of pathotypes (corrected for sample size) increased significantly during the course of this study. On average, six (ranging from 0 to 20) new commercial cultivars were introduced annually between 1981 and 1998. However, the overall cultivar diversity, estimated using the Shannon index, was low. Most of the new cultivars were not planted to a large area because the seven most common cultivars each year occupied over 70% of the rice-cultivated area. The frequencies of the nine dominant patho-types from these seven cultivars were highly correlated with those from the entire set of cultivars. To understand genetic diversity within and between pathotypes, 176 isolates collected from 1984 to 1999 were randomly sampled and analyzed by DNA fingerprinting. High similarities were observed among isolates; overall similarities were greater than 63% in combined MGR586 and MAGGY DNA fingerprints. Unlike most other populations of M. grisea, DNA fingerprints showed no clear lineage structure. No groups were supported by bootstrap values greater than 10%. Furthermore, there was no significant correlation between DNA fingerprint similarities and pathotypes. Genetic similarity was significantly greater (P < 0.001) within years than between years, although the difference was small. Our data suggest that M. grisea populations in Korea have been mostly dominated by a single clonal lineage. We cannot conclude from these data that selection by the host population has been a major force in the evolution of M. grisea in Korea.     

Comparison of Genetic and Virulence Diversity of Melampsora larici-populina Populations on Wild and Cultivated Poplar and Influence of the Alternate Host

ABSTRACT The aims of this study were, first, to compare the genetic and virulence diversity between populations of the rust fungus Melampsora larici-populina on wild and cultivated poplar stands and, second, to investigate the influence of the presence of the alternate host of the pathogen, larch, on which its sexual reproduction occurs, on these diversities. Nine French M. larici-populina populations collected from poplar trees in autumn and four populations collected from larch trees during the following spring were analyzed using both virulence factors and neutral markers. In all, 30 pathotypes were identified within the 13 populations studied. The pathotypic structure clearly distinguished the cultivated stands with high richness and complexity from the wild stands with low richness and complexity. High linkage disequilibria between virulences indicated preferential virulence associations, probably due to selection by the host. In all, 19 random amplified polymorphic DNA (RAPD) markers were used, which revealed a very high genetic diversity in the 743 isolates analyzed. The nine populations from poplar appeared moderately differentiated, indicating long-distance gene flow, and no isolation by distance was found. Linkage disequilibria between RAPD markers generally were low, indicating frequent recombination, but they were not lower in populations located near larch, probably due to long-distance dispersal.     

Epidemiology in mixed host populations

ABSTRACT Although plant disease epidemiology has focused on populations in which all host plants have the same genotype, mixtures of host genotypes are more typical of natural populations and offer promising options for deployment of resistance genes in agriculture. In this review, we discuss Leonard's classic model of the effects of host genotype diversity on disease and its predictions of disease level based on the proportion of susceptible host tissue. As a refinement to Leonard's model, the spatial structure of host and pathogen population can be taken into account by considering factors such as autoinfection, interaction between host size and pathogen dispersal gradients, lesion expansion, and host carrying capacity for disease. The genetic composition of the host population also can be taken into account by considering differences in race-specific resistance among host genotypes, compensation, plant competition, and competitive interactions among pathogen genotypes. The magnitude of host-diversity effects for particular host-pathogen systems can be predicted by considering how the inherent characteristics of a system causes it to differ from the assumptions of the classic model. Because of the limited number of studies comparing host-diversity effects in different systems, it is difficult at this point to make more than qualitative predictions. Environmental conditions and management decisions also influence host-diversity effects on disease through their effect on factors such as host density and epidemic length and intensity.     

