Fusarium pseudograminearum as an emerging pathogen of crown rot of wheat in eastern China

Fusarium crown rot of wheat has been spreading in the Huanghuai wheat-growing area in China since 2010, leading to a potential yield loss. To investigate the pathogens associated with this disease in Jiangsu and Shandong provinces in recent years, 617 Fusarium isolates were isolated from nine sites in these two provinces between 2014 and 2016. Of these isolates, 372 were identified as Fusarium pseudograminearum, and the remaining isolates were identified as F. asiaticum and F. graminearum, suggesting that F. pseudograminearum is becoming a predominant causative pathogen of crown rot of wheat in eastern China. Trichothecene gene detection and chemical analyses of trichothecenes indicated that the F. pseudograminearum isolates belonged to the 3-ADON or 15-ADON chemotype, and one isolate had the NIV genotype but produced no detectable NIV. 3-ADON isolates were predominant in Jiangsu, whereas 15-ADON isolates were prevalent in Shandong. The mating type of the F. pseudograminearum isolates were identified. MAT-1 and MAT-2 existed, but in most collections, particularly those in Jiangsu, the ratios of the two mating types deviated significantly from an expected 1:1 ratio. The reason for the occurrence of F. pseudograminearum is hypothesized, and the chemotype and mating type distribution of this species in these two provinces are analysed.     

A comparison of aggressiveness and deoxynivalenol production between Canadian <Emphasis Type="Italic">Fusarium graminearum</Emphasis> isolates with 3-acetyl and 15-acetyldeoxynivalenol chemotypes in field-grown spring wheat

Twenty four isolates of Fusarium graminearum, half of which were 3-acetyldeoxynivalenol (3-ADON) and half 15-acetyldeoxynivalenol (15-ADON) chemotypes, were tested for their ability to produce deoxynivalenol and to cause Fusarium head blight (FHB) in spring wheat cultivars. The objectives of this study were to determine (1) whether 3-ADON isolates differ in aggressiveness, as measured by the FHB index, and DON production from 15-ADON isolates under field conditions, and (2) whether the performance of resistant host cultivars was stable across isolates. Field tests of all isolates were conducted with three replicates at each of two locations in Canada and Germany in 2008 with three host genotypes differing in FHB resistance level. The resistant host genotype showed resistance regardless of the chemotype or location. The differences between mean FHB indices of 3-ADON and 15-ADON isolates were not significant for any wheat genotype. In contrast, average DON production by the 3-ADON isolates (10.44 mg kg⁻¹) was significantly (P < 0.05) higher than for the 15-ADON isolates (6.95 mg kg⁻¹) at three of the four locations where moderately resistant lines were tested, and at both locations where susceptible lines were evaluated. These results indicate that 3-ADON isolates could pose a greater risk to food safety. However, as the mean aggressiveness and DON production of 3-ADON and 15-ADON chemotypes was similar on highly resistant lines, breeding and use of highly resistant lines is still the most effective measure of reducing the risks associated with DON in wheat.     

Genotypic diversity in Fusarium pseudograminearum populations in Australian wheat fields

Despite being closely related to Fusarium graminearum, which has been extensively characterized in many countries around the world, the population biology of Fusarium pseudograminearum, the main causal agent of crown rot of wheat in Australia and many other wheat‐growing regions, has been comparatively poorly studied to date. A simple sequence repeat analysis of 163 F. pseudograminearum isolates from three field sites in NSW, Australia identified 128 distinct multilocus genotypes. Observed genetic diversity within fields was high, whilst genetic variation between fields was low. Across all fields genetic linkage disequilibrium was detected, but of the three individual fields, only one also displayed linkage disequilibrium. These results indicate that the isolates obtained were part of the same, highly diverse, population. However, this population may not be freely interbreeding. Whilst isolation incidence of F. pseudograminearum was found to be spatially aggregated within fields, spatial aggregation of genotypes within fields was weak. The study suggests that processes influencing population dynamics may operate at a scale larger than the narrow geographical scale covered in the fields sampled.     

Comparison of pathogenicity of the Fusarium crown rot (FCR) complex (<Emphasis Type="Italic">F. culmorum</Emphasis>, <Emphasis Type="Italic">F. pseudograminearum</Emphasis> and <Emphasis Type="Italic">F. graminearum)</Emphasis> on hard red spring and durum wheat

Fusarium species involved in the Fusarium crown rot (FCR) complex affect wheat in every stage of development from seedling to grain fill. This study was designed to compare the aggressiveness of the FCR complex members including F. culmorum, F. pseudograminearum and F. graminearum in causing seedling blight, decreased plant vigour and crown rot. To assess their relative pathogenicity, two hard red spring wheat cultivars and two durum wheat cultivars were inoculated in the field with five isolates from each of the three species for two years. Significant differences in patterns of pathogenicity were identified. In particular, F. culmorum caused greater seedling blight while F. pseudograminearum and F. graminearum caused greater crown rot. Greatest yield reductions were caused by F. pseudograminearum. Cultivar differences were identified with respect to seedling disease and late season crown rot. No interactions were identified between cultivar performance and isolates or species with which they were challenged.     

Mycotoxin Production and Molecular Variability of European and American Isolates of Fusarium Culmorum

The main causative agents of Fusarium head blight are Fusarium graminearum and Fusarium culmorum. We examined the mycotoxin-producing abilities and molecular variability of 37 Fusarium culmorum isolates collected from the Pan-Northern Hemisphere, together with isolates representing related species. Mycotoxin-producing abilities of the isolates were tested by thin layer chromatography and by PCR using primer pairs specific for the Tri7 and Tri13 genes. Thirty isolates belonged to chemotype I (producing deoxynivalenol and 3-acetyl-deoxynivalenol), while seven represented chemotype II (producing nivalenol and/or fusarenone X). The presence of a functional Tri7 gene correlated well with nivalenol production. Isolates belonging to chemotype I were in general more pathogenic in in vitro tests than those belonging to chemotype II. Phylogenetic analysis of the random amplified polymorphic DNA profiles (RAPD) of the isolates enabled the isolates to be clustered into different groups. Most isolates from Hungary exhibited identical RAPD profiles. A similar clustering was found on the tree based on restriction analysis of the intergenic spacer region data. Sequence analysis of a putative reductase gene fragment of the isolates was also carried out. A correlation was detected between the geographic origin of the isolates and their position on the cladogram produced based on sequence data. The presence of mating type gene homologues was also tested with primer pairs specific for MAT1-1 and MAT1-2. The isolates carried either MAT1-1 or MAT1-2 homologues. No correlation was observed between clustering of the isolates based on RAPD, restriction analysis of the intergenic spacer region or sequence data and the distribution of MAT idiomorphs. Similarly, no correlation was detected between mycotoxin-producing abilities or aggressiveness and molecular characteristics of the isolates. Statistical analysis of RAPD data and lack of strict correlation between trees based on different data sets supported the view that Fusarium culmorum has a recombining population structure. The presence of mating type gene homologues in the isolates indicates that the recombining population structure is caused by ongoing or past meiotic exchanges.     

Diversity of the Fusarium graminearum species complex on French cereals

Fusarium graminearum is an important pathogen causing Fusarium head blight (FHB) on wheat and barley and Fusarium ear rot (FER) on maize, and harvested grains often are contaminated with trichothecenes such as deoxynivalenol (DON) and nivalenol (NIV) that are a major health and food safety concern due to their toxicity to humans and farm animals. In this study, species identity and trichothecene toxin potential of 294 members of the Fusarium graminearum species complex (FGSