Genetic differentiation at microsatellite loci among populations of Mycosphaerella graminicola from California, Indiana, Kansas, and North Dakota

Mycosphaerella graminicola causes Septoria tritici blotch (STB) in wheat (Triticum aestivum) and is considered one of the most devastating pathogens of that crop in the United States. Although the genetic structures of M. graminicola populations from different countries have been analyzed using various molecular markers, relatively little is known about M. graminicola populations from geographically distinct areas of the United States and, in particular, of those from spring versus winter wheat. These are exposed to great differences in environmental conditions, length and season of host-free periods, and resistance sources used in geographically separated wheat breeding programs. Thus, there is more likely to be genetic differentiation between populations from spring versus winter wheat than there is among those within each region. To test this hypothesis, 330 single-spore isolates of M. graminicola representing 11 populations (1 from facultative winter wheat in California, 2 from spring wheat in North Dakota, and 8 from winter wheat in Indiana and Kansas) were analyzed for mating type frequency and for genetic variation at 17 microsatellite or simple-sequence repeat (SSR) loci. Analysis of clone-corrected data revealed an equal distribution of both mating types in the populations from Kansas, Indiana, and North Dakota, but a deviation from a 1:1 ratio in the California population. In total, 306 haplotypes were detected, almost all of which were unique in all 11 populations. High levels of gene diversity (H = 0.31 to 0.56) were observed within the 11 populations. Significant (P ≤ 0.05) gametic disequilibrium, as measured by the index of association (rBarD), was observed in California, one Indiana population (IN1), and three populations (KS1, KS2, and KS3) in Kansas that could not be explained by linkage. Corrected standardized fixation index (G″(ST)) values were 0.000 to 0.621 between the 11 populations and the majority of pairwise comparisons were statistically significant (P ≤ 0.001), suggesting some differentiation between populations. Analysis of molecular variance showed that there was a small but statistically significant level of genetic differentiation between populations from spring versus winter wheat. However, most of the total genetic variation (>98%) occurred within spring and winter wheat regions while <2% was due to genetic differentiation between these regions. Taken together, these results provide evidence that sexual recombination occurs frequently in the M. graminicola populations sampled and that most populations are genetically differentiated over the major spring- and winter-wheat-growing regions of the United States.     

Association mapping of quantitative resistance to Phaeosphaeria nodorum in spring wheat landraces from the USDA National Small Grains Collection

Stagonospora nodorum blotch (SNB), caused by Phaeosphaeria nodorum, is a destructive disease of wheat (Triticum aestivum) found throughout the United States. Host resistance is the only economically feasible option for managing the disease; however, few SNB-resistant wheat cultivars are known to exist. In this study, we report findings from an association mapping (AM) of resistance to P. nodorum in 567 spring wheat landraces of diverse geographic origin. The accessions were evaluated for seedling resistance to P. nodorum in a greenhouse. Phenotypic data and 625 polymorphic diversity array technology (DArT) markers have been used for linkage disequilibrium (LD) and association analyses. The results showed that seven DArT markers on five chromosomes (2D, 3B, 5B, 6A, and 7A) were significantly associated with resistance to P. nodorum. Genetic regions on 2D, 3B, and 5B correspond to previously mapped quantitative trait loci (QTL) conferring resistance to P. nodorum whereas the remaining QTL appeared to be novel. These results demonstrate that the use of AM is an effective method for identifying new genomic regions associated with resistance to P. nodorum in spring wheat landraces. Additionally, the novel resistance found in this study could be useful in wheat breeding aimed at controlling SNB.     

Novel necrotrophic effectors from Stagonospora nodorum and corresponding host sensitivities in winter wheat germplasm in the southeastern United States

Stagonospora nodorum blotch (SNB), caused by the necrotrophic fungus Stagonospora nodorum (teleomorph: Phaeosphaeria nodorum), is among the most common diseases of winter wheat in the United States. New opportunities in resistance breeding have arisen from the recent discovery of several necrotrophic effectors (NEs, also known as host-selective toxins) produced by S. nodorum, along with their corresponding host sensitivity (Snn) genes. Thirty-nine isolates of S. nodorum collected from wheat debris or grain from seven states in the southeastern United States were used to investigate the production of NEs in the region. Twenty-nine cultivars with varying levels of resistance to SNB, representing 10 eastern-U.S. breeding programs, were infiltrated with culture filtrates from the S. nodorum isolates in a randomized complete block design. Three single-NE Pichia pastoris controls, two S. nodorum isolate controls, and six Snn-differential wheat controls were also used. Cultivar-isolate interactions were visually evaluated for sensitivity at 7 days after infiltration. Production of NEs was detected in isolates originating in each sampled state except Maryland. Of the 39 isolates, 17 produced NEs different from those previously characterized in the upper Great Plains region. These novel NEs likely correspond to unidentified Snn genes in Southeastern wheat cultivars, because NEs are thought to arise under selection pressure from genes for resistance to biotrophic pathogens of wheat cultivars that differ by geographic region. Only 3, 0, and 23% of the 39 isolates produced SnToxA, SnTox1, and SnTox3, respectively, by the culture-filtrate test. A Southern dot-blot test showed that 15, 74, and 39% of the isolates carried the genes for those NEs, respectively; those percentages were lower than those found previously in larger international samples. Only two cultivars appeared to contain known Snn genes, although half of the cultivars displayed sensitivity to culture filtrates containing unknown NEs. Effector sensitivity was more frequent in SNB-susceptible cultivars than in moderately resistant (MR) cultivars (P = 0.008), although some susceptible cultivars did not exhibit sensitivity to NEs produced by isolates in this study and some MR cultivars were sensitive to NEs of multiple isolates. Our results suggest that NE sensitivities influence but may not be the only determinant of cultivar resistance to S. nodorum. Specific knowledge of NE and Snn gene frequencies in this region can be used by wheat breeding programs to improve SNB resistance.     

Meta-analysis of regression coefficients for the relationship between fusarium head blight and deoxynivalenol content of wheat

ABSTRACT A total of 126 field studies reporting deoxynivalenol (DON; ppm) content of harvested wheat grain and Fusarium head blight index (IND; field or plot-level disease severity) were analyzed to determine the overall mean regression slope and intercept for the relationship between DON and IND, and the influence of study-specific variables on the slope and intercept. A separate linear regression analysis was performed to determine the slope and intercept for each study followed by a meta-analysis of the regression coefficients from all studies. Between-study variances were significantly (P < 0.05) greater than 0, indicating substantial variation in the relationship between the variables. Regression slopes and intercepts were between -0.27 and 1.48 ppm per unit IND and -10.55 to 32.75 ppm, respectively. The overall mean regression slope and intercept, 0.22 ppm per unit IND and 2.94 ppm, respectively, were significantly different from zero (P < 0.001), and the width of the 95% confidence interval was 0.07 ppm per unit IND for slope and 1.44 ppm for intercept. Both slope and intercept were significantly affected by wheat type (P < 0.05); the overall mean intercept was significantly higher in studies conducted using winter wheat cultivars than in studies conducted using spring wheat cultivars, whereas the overall mean slope was significantly higher in studies conducted using spring wheat cultivars than in winter wheat cultivars. Study location had a significant effect on the intercept (P < 0.05), with studies from U.S. winter wheat-growing region having the highest overall mean intercept followed by studies from Canadian wheat-growing regions and U.S. spring wheat-growing regions. The study-wide magnitude of DON and IND had significant effects on one or both of the regression coefficients, resulting in considerable reduction in between-study variances. This indicates that, at least indirectly, environment affected the relationship between DON and IND.     

Relative Contribution of Seed-Transmitted Inoculum to Foliar Populations of Phaeosphaeria nodorum

ABSTRACT A marked-isolate, release-recapture experiment was conducted to assess the relative contributions of seed-transmitted (released isolates) versus all other inocula to foliar and grain populations of Phaeosphaeria nodorum in winter wheat rotated with nonsusceptible crops in New York and Georgia, United States. Seed infected with two distinct groups of marked isolates of P. nodorum containing rare alleles (identified by amplified fragment length polymorphisms [AFLPs]) and balanced for mating type were planted in experimental field plots in two locations in each state. Recapture was done by isolating P. nodorum from leaves showing necrotic lesions at spring tillering and flowering stages, and mature grains from spikes showing glume blotch. Isolates from these samples were genotyped by AFLPs and categorized as released or nonreleased to infer sources of inoculum. Both infected seed and other sources of the pathogen contributed significant primary inocula to populations recovered from leaves and harvested grain. Seed-transmitted genotypes accounted for a total of 57% of all isolates recovered from inoculated plots, with a range of 15 to 90% of the populations of P. nodorum collected over the season in individual, inoculated plots at the four locations. Plants in the noninoculated control plots also became diseased and 95% or more of the isolates recovered from these plots were nonreleased genotypes. Although other potential sources of P. nodorum within and adjacent to experimental plots were not ruled out, nonreleased genotypes likely were derived from immigrant ascospores potentially from sources at a considerable distance from the plots. Our results suggest that, although reduction of seedborne inoculum of P. nodorum may delay foliar epidemics, this strategy by itself is unlikely to result in high levels of control in eastern North America because of the additional contribution from alternative sources of inoculum.     

中国三麦区小麦抗叶锈基因部署研究

 利用已知抗叶锈基因的小麦近等基因系(或单基因系),推导了来自中国3大麦区(东北春麦区、华北平原冬麦区、江淮半冬麦区)的主要小麦品种,待推广品种(系)和抗源材料中可能含有的抗叶锈基因,分析了3大麦区小麦叶锈菌群体的毒性基因。研究结果表明,东北春麦区的小麦品种可能含有的抗叶锈基因主要有Lr16、Lr20、Lr17等11个;华北平原冬麦区的小麦品种,可能含有的抗叶锈基因主要是Lr26等7个,江淮麦区的小麦品种可能含有的抗叶锈基因主要是Lr3Bg、Lr10、Lr26等9个。对3大麦区小麦叶锈菌群体的毒性基因分析表明,V1、V2d、V3、V10、V26、V33等13个基因是3大麦区的优势毒性基因,但个别毒性基因在3大麦区的出现频率仍有较大差别。根据基因对基因概念,在基因部署上建议,东北麦区减少含单个无效抗病基因的品种如90-05744、91-1179、东农88-5797等的种植面积,适当控制含Lr20、17、15、30、3Ka的品种,增加沈免系统及小冰麦系统的种植面积;华北麦区增加含Lr14a、14ab、15的品种的种植面积,如CA9069、CA8646、CA9070、C₄⁸⁹和冀87-5108等,减少含单个Lr26的品种的种植面积,江淮麦区适当减少肖农12(Lr3Bg)、偃师9号(Lr26)、8870(Lr26)等含单个无效抗病基因的品种,适当增加含Lr2b、3Ka、30的品种如内乡5号、宁8931等的种植面积。本文初步讨论了不同生态区小麦叶锈菌的基因鉴定系统。     

Frequency of Phaeosphaeria nodorum, the Sexual Stage of Stagonospora nodorum, on Winter Wheat in North Carolina

ABSTRACT Ascocarps of Phaeosphaeria nodorum, which causes Stagonospora nodorum blotch (SNB) of wheat, have not been found by others in the eastern United States despite extensive searches. We sampled tissues from living wheat plants or wheat debris in Kinston, NC, each month except June from May to October 2003. Additional wheat samples were gathered in Kinston, Salisbury, and Plymouth, NC, in 2004 and 2005. For the 3 years, in all, 2,781 fruiting bodies were dissected from the wheat tissues and examined microscopically. Fruiting bodies were tallied as P. nodorum pycnidia or ascocarps, "unknown" (not containing spores, potentially P. nodorum or other fungi), or "other fungi." In the 2003 sample, asco-carps of P. nodorum were present each month after May at a frequency of 0.8 to 5.4%, and comprised a significantly higher percentage of fruiting bodies from wheat spikes than of those from lower stems and leaves. Ascocarps also were found at frequencies <10% in some wheat debris samples from 2004 and 2005. Analysis of the nucleotide sequences of internally transcribed spacer regions of 18 genetically distinct North Carolina isolates from 2003 suggested that all were P. nodorum, not the morphologically similar P. avenaria f. sp. triticea. Neither the 18 isolates from 2003 nor a set of 77 isolates derived from 2004 Kinston leaf samples gave reason to suspect a mating-type imbalance in the larger P. nodorum population (P >/= 0.4). We conclude that, in North Carolina, sexual reproduction plays a role in initiation of SNB epidemics and the creation of adaptively useful genetic variability, although its relative importance in structuring this population is uncertain.     

Distribution and Pathogenic Characterization of Pyrenophora tritici-repentis and Stagonospora nodorum in Ohio

ABSTRACT To determine the distribution of Stagonospora nodorum and Pyrenophora tritici-repentis on wheat in Ohio, flag leaves with lesions were collected from wheat-producing counties in 2002 and 2003. Counties were arbitrarily grouped into seven regions. Log-linear analysis of pathogen presence within regions indicated that the presence of S. nodorum was independent of the presence of P. tritici-repentis. A logistic analysis revealed that the occurrence of both pathogens varied by region in one or both years. The aggressiveness of S. nodorum isolates was determined by inoculating two susceptible genotypes with a subsample of isolates from each region from both years. S. nodorum isolates obtained from northeast Ohio, with fewer wheat fields, were less aggressive than those from other regions. Isolates obtained from west-central Ohio, surrounded by regions with high wheat production annually, were significantly more aggressive than those obtained in the remaining five regions. Isolates from the five other regions did not differ significantly (P > 0.05) in aggressiveness. Races 1 and 2, and a few race 3 isolates, of P. tritici-repentis were detected in Ohio. The distribution of P. tritici-repentis races 1 and 2 was not associated with any region, although the prevalence of race 1 was three times greater than race 2. The rarer race 3 was associated with three dispersed regions. Results indicate that S. nodorum was the major wheat leaf-blotching pathogen. There were no positive or negative associations of S. nodorum and P. tritici-repentis or individual races of P. tritici-repentis in any of the tested regions, which indicates that neither pathogen can be used to predict the presence of the other. The isolated northeastern corner of Ohio appeared to contain isolates of S. nodorum with unique characteristics and potentially only one race of P. tritici-repentis, indicating that this area may be genetically isolated from the remaining tested areas of the state.     

中国主要小麦种植区雨养条件下水分胁迫发生规律模拟

运用EPIC(Environmental Policy Integrated Climate)作物生长模型模拟了1961—2011年中国各小麦种植区小麦生长过程,对中国各小麦种植区小麦生育期内水分胁迫规律进行了分析。结果表明:雨养条件下,全国11个小麦种植区的水分胁迫程度可分为3个等级;冬麦区(新疆冬麦区除外)和东北春麦区水分胁迫均值、发生频率均明显低于其他春麦区;南方的3个冬麦区(华南、西南、长江中下游),水分胁迫的程度、发生频率均低于其他麦区;黄淮、北部冬麦区水分胁迫规律相似,在1、2月和5、6月较高,而北部冬麦区水分胁迫程度和发生频率略高于黄淮冬麦区;北部、西北、北疆春麦区、新疆冬麦区在雨养条件下水分胁迫最为严重,如果没有灌溉,小麦几乎绝收,尤其是新疆冬麦区。     

Virulence associations in oat crown rust

ABSTRACT Isolates of Puccinia coronata obtained from natural populations of Avena sterilis in Israel, winter oat (A. sativa) cultivars in Texas, and spring oat cultivars in the Northern Plains states of Minnesota, North Dakota, and South Dakota were analyzed for significance of pairwise virulence associations. Isolates from all three regions were tested on 25 oat lines with single P. coronata (Pc) genes for crown rust resistance from A. sterilis and one line with a Pc gene from A. sativa. Isolates from Israel were tested also on 11 Iowa backcross lines with undesignated crown rust resistance genes from A. sterilis. Four associated virulence groups were identified from significant positive virulence associations that were consistent across all three regions. Group 38 included virulence to Pc-38, Pc-39, Pc-55, Pc-63, and Pc-71; group 45 included virulence to Pc-45, Pc-46, Pc-48, Pc-52, Pc-54, and Pc-57; group 58 included virulence to Pc-35, Pc-40, Pc-58, and Pc-59; and group 61 included virulence to Pc- 36, Pc-51, Pc-56, Pc-60, and Pc-61. Virulence to Pc-70 showed the strongest association to virulences in group 38 but also showed significant association with virulence to Pc-45, Pc-35, and Pc-58. Virulences in group 61 were consistently negatively associated with virulences in group 38 in each region. In Israel, virulences to five of the Iowa lines showed positive associations to virulences in group 61 and negative associations to virulences in groups 38 and 45. Close similarity of reactions of nearly all isolates to Pc-39, Pc-55, and Pc-71 suggest that these genes may be identical or nearly identical alleles.     

Pathogenic and genetic diversity of Xanthomonas translucens pv. undulosa in North Dakota

Bacterial leaf streak (BLS), caused by Xanthomonas translucens pv. undulosa, has become more prevalent recently in North Dakota and neighboring states. From five locations in North Dakota, 226 strains of X. translucens pv. undulosa were collected and evaluated for pathogenicity and then selected strains were inoculated on a set of 12 wheat cultivars and other cereal hosts. The genetic diversity of all strains was determined using repetitive sequence-based polymerase chain reaction (rep-PCR) and insertion sequence-based (IS)-PCR. Bacterial strains were pathogenic on wheat and barley but symptom severity was greatest on wheat. Strains varied greatly in aggressiveness, and wheat cultivars also showed differential responses to several strains. The 16S ribosomal DNA sequences of the strains were identical, and distinct from those of the other Xanthomonas pathovars. Combined rep-PCR and IS-PCR data produced 213 haplotypes. Similar haplotypes were detected in more than one location. Although diversity was greatest (≈92%) among individuals within a location, statistically significant (P ≤ 0.001 or 0.05) genetic differentiation among locations was estimated, indicating geographic differentiation between pathogen populations. The results of this study provide information on the pathogen diversity in North Dakota, which will be useful to better identify and characterize resistant germplasm.     

Population Subdivision of Fusarium graminearum Sensu Stricto in the Upper Midwestern United States

ABSTRACT A collection of 712 Fusarium graminearum sensu stricto (s.s.) strains, predominantly gathered between 1999 and 2000 from nine states within the United States, was examined for population structure and polymerase chain reaction-based trichothecene type. Most strains belonged to a cohesive genetic population characterized by a 15-acetyldeoxynivalenol (15ADON) trichothecene type. However, using a Bayesian model-based clustering method, we also identified genetically divergent groups of strains in some sampled locations of Minnesota and North Dakota. Strains of the major group of divergent populations were of a 3ADON trichothecene type and formed a distinct cluster with a collection of previously gathered strains from Italy, which displayed all three trichothecene types (15ADON, 3ADON, and nivalenol). The co-existence of genetically divergent populations of F. graminearum s.s. in the Upper Midwest allows for the rejection of the hypothesis that F. graminearum s.s. in the United States consists of a single population. These results also suggest that recombination has been insufficiently frequent in this homothallic (selfing) fungal species to homogenize the divergent populations observed in the Upper Midwest.     

Population Genetic Structure of Septoria passerinii in Northern Great Plains Barley

ABSTRACT The genetic structure of Septoria passerinii from nine field populations was examined at several scales (within lesions, among lesions in a leaf, among leaves in a field, and among fields in North Dakota and western Minnesota) by using amplified fragment length polymorphism (AFLP) markers. A total of 390 isolates were sampled from seven barley fields located in North Dakota and two barley fields located nearby in western Minnesota in 2003 and 2004. Based on 57 polymorphic AFLP markers, AFLP DNA fingerprints identified 176 different genotypes among 390 (non-clone-corrected) isolates in nine different fields. In two intensively sampled sites, ND16 (Williston, ND) and ND17 (Langdon, ND), only one to four different genotypes were found within a lesion. A higher level of genetic and genotypic diversity was found within a leaf in which six to nine different genotypes were found from lesions on a leaf. The genetic diversity within a leaf was similar to the genetic diversity within a field. The average genetic diversity (H) within a field across all AFLP loci was approximately 0.3, except at site ND12 (Carrington, ND) where it was 0.16. Genotypic diversity was high in all populations, and with the exception of ND15 (Rothsay, MN), very low multilocus linkage disequilibrium values ( r(d)) were found in all populations. The population differentiation, G(ST), was relatively high (G(ST) = 0.238) among the nine populations due to the high G(ST) in ND12, ND14 (Twin Valley, MN), and ND15. Population differentiation without those three populations was 0.09. A lack of correlation between geographical distance and genetic distance was found, suggesting the potential for a high level of gene flow between different geographical regions. The population genetic structure described in this study for S. passerinii in North Dakota and western Minnesota is consistent with that of a sexually reproducing fungus.     

Reaction of selected winter wheat cultivars from Europe and United States to Karnal bunt

A study was conducted to determine the susceptibility of popular United States (U.S.) and European winter wheat cultivars to the fungal pathogen Tilletia indica. Historically, the disease has been limited to autumn-sown spring-habit wheat areas and not associated with winter wheat. In 1997, Karnal bunt was observed on winter wheat in limited regions of Texas. This region marks the southern end of the contiguous U.S. central winter wheat belt, which extends north into Canada. The aim of this study was to assess the levels of disease resistance in winter wheat. Fifty U.S. and European winter wheat cultivars were tested using two different greenhouse inoculation procedures. For each cultivar, 12 spikes in boot were inoculated by boot-injection with a sporidial suspension (1.0 ml/boot, 10,000 spores ml⁻¹), and 12 other emerged spikes were spray-inoculated with the same concentration. The experiment was repeated for three seasons. Among cultivars, mean seed infection ranged from 2.1 to 87.2% and 0 to 15.6% for boot-injected and spray-inoculated treatments, respectively. Results showed that the majority of winter wheat cultivars tested were susceptible to Karnal bunt.     

Relationship between visual estimates of fusarium head blight intensity and deoxynivalenol accumulation in harvested wheat grain: a meta-analysis

ABSTRACT The association between Fusarium head blight (FHB) intensity and deoxynivalenol (DON) accumulation in harvested grain is not fully understood. A quantitative review of research findings was performed to determine if there was a consistent and significant relationship between measures of Fusarium head blight intensity and DON in harvested wheat grain. Results from published and unpublished studies reporting correlations between DON and Fusarium head blight "index" (IND; field or plot-level disease severity), incidence (INC), diseased-head severity (DHS), and Fusarium-damaged kernels (FDK) were analyzed using meta-analysis to determine the overall magnitude, significance, and precision of these associations. A total of 163 studies was analyzed, with estimated correlation coefficients (r) between -0.58 and 0.99. More than 65% of all r values were >0.50, whereas less that 7% were <0. The overall mean correlation coefficients for all relationships between DON and disease intensity were significantly different from zero (P < 0.001). Based on the analysis of Fisher-transformed r values ( z(r) values), FDK had the strongest relationship with DON, with a mean r of 0.73, followed by IND (r = 0.62), DHS (r = 0.53), and INC (r = 0.52). The mean difference between pairs of transformed z(r) values (z(d) ) was significantly different from zero for all pairwise comparisons, except the comparison between INC and DHS. Transformed correlations were significantly affected by wheat type (spring versus winter wheat), study type (fungicide versus genotype trials), and study location (U.S. spring- and winter-wheat-growing regions, and other wheat-growing regions). The strongest correlations were observed in studies with spring wheat cultivars, in fungicide trials, and in studies conducted in U.S. spring-wheat-growing regions. There were minor effects of magnitude of disease intensity (and indirectly, environment) on the transformed correlations.     

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.     

Effects of treatments to perennial ryegrass on the development of Septoria spp. in a subsequent crop of winter wheat

An experiment started in spring 1979 tested the effects of different treatments to perennial ryegrass, established as pure stands or undersown in spring wheat, on the severity of septoria diseases on the grass and in a following crop of winter wheat, sown in autumn 1980. Other plots were fallowed in 1979 and 1980 before sowing to winter wheat, also in autumn 1980.
Septoria tritici was most severe in winter wheat after fallow and least severe in wheat that had been direct-drilled after ryegrass. These effects were attributed to differences in amounts of available nitrogen, and consequent differences in crop growth, rather than to any differences in primary inoculum.
Symptoms attributed to S. nodorum on the ryegrass were more common where the grass had been established under spring wheat than where it had been sown as a pure stand. They also tended to be more common where ryegrass had been inoculated with spores of S. nodorum than where it had been sprayed with captafol. Similar effects on symptoms attributed to Septoria spp. (mostly S. tritici ) on the wheat were apparent in July. The results support the conclusion that the greater severity of septoria diseases that often occurs on wheat after grass than after non-graminaceous breaks, is probably due in part to survival of the pathogens on grass and, in some circumstances, on debris remaining from a previous wheat crop. However, other factors are also likely to be involved.     

Trichothecene profiling and population genetic analysis of Gibberella zeae from barley in North Dakota and Minnesota

Gibberella zeae, the principal cause of Fusarium head blight (FHB) of barley, contaminates grains with several mycotoxins, which creates a serious problem for the malting barley industry in the United States, China, and Europe. However, limited studies have been conducted on the trichothecene profiles and population genetic structure of G. zeae isolates collected from barley in the United States. Trichothecene biosynthesis gene (TRI)-based polymerase chain reaction (PCR) assays and 10 variable number tandem repeat (VNTR) markers were used to determine the genetic diversity and compare the trichothecene profiles of an older population (n = 115 isolates) of G. zeae collected in 1997 to 2000 with a newer population (n = 147 isolates) collected in 2008. Samples were from across the major barley-growing regions in North Dakota and Minnesota. The results of TRI-based PCR assays were further validated using a subset of 32 and 28 isolates of G. zeae by sequence analysis and gas chromatography, respectively. TRI-based PCR assays revealed that all the G. zeae isolates in both populations had markers for deoxynivalenol (DON), and the frequencies of isolates with a 3-acetyldeoxynivalenol (3-ADON) marker in the newer population were ≈11-fold higher than those among isolates in the older population. G. zeae populations from barley in the Midwest of the United States showed no spatial structure, and all the isolates were solidly in clade 7 of G. zeae, which is quite different from other barley-growing areas of world, where multiple species of G. zeae are commonly found in close proximity and display spatial structure. VNTR analysis showed high gene diversity (H = 0.82 to 0.83) and genotypic diversity but low linkage disequilibrium (LD = 0.02 to 0.07) in both populations. Low genetic differentiation (F(ST) = 0.013) and high gene flow (Nm = 36.84) was observed between the two populations and among subpopulations within the same population (Nm = 12.77 to 29.97), suggesting that temporal and spatial variations had little influence on population differentiation in the Upper Midwest. Similarly, low F(ST) (0.02) was observed between 3-ADON and 15-acetyldeoxynivalenol populations, indicating minor influence of the chemotype of G. zeae isolates on population subdivision, although there was a rapid increase in the frequencies of isolates with the 3-ADON marker in the Upper Midwest between the older collection made in 1997 to 2000 and the newer collection made in 2008. This study provides information to barley-breeding programs for their selection of isolates of G. zeae for evaluating barley genotypes for resistance to FHB and DON accumulation.     

High Genetic Similarity Among Populations of Phaeosphaeria nodorum Across Wheat Cultivars and Regions in Switzerland

ABSTRACT Phaeosphaeria nodorum was sampled from nine wheat fields across a 30-km transect representing three geographical regions in Switzerland to determine the scale of genetic differentiation among subpopulations. Three different wheat cultivars were sampled three times to determine whether differences in host genotype correlated with differences among corresponding pathogen populations. Seven restriction fragment length polymorphism (RFLP) loci and one DNA fingerprint were assayed for each of the 432 isolates in the collection. DNA fingerprints differentiated 426 unique genotypes. Though absolute differences were small, five RFLP loci exhibited significant differences in allele frequencies across the nine sub-populations. Gene diversity within all subpopulations was high (H(T) = 0.51), but only 3% of the total genetic variation was distributed among the nine subpopulations. When subpopulations were grouped according to geographical region or host cultivar, less than 1% of the genetic variation was distributed among groups, suggesting widespread gene flow and the absence of pathogen adaptation to specific wheat cultivars. Tests for gametic equilibrium within subpopulations and across the entire Swiss population supported the hypothesis of random mating.     

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.