Genetic structure of Phaeosphaeria nodorum populations in the north-central and midwestern United States

Stagonospora nodorum blotch, caused by Phaeosphaeria nodorum, is considered one of the most destructive foliar diseases of wheat in the United States. However, relatively little is known about the population biology of this fungus in the major wheat-growing regions of the central United States. To rectify this situation, 308 single-spore isolates of P. nodorum were analyzed from 12 populations, five from hard red spring wheat cultivars in Minnesota and North Dakota and seven from soft red winter wheat in Indiana and Ohio. The genetic structure of the sampled populations was determined by analyzing polymorphisms at five microsatellite or simple-sequence repeat (SSR) loci and the mating type locus. Although a few clones were identified, most P. nodorum populations had high levels of gene (H(S) = 0.175 to 0.519) and genotype (D = 0.600 to 0.972) diversity. Gene diversity was higher among isolates collected from spring wheat cultivars in North Dakota and Minnesota (mean H(S) = 0.503) than in those from winter wheat cultivars in Indiana and Ohio (H(S) = 0.269). Analyses of clone-corrected data sets showed equal frequencies of both mating types in both regional and local populations, indicating that sexual recombination may occur regularly. However, significant gametic disequilibrium occurred in three of the four populations from North Dakota, and there was genetic differentiation both within and among locations. Genetic differentiation between the hard red spring and soft red winter wheat production regions was moderate (F(ST) = 0.168), but whether this is due to differences in wheat production or to geographical variation cannot be determined. These results suggest that sexual reproduction occurs in P. nodorum populations in the major wheat-growing regions of the central United States, and that geographically separated populations can be genetically differentiated, reflecting either restrictions on gene flow or selection.     

Comparison of natural populations of Mycosphaerella graminicola from single fields in Kansas and California

Kansas and California wheat-growing regions differ dramatically in soils, climate, wheat cultivars, crop rotation patterns, and cultural practices, which could select for different fungal populations of Mycosphaerella graminicola. Our objective in this study was to use amplified fragment length polymorphism (AFLP) loci to assess the genetic diversity of M. graminicola populations within single fields in two widely separated, and geographically isolated sites in Kansas and California. Three primer-pair combinations were used to resolve polymorphism at 177 loci in 67 and 63 isolates from Kansas and California, respectively. Genotypic variability was high, which is consistent with a genetically diverse initial inoculum. There was no evidence of genetic disequilibrium in either population, with only 4.6% of the locus pairs in Kansas, and 5.4% of the locus pairs in California in detectable disequilibrium. The migration rate calculated between the two sites was as low as 1.8 individuals per generation, and significant differences in allele frequencies were observed. Therefore, these two populations do not represent mere subsamples of a larger, randomly mating population. This is a rare report of isolation by distance occurring between two North American populations of M. graminicola, indicating that at least some of these populations may be differentiating. Although genetic isolation by distance may occur, we cannot exclude movement of new gene combinations such as fungicide resistance or virulence between these two locations.     

Genetic relationships among populations of Gibberella zeae from barley, wheat, potato, and sugar beet in the upper Midwest of the United States

Gibberella zeae, a causal agent of Fusarium head blight (FHB) in wheat and barley, is one of the most economically harmful pathogens of cereals in the United States. In recent years, the known host range of G. zeae has also expanded to noncereal crops. However, there is a lack of information on the population genetic structure of G. zeae associated with noncereal crops and across wheat cultivars. To test the hypothesis that G. zeae populations sampled from barley, wheat, potato, and sugar beet in the Upper Midwest of the United States are not mixtures of species or G. zeae clades, we analyzed sequence data of G. zeae, and confirmed that all populations studied were present in the same clade of G. zeae. Ten variable number tandem repeat (VNTR) markers were used to determine the genetic structure of G. zeae from the four crop populations. To examine the effect of wheat cultivars on the pathogen populations, 227 strains were sampled from 10 subpopulations according to wheat cultivar types. The VNTR markers also were used to analyze the genetic structure of these subpopulations. In all populations, gene (H = 0.453 to 0.612) and genotype diversity (GD = or >0.984) were high. There was little or no indication of linkage disequilibrium (LD) in all G. zeae populations and subpopulations. In addition, high gene flow (Nm) values were observed between cereal and noncereal populations (Nm = 10.69) and between FHB resistant and susceptible wheat cultivar subpopulations (Nm = 16.072), suggesting low population differentiation of G. zeae in this region. Analysis of molecular variance also revealed high genetic variation (>80%) among individuals within populations and subpopulations. However, low genetic variation (<5%) was observed between cereal and noncereal populations and between resistant and susceptible wheat subpopulations. Overall, these results suggest that the populations or subpopulations are likely a single large population of G. zeae affecting crops in the upper Midwest of the United States.     

Using Restriction Fragment Length Polymorphisms to Assess Temporal Variation and Estimate the Number of Ascospores that Initiate Epidemics in Field Populations of Mycosphaerella graminicola

ABSTRACT Restriction fragment length polymorphisms (RFLPs) and DNA fingerprints were used to assess temporal variation and estimate the effective population size of the wheat pathogen Mycosphaerella graminicola over a 6-year period. In each year, the fungal population was founded by ascospores originating from outside the sampled fields. A total of 605 fungal isolates were included in this study. Our results indicate that the genetic structure of these M. graminicola populations were stable over the 6-year period. The common alleles at each RFLP locus were present at similar frequencies each year. More than 99% of gene diversity was distributed within populations sampled from the same year and less than 1% was attributed to differences among years. The lack of population differentiation among collections taken in different years indicated that the effective size of the source population was sufficiently large that genetic drift was insignificant in this location. It also suggests that the initial colonists from ascospore founder populations were a fair reflection of the source population. We estimate that the effective sizes of these field populations ranged from 3,400 to 700,000 individuals, depending on the size of the field sampled and assumptions about mutation rates. Estimates of the number of ascospores initiating epidemics of leaf blotch disease in each field plot and factors that contribute to the large effective population size of M. graminicola are discussed.     

Role of Ascospores in Further Spread of QoI-Resistant Cytochrome b Alleles (G143A) in Field Populations of Mycosphaerella graminicola

ABSTRACT Strobilurin fungicides or quinone outside inhibitors (QoIs) have been used successfully to control Septoria leaf blotch in the United Kingdom since 1997. However, QoI-resistant isolates of Mycosphaerella graminicola were reported for the first time at Rothamsted during the summer of 2002. Sequence analysis of the cytochrome b gene revealed that all resistant isolates carried a mutation resulting in the replacement of glycine by alanine at codon 143 (G143A). Extensive monitoring using real-time polymerase chain reaction (PCR) testing revealed that fungicide treatments based on QoIs rapidly selected for isolates carrying resistant A143 (R) alleles within field populations. This selection is driven mainly by polycyclic dispersal of abundantly produced asexual conidia over short distances. In order to investigate the role of sexually produced airborne ascospores in the further spread of R alleles, a method integrating spore trapping with real-time PCR assays was developed. This method enabled us to both quantify the number of M. graminicola ascospores in air samples as well as estimate the frequency of R alleles in ascospore populations. As expected, most ascospores were produced at the end of the growing season during senescence of the wheat crop. However, a rapid increase in R-allele frequency, from 35 to 80%, was measured immediately in airborne ascospore populations sampled in a wheat plot after the first QoI application at growth stage 32. After the second QoI application, most R-allele frequencies measured for M. graminicola populations present in leaves and aerosols sampled from the treated plot exceeded 90%. Spatial sampling and testing of M. graminicola flag leaf populations derived from ascospores in the surrounding crop showed that ascospores carrying R alleles can spread readily within the crop at distances of up to 85 m. After harvest, fewer ascospores were detected in air samples and the R-allele frequencies measured were influenced by ascospores originating from nearby wheat fields.     

Diversity of Epidemic Populations of Gibberella zeae from Small Quadrats in Kansas and North Dakota

ABSTRACT Gibberella zeae (anamorph Fusarium graminearum) causes Fusarium head blight (FHB) of wheat and barley and has been responsible for several billion dollars of losses in the United States since the early 1990s. We isolated G. zeae from the top, middle, and bottom positions of wheat spikes collected from 0.25-m(2) quadrats during severe FHB epidemics in a single Kansas (KS) field (1993) and in a single North Dakota (ND) field (1994). Three amplified fragment length polymorphism (AFLP) primer pairs were used to resolve 94 polymorphic loci from 253 isolates. Members of a subset of 26 isolates also were tested for vegetative compatibility groups (VCGs). Both methods indicated high levels of genotypic variability and identified the same sets of isolates as probable clones. The mean number of AFLP multilocus haplotypes per head was approximately 1.8 in each population, but this value probably underestimates the true mean due to the small number of samples taken from each head. Isolates with the same AFLP haplotype often were recovered from different positions in a single head, but only rarely were such apparently clonal isolates recovered from more than one head within a quadrat, a pattern that is consistent with a genetically diverse initial inoculum and limited secondary spread. The KS and ND samples had no common AFLP haplotypes. All G. zeae isolates had high AFLP fingerprint similarity (>70%, unweighted pair group method with arithmetic means similarity) to reference isolates of G. zeae lineage 7. The genetic identity between the KS and ND populations was >99% and the estimated effective migration rate was high (Nm approximately 70). Tests for linkage disequilibrium provide little evidence for nonrandom associations between loci. Our results suggest that these populations are parts of a single, panmictic population that experiences frequent recombination. Our results also suggest that a variety of population sampling designs may be satisfactory for assessing diversity in this fungus.     

Molecular Mapping of the Stb4 Gene for Resistance to Septoria tritici Blotch in Wheat

ABSTRACT Breeding wheat for resistance is the most effective means to control Septoria tritici blotch (STB), caused by the ascomycete Mycosphaerella graminicola (anamorph Septoria tritici). At least eight genes that confer resistance to STB in wheat have been identified. Among them, the Stb4 locus from the wheat cv. Tadinia showed resistance to M. graminicola at both seedling and adult-plant stages. However, no attempt has been made to map the Stb4 locus in the wheat genome. A mapping population of 77 F10 recombinant-inbred lines (RILs) derived from a three-way cross between the resistant cv. Tadinia and the susceptible parent (Yecora Rojo x UC554) was evaluated for disease resistance and molecular mapping. The RILs were tested with Argentina isolate I 89 of M. graminicola for one greenhouse season in Brazil during 1999, with an isolate from Brazil (IPBr1) for one field season in Piracicaba (Brazil) during 2000, and with Indiana tester isolate IN95-Lafayette-1196-WW-1-4 in the greenhouse during 2000 and 2001. The ratio of resistant:susceptible RILs was 1:1 in all three tests, confirming the single-gene model for control of resistance to STB in Tadinia. However, the patterns of resistance and susceptibility were different between the Indiana isolate and those from South America. For example, the ratio of RILs resistant to both the Indiana and Argentina isolates, resistant to one but susceptible to the other, and susceptible to both isolates was approximately 1:1:1:1, indicating that Tadinia may contain at least two genes for resistance to STB. A similar pattern was observed between the Indiana and Brazil isolates. The gene identified with the Indiana tester isolate was assumed to be the same as Stb4, whereas that revealed by the South American isolates may be new. Bulked-segregant analysis was used to identify amplified fragment length polymorphism (AFLP) and microsatellite markers linked to the presumed Stb4 gene. The AFLP marker EcoRI-ACTG/MseI-CAAA5 and microsatellite Xgwm111 were closely linked to the Stb4 locus in coupling at distances of 2.1 and 0.7 centimorgans (cM), respectively. A flanking marker, AFLP EAGG/ M-CAT10, was 4 cM from Stb4. The Stb4 gene was in a potential supercluster of resistance genes near the centromere on the short arm of wheat chromosome 7D that also contained Stb5 plus five previously identified genes for resistance to Russian wheat aphid. The microsatellite marker Xgwm111 identified in this study may be useful for facilitating the transfer of Stb4 into improved cultivars of wheat.     

Airborne inoculum as a major source of Septoria tritici (Mycosphaerella graminicola) infections in winter wheat crops in the UK

The pattern and extent of primary infection by Septoria tritici were compared over a period of 3 years in winter wheat grown at sites with differing histories and from seed stocks obtained in different countries, in the open, under airtight cover and in sterilized soil. Only the airtight cover altered the number of lesions found, substantially reducing it. Lesions were evenly distributed. Lesions were found throughout the autumn and occasionally in the winter and spring on wheat seedlings exposed in trays to the open air for periods of 1 week, then given good conditions for infection to occur. This was true even at a site 0.4 km distant from wheat residues. The results show that the main source of primary infection of winter wheat in these experiments was evenly dispersed and airborne. It probably consisted mainly of ascospores of Mycosphaerella graminicola.     

Population genetic structure of Mycosphaerella graminicola and Quinone Outside Inhibitor (QoI) resistance in the Czech Republic

Damage caused by the wheat pathogen Mycosphaerella graminicola increased rapidly during the last two decades in the Czech Republic. We collected isolates from naturally infected fields in seven wheat-growing locations and analysed these using eight microsatellite markers. All markers were highly polymorphic. We found a high degree of genetic diversity and low clonality within all sampled Czech populations. We identified 158 unique multilocus haplotypes among 184 isolates. Field populations showed weak genetic structure but we detected more differentiation between climatic regions within the Czech Republic. We compared the Czech field populations to populations from the United Kingdom, Germany and Switzerland and found a marked differentiation between Czech populations and Western European populations. We hypothesize that decades of different agricultural practices, including the use of different wheat cultivars, may explain this genetic differentiation. We detected a rapid increase in QoI fungicide resistance during the sampling period from 2005 to 2011, coinciding with the widespread application of this class of fungicides in the Czech Republic. M. graminicola populations in the Czech Republic underwent a rapid adaptive evolution from sensitivity to resistance similar to what was described earlier in Western Europe.     

Frequency of sexual recombination by Mycosphaerella graminicola in mild and severe epidemics

The importance of sexual recombination in determining fungal population structure cannot be inferred solely from the relative abundance of sexual and asexual spores and reproductive structures. To complement a previously reported study of proportions of Mycosphaerella graminicola ascocarps and pycnidia, we investigated the share of sexual recombinants among isolates randomly derived from the same field at the same time. Early in three successive growing seasons (those ending in 1998, 1999, and 2000), field plots of the susceptible winter wheat cultivar Stephens were inoculated with suspensions of two M. graminicola isolates that each had rare alleles at restriction fragment length polymorphism (RFLP) loci. Near harvest time, leaves were randomly sampled from the same plots, and a population of over 100 monopycnidial isolates was created for each year of the experiment. Natural populations were also sampled from noninoculated plots in the 1999 and 2000 seasons, in order to compare allele frequencies. Based on RFLP haplotypes and DNA fingerprints, isolates from the inoculated plots were categorized by both inspection and Bayesian methods as inoculant clones, recombinants, or immigrants. Inoculation in the 2000 season was delayed, and the recovery rate of inoculant types was just 1 to 2%. In 1998, a high-disease year, and 1999, a low-disease year, inoculants comprised 36 and 22 to 23% of end-of-season samples, respectively. In those 2 years, recombinants as a percentage of inoculant descendants (both sexual and asexual) were 35 and 32%, respectively. By comparison, the study of fruiting bodies had found 93 and 32% of M. graminicola fruiting bodies were ascocarps in 1998 and 1999, respectively. These findings support the hypothesis that sexual recombination makes a relatively consistent contribution to M. graminicola population structure, despite differences in epidemic severity and ascocarp proportions.     

Effects of Wheat Cultivar Mixtures on Epidemic Progression of Septoria Tritici Blotch and Pathogenicity of Mycosphaerella graminicola

ABSTRACT The effects of host genotype mixtures on disease progression and pathogen evolution are not well understood in pathosystems that vary quantitatively for resistance and pathogenicity. We used four mixtures of moderately resistant and susceptible winter wheat cultivars naturally inoculated with Mycosphaerella graminicola to investigate impacts on disease progression in the field, and effects on pathogenicity as assayed by testing isolate populations sampled from the field on greenhousegrown seedlings. Over 3 years, there was a correspondence between the mixtures' disease response and the pathogenicity of isolates sampled from them. In 1998, with a severe epidemic, mixtures were 9.4% less diseased than were their component pure stands (P = 0.0045), and pathogen populations from mixtures caused 27% less disease (P = 0.085) in greenhouse assays than did populations from component pure stands. In 1999, the epidemic was mild, mixtures did not reduce disease severity (P = 0.39), and pathogen populations from mixtures and pure stands did not differ in pathogenicity (P = 0.42). In 2000, epidemic intensity was intermediate, mixture plots were 15.2% more diseased than the mean of component pure stands (P = 0.053), and populations from two of four mixtures were 152 and 156% more pathogenic than the mean of populations from component pure stands (P = 0.043 and 0.059, respectively). Mixture yields were on average 2.4 and 6.2% higher than mean component pure-stand yields in 1999 and 2000, respectively, but the differences were not statistically significant. The ability of mixtures challenged with M. graminicola to suppress disease appears to be inconsistent. In this system, host genotype mixtures evidently do not consistently confer either fitness benefits or liabilities on pathogen populations.     

Measuring Immigration and Sexual Reproduction in Field Populations of Mycosphaerella graminicola

ABSTRACT A field experiment was conducted to determine the relative contributions of immigration and sexual reproduction to the genetic structure of Mycosphaerella graminicola populations during the course of an epidemic. The genetic structure of M. graminicola populations sampled from wheat plots inoculated artificially with 10 isolates was compared with control plots infected naturally by airborne ascospores. Restriction fragment length polymorphisms (RFLPs) were used to test the randomness of associations among loci, and DNA fingerprints were used to identify clones. All isolates in the control plots had unique genotypes and RFLP loci were at gametic equilibrium, findings consistent with random mating. The proportion of isolates in the inoculated plots with DNA fingerprints that differed from the 10 inoculated isolates increased from 3% in the early to 39 and 34% in the mid- and late season, respectively. The degree of gametic disequilibrium was higher in the mid-season than in the late-season population. By the end of the growing season, we estimate that 66% of the isolates in the inoculated plots were asexual progeny of the 10 inoculated isolates, 10% were immigrants, and 24% were sexual recombinants. The proportion of infections caused by ascospores increased over the growing season.     

Simple Sequence Repeat Diversity of a Worldwide Collection of Puccinia triticina from Durum Wheat

ABSTRACT Isolates of Puccinia triticina collected from durum wheat from Argentina, Chile, Ethiopia, France, Mexico, Spain, and the United States were analyzed with 11 simple sequence repeat (SSR) markers in order to determine the genetic relationship among isolates. These isolates also were compared with P. triticina isolates from common wheat from North America, and an isolate collected from Aegilops speltoides from Israel, to determine genetic relationships among groups of P. triticina found on different telial hosts. The large majority of isolates from durum wheat were identical for SSR markers or had <8% genetic dissimilarity, except for isolates from Ethiopia, which had 55% dissimilarity with respect to the other durum isolates. Isolates from common wheat had >70% genetic dissimilarity from isolates from durum wheat, and the isolate from A. speltoides was >90% dissimilar from all isolates tested. Analysis of molecular variance tests showed significant levels (P = 0.001) of genetic differentiation among regions and among isolates within countries. Isolates of P. triticina from durum wheat from South America, North America, and Europe were closely related based on SSR genotypes, suggesting a recent common ancestor, whereas P. triticina from Ethiopia, common wheat, and A. speltoides each had distinct SSR genotypes, which suggested different origins.     

Identification of isolate-specific and partial resistance to septoria tritici blotch in 238 European wheat cultivars and breeding lines

From a total of 238 European cultivars and breeding lines screened for isolate-specific resistance to septoria tritici blotch (STB) with eight Mycosphaerella graminicola isolates from five different countries, 142 lines were resistant to Ethiopian isolate IPO88004, and 43 lines were specifically resistant to IPO323, with little or no leaf area bearing pycnidia of M. graminicola . These lines probably all have the resistance gene Stb6 . Specific resistances to isolates CA30JI, IPO001, IPO89011, IPO92006 and ISR398 were less common. Seventy-three per cent of the lines were specifically resistant to at least one isolate and 36 lines were resistant to more than one isolate. The line with the greatest number of specific resistances was the spring cultivar Raffles, with five. The most resistant line in which no specific resistance was identified was the Italian landrace Rieti, an ancestor of many modern European wheat cultivars. There was also a wide range of partial resistance among the lines tested, expressed in detached seedling leaves. Information about the resistance of wheat lines to M. graminicola isolates will assist breeders to choose parents of crosses from which progeny with superior resistance to STB may be selected.     

Genetic Differentiation of Puccinia triticina Populations in Central Asia and the Caucasus

ABSTRACT Isolates of Puccinia triticina collected from common wheat in the Central Asia countries of Kazakhstan, Uzbekistan, Tajikistan, and Kyrgyzstan and the Caucasus countries of Azerbaijan, Georgia, and Armenia were tested for virulence to 20 isolines of Thatcher wheat with different leaf rust resistance genes and molecular genotype at 23 simple sequence repeat (SSR) loci. After clone correction within each country, 99 isolates were analyzed for measures of population diversity, variation at single SSR loci, and for genetic differentiation of virulence phenotypes and SSR genotypes. Isolates from Central Asia and the Caucasus were also compared with 16 P. triticina isolates collected from common wheat in North America that were representative of the virulence and molecular variation in this region and two isolates collected from durum wheat in France and the United States. Populations from the Caucasus, Uzbekistan, Tajikistan, and Kyrgyzstan were not significantly (P > 0.05) differentiated for SSR variation with F(st) and R(st) statistics. Populations from the Caucasus, Uzbekistan, Tajikistan, and Kyrgyzstan were significantly (P < 0.05) differentiated from the populations in South and North Kazakhstan for SSR variation. All populations from Central Asia and the Caucasus were significantly differentiated from the North American isolates and isolates from durum wheat for SSR variation and virulence phenotypes. There was a correlation between virulence phenotype and SSR genotype among individual isolates and at the population level. Mountain barriers may account for the differentiation of P. triticina geographic populations in Central Asia and the Caucasus.     

SEED PRODUCTION OF AGROPYRON REPENS (L.) BEAUV. IN ARABLE CROPS IN ENGLAND AND WALES IN 1969

Summary. Spikes oi Agropyron repens (L.) Beauv. were collected from field crops (mostly spring barley and winter wheat) in England and Wales near the time of crop harvest. Seeds per spike were counted and their germination tested in soil in the glasshouse.
About 95% of the samples contained viable seeds; about one third of the samples had fewer than 5 viable seeds/spike, a third between 6 and 15 and a third more than 15. The average number of viable seeds/spike for all samples was 13. Spikes from spring barley (152 samples) had an average of 11 and a maximum of 51 viable seeds and those from winter wheat (42 samples), 20 and 48 respectively. Samples collected within 3 weeks after mid-July had fewer viable seeds/spike than those collected later. Samples containing morphologically-different spikes had more viable seeds/spike than apparently uniform samples, and spikes from dense field populations more seeds than those from sparse populations.     

Identification and location of Stb9, a gene for resistance to septoria tritici blotch in wheat cultivars Courtot and Tonic

This study reports the discovery of a gene for resistance to septoria tritici blotch (STB) in two spring wheat cultivars, Courtot and Tonic. The gene, named Stb9 , confers resistance to Mycosphaerella graminicola isolate IPO89011. It was mapped by quantitative trait loci (QTL) analysis using an existing map of Courtot × Chinese Spring and was located between markers Xfbb226 (3·6 cM) and XksuF1b (9 cM) on the long arm of chromosome 2B. Markers linked to Stb9 in Courtot were then shown to be linked to resistance to IPO89011 in F₃ families of Tonic × Longbow. Allelism tests in which Tonic was crossed with Courtot confirmed that Tonic has a gene for resistance to IPO89011 at or very close to the Stb9 locus. SSR markers flanking Stb9 may be used in marker-assisted selection to introgress this gene into winter cultivars or in spring wheat breeding programmes outside Europe.     

Association study of resistance to Soilborne wheat mosaic virus in U.S. winter wheat

Soilborne wheat mosaic virus (SBWMV) is one of the most important winter wheat pathogens worldwide. To identify genes for resistance to the virus in U.S. winter wheat, association study was conducted using a selected panel of 205 elite experimental lines and cultivars from U.S. hard and soft winter wheat breeding programs. Virus symptoms were evaluated twice in virus-infected fields for the panel at Manhattan, KS in spring 2010 and 2011 and for a subpanel of 137 hard winter wheat accessions at Stillwater, OK in spring 2008. At the two locations, 69.8 and 79.5% of cultivars were resistant or moderately resistant to the disease, respectively. After 282 simple-sequence repeat markers covering all wheat chromosome arms were scanned for association in the panel, marker Xgwm469 on the long arm of chromosome 5D (5DL) showed a significant association with the disease rating. Three alleles (Xgwm469-165bp, -167bp, and -169bp) were associated with resistance and the null allele was associated with susceptibility. Correlations between the marker and the disease rating were highly significant (0.80 in Manhattan at P < 0.0001 and 0.63 in Stillwater at P < 0.0001). The alleles Xgwm469-165bp and Xgwm469-169bp were present mainly in the hard winter wheat group, whereas allele Xgwm469-167bp was predominant in the soft winter wheat. The 169 bp allele can be traced back to 'Newton', and the 165 bp allele to Aegilops tauschii. In addition, a novel locus on the short arm of chromosome 4D (4DS) was also identified to associate with the disease rating. Marker Xgwm469-5DL is closely linked to SBWMV resistance and highly polymorphic across the winter wheat accessions sampled in the study and, thus, should be useful in marker-assisted selection in U.S. winter wheat.     

Sources of Verticillium dahliae Affecting Lettuce

ABSTRACT Since 1995, lettuce in coastal California, where more than half of the crop in North America is grown, has consistently suffered from severe outbreaks of Verticillium wilt. The disease is confined to this region, although the pathogen (Verticillium dahliae) and the host are present in other crop production regions in California. Migration of the pathogen with infested spinach seed was previously documented, but the geographic sources of the pathogen, as well as the impact of lettuce seed sparsely infested with V. dahliae produced outside coastal California on the pathogen population in coastal California remain unclear. Population analyses of V. dahliae were completed using 16 microsatellite markers on isolates from lettuce plants in coastal California, infested lettuce seed produced in the neighboring Santa Clara Valley of California, and spinach seed produced in four major spinach seed production regions: Chile, Denmark, the Netherlands, and the United States (Washington State). California produces 80% of spinach in the United States and all seed planted with the majority infested by V. dahliae comes from the above four sources. Three globally distributed genetic populations were identified, indicating sustained migration among these distinct geographic regions with multiple spinach crops produced each year and repeated every year in coastal California. The population structure of V. dahliae from coastal California lettuce plants was heavily influenced by migration from spinach seed imported from Denmark and Washington. Conversely, the sparsely infested lettuce seed had limited or no contribution to the Verticillium wilt epidemic in coastal California. The global trade in plant and seed material is likely contributing to sustained shifts in the population structure of V. dahliae, affecting the equilibrium of native populations, and likely affecting disease epidemiology.     

东北春麦区小麦白粉菌的侵染循环

 小麦白粉菌(Blumeria graminis DC.Speer)可以在我国东北的南部冬麦区以菌丝垫形态潜伏在小麦苗基部叶片和叶鞘上越冬,以混在干燥保存的小麦种子中的闭囊壳越夏。自生麦苗和白粉病的野生寄主在侵染循环中不起作用。小麦白粉菌的毒力频率(Vf)分析表明,公主岭、沈阳、海城、大连和烟台的小麦白粉菌同属于一个群体。东北春麦区小麦白粉病初发日期和高空天气图分析表明,东北春麦区小麦白粉病的初侵染菌源来自胶东半岛冬麦区。在一定的天气系统控制下,小麦白粉菌的分生孢子随夏初的偏南气流,在700毫巴高度层向北传播,进入东北春麦区。随降雨沉降,并侵染小麦植株。