Intraspecific variation in host susceptibility and climatic factors mediate epidemics of sudden oak death in western US forests

Umbellularia californica is one of the key infectious hosts of the exotic Phytophthora ramorum, which causes sudden oak death (SOD) in California and Oregon forests. This study provides a comprehensive analysis of the epidemiologically relevant parameters for SOD in California and southern Oregon, including potential differences between the two states. Experimental infection of U. californica leaves was optimal when leaves were wet for 6–12 h, temperature was approximately 19°C and pathogen concentration was approximately 2·7 × 10⁴ zoospores mL^?1. Seasonal variation in host susceptibility and disease incidence was examined for two populations by inoculating detached leaves at 12 dates and by monitoring naturally infected leaves, respectively. Susceptibility of U. californica and disease incidence varied significantly in time and the variation was highest for both in spring. Susceptibility of trees from 17 natural populations from California and southern Oregon was assessed in detached leaf inoculations. One California and three southern Oregon populations had significantly and repeatable lower average susceptibility in artificial inoculations, but differences among three selected California and Oregon populations were not significant in inoculations of seedlings grown from seed in a common garden. This study concludes that U. californica susceptibility has a large environmental component, yet still predicts potential disease severity in different sites especially where infestations are young or the pathogen has not yet arrived. The accuracy and utility of predictive risk models for P. ramorum will be enhanced by the inclusion of both the environmental and host susceptibility components.     

Influence of plant host species on intraspecific competition during infection by Aspergillus flavus

Compositions of Aspergillus flavus populations determine the extent to which crops become contaminated with aflatoxins. In the current study, influences of diverse crop hosts on competition among A. flavus isolates were quantified with pyrosequencing. Maize, cotton, soyabean and sorghum supported different levels of sporulation, but intraspecific differences in sporulation were not detected on any host. However, hosts differentially influenced competition during infection, allowing greater sporulation by some isolates and increased host tissue invasion by others. Furthermore, competitive interactions during host invasion did not predict isolate success during sporulation. Isolates were similarly competitive on maize and sorghum, the two most closely related hosts. Host‐specific influences on intraspecific competition may dictate compositions of A. flavus populations and, as a result, the severity of aflatoxin contamination. Host factors should be considered when designing and implementing aflatoxin management strategies including biocontrol with atoxigenic strains.     

Pathotypes and phylogenetic variation determine downy mildew epidemics in Brassica spp. in Australia

Isolates of Hyaloperonospora brassicae inoculated onto cotyledons of 28 diverse Brassicaceae genotypes, 13 from Brassica napus, two from B. juncea, five from B. oleracea, two from Eruca vesicaria, and one each from B. nigra, B. carinata, B. rapa, Crambe abyssinica, Raphanus sativus and R. raphanistrum, showed significant effects (P ≤ 0.001) of isolate, host and their interaction. Host responses ranged from no visible symptom or a hypersensitive response, to systemic spread and abundant pathogen sporulation. Isolates were generally most virulent on their host of origin. Using an octal classification, six host genotypes were identified as suitable host differentials to characterize pathotypes of H. brassicae and distinguished eight distinct pathotypes. There were fewer, but more virulent, pathotypes in 2015–2016 isolates than 2006–2008 pathogen populations, probably explaining the increase in severity of canola downy mildew over the past decade. Phylogenetic relationships determined across 20 H. brassicae isolates collected in 2006–2008 and 88 isolates collected in 2015–2016 showed seven distinct clades, with 70% of 2006–2008 isolates distributed within clade I (bootstrap value (BVs) of 100%) and the remaining 30% in clade V (BVs 83.3%). This is the first study to define phylogenetic relationships of H. brassicae isolates in Australia, setting a benchmark for understanding current and future genetic shifts within pathogen populations; it is also the first to use octal classification to characterize pathotypes of H. brassicae, providing a novel basis for standardizing phenotypic characterization and monitoring of pathotypes on B. napus and some crucifer species in Australia.     

Diversity,spatial variation,and temporal dynamics of virulences in the German leaf rust (<Emphasis Type="Italic">Puccinia recondita</Emphasis> f. sp. <Emphasis Type="Italic">secalis</Emphasis>) population in winter rye

A large collection of German rye leaf rust isolates was analysed to characterize the diversity, spatial variation and temporal dynamics of virulences. Virulence-avirulence phenotypes (=pathotypes) were determined on 23 host differentials. We found 93 pathotypes among 177 single-uredinial isolates in 2000, 201 pathotypes among 437 isolates in 2001, and 125 pathotypes among 213 isolates in 2002. In total, the 827 analyzed isolates represented 317 pathotypes. Frequency of virulences on the individual differentials varied from 2% to 97%. Eight of the differentials showed a high resistance level with virulence frequencies <10%. Virulence complexity of the isolates ranged from 3 to 21 with a mean of nine. The percentages of highly virulent isolates (>14 virulences) increased from 4 to 15% during the sampling period. A high level of virulence diversity was observed within and between individual sampling sites with Simpson indices around 0.9. Evenness indices ranged from 0.88 to 0.92. Four of the five most frequent pathotypes were found in each year but their frequency never exceeded 10%. Isolates with unusual virulence combinations could be clearly separated by principal component analysis. Location-specific pathotype frequencies were revealed in each year, but the frequency patterns varied across years. On four fields a considerable increase of highly virulent pathotypes occurred within 6 weeks during the epidemic. The high diversity of pathotypes as well as the fast accumulation of highly virulent pathotypes favour the adaptation of the pathogen to race-specific host resistances. More durable resistance might be achievable by combining new effective race-specific resistances with adult-plant and/or race-non-specific quantitative resistances.     

Pathotypes of <Emphasis Type="Italic">Plasmodiophora brassicae</Emphasis> causing damage to oilseed rape in the Czech Republic and Poland

Winter oilseed rape (Brassica napus) is an important crop in the Czech Republic and Poland. Clubroot disease caused by the pathogen Plasmodiophora brassicae is a serious and still-growing problem for oilseed rape growers in both countries. The aim of this study was to evaluate the pathotype composition of P. brassicae populations from the Czech Republic and Poland, according to the three evaluation systems, and to determine soil inoculum loads for representative fields via traditional end-point PCR as well as quantitative PCR analysis. There were considerable differences between the populations of P. brassicae from both countries, and the number of pathotypes varied depending on the evaluation system and the threshold used to distinguish susceptible vs. resistant plant reactions. This is the first study comparing the effect of different thresholds. Using an index of disease (ID) of 25 % to distinguish susceptible vs. resistant reactions, there was a total of seven pathotypes identified based on the differentials of Williams, five with the system of Somé et al., and 18 with the European Clubroot Differential (ECD) set. However, based on a threshold of 50 %, there were nine pathotypes according to the evaluation system by Williams, four based on the differentials of Somé et al., and 15 with the ECD set. Changing of the thresholds led to the reclassification of some pathotypes. Several pathotypes were common in both countries. High amounts of pathogen DNA were found in many of the field soils analysed by quantitative PCR. There was a weak correlation between soil pH and infestation of P. brassicae for the Polish soils.     

Studies on the origin,spread, and evolution of an important group ofPuccinia recondita f. sp.tritici pathotypes in Australasia

Wheat brown rust pathotype (pt) 104-2,3,(6),(7), 11 was first detected in Australasia in Victoria during 1984. Although it appeared similar to a pre-existing pathotype, 104-2,3,6,(7), detailed greenhouse test revealed nine pathogenic differences between the two rusts. Six differences involved contrasting virulence/avirulence for the resistance genes/specificitiesLr12, Lr27+Lr31 andLr16, and three uncharacterised genes, present in the wheat cultivars Gaza and Harrier, and in triticale cultivar Lasko. Differences in partial virulence between the pathotypes were found for the genesLr2a, Lr13 andLr26. A comparison of the phenotypes for 13 isozyme systems in the two pathotypes revealed two differences, including aPgm2 allele in pt 104-2,3,(6),(7),11 not found in other contemporary AustralasianPuccinia recondita f. sp.tritici pathotypes. On the basis of these differences, it was concluded that pt 104-2,3,(6),(7),11 was introduced into the Australasian region before or during 1984.Seven variants of pt 104-2,3,(6),(7),11, that differed by single virulences, were detected during 1984–1992. Pt 104-2,3,(6),(7),11 and a derivative pathotype with virulence forLr20 underwent rapid increases in frequency, largely displacing pathotypes which predominated before 1984. Although first detected in eastern Australia, both pathotypes spread to New Zealand, and the derivative pathotype appeared in Western Australia. The rapid spread and increase of these pathotypes could not be explained by host selection. Pt 104-2,3,(6),(7),11 and derivatives may therefore be more aggressive than other contemporary Australasian pathotypes.Abbreviations NSW New South Wales - Prt Puccinia recondita f. sp.tritici - Qld Queensland - SA South Australia - WA Western Australia     

Intraspecific Relationship of Plasmopara halstedii Isolates Differing in Pathogenicity and Geographic Origin Based on ITS Sequence Data

Sequence parts of the internal transcribed spacer (ITS) region of nuclear ribosomal DNA were analysed to screen for the intraspecific variability of a non-coding genomic region in 15 Plasmopara halstedii populations of different pathotype and geographic origin. Samples revealed uniformity in a ca. 790 Bp fragment comprising of the ITS-1, 5.8S and front parts of the ITS-2. In contrast, clear differences were found in a ca. 810 Bp fragment of the ITS-2 thus allowing differentiation between populations of pathotype 100, 310 and 330 and a group of populations representing pathotypes 700, 701, 703, 710 and 730. Samples of pathotypes 700 to730 originated from Slovakia, France, and Germany, but were uniform in both ITS sequence parts, thus indicating very recent origin of these highly aggressive physiological races. The potential use of ITS sequences for pathotype differentiation and phylogenetic studies in P. halstedii is discussed.     

Is virulence phenotype evolution driven exclusively by Lr gene deployment in French Puccinia triticina populations?

Puccinia triticina is a highly damaging wheat pathogen. The efficacy of leaf rust control by genetic resistance is mitigated by the adaptive capacity of the pathogen, expressed as changes in its virulence combinations (pathotypes). An extensive P. triticina population survey has been carried out in France over the last 30 years, describing the evolutionary dynamics of this pathogen in response to cultivar deployment. We analysed the data set for the 2006–2016 period to determine the relationship between the Lr genes in the cultivars and virulence in the pathotypes. Rust populations were dominated by a small number of pathotypes, with variations in most of the virulence frequencies related to the corresponding Lr gene frequencies in the cultivated landscape. Furthermore, the emergence and spread of a new virulence matched the introduction and use of the corresponding Lr gene (Lr28), confirming that the deployment of qualitative resistance genes is an essential driver of evolution in P. triticina populations. However, principal component analysis (PCA) revealed that certain pathotype–cultivar associations cannot be explained solely by the distribution of Lr genes in the landscape. This conclusion is supported by the predominance of a few pathotypes on some cultivars, with the persistence of several other compatible pathotypes at low frequencies. Specific interactions are not, therefore, sufficient to explain the distribution of virulence in rust populations. The hypothesis that quantitative interactions between P. triticina populations and bread wheat cultivars—based on differences in aggressiveness—is also a driver of changes in pathotype frequencies deserves further investigation.     

Diagnostics,genetic diversity and pathogenic variation of ascochyta blight of cool season food and feed legumes

Molecular diagnostic techniques have been developed to differentiate the Ascochyta pathogens that infect cool season food and feed legumes, as well as to improve the sensitivity of detecting latent infection in plant tissues. A seed sampling technique was developed to detect a 1% level of infection by Ascochyta rabiei in commercial chickpea seed. The Ascochyta pathogens were shown to be genetically diverse in countries where the pathogen and host have coexisted for a long time. However, where the pathogen was recently introduced, such as A. rabiei to Australia, the level of diversity remained relatively low, even as the pathogen spread to all chickpea-growing areas. Pathogenic variability of A. rabiei and Ascochyta pinodes pathogens in chickpea and field pea respectively, appears to be quantitative, where measures of disease severity were based on aggressiveness (quantitative level of infection) rather than on true qualitative virulence. In contrast, qualitative differences in pathogenicity in lentil and faba bean genotypes indicated the existence of pathotypes of Ascochyta lentis and Ascochyta fabae. Therefore, reports of pathotype discrimination based on quantitative differences in pathogenicity in a set of specific genotypes is questionable for several of the ascochyta-legume pathosystems such as A. rabiei and A. pinodes. This is not surprising since host resistance to these pathogens has been reported to be mainly quantitative, making it difficult for the pathogen to overcome specific resistance genes and form pathotypes. For robust pathogenicity assessment, there needs to be consistency in selection of differential host genotypes, screening conditions and disease evaluation techniques for each of the Ascochyta sp. in legume-growing countries throughout the world. Nevertheless, knowledge of pathotype diversity and aggressiveness within populations is important in the selection of resistant genotypes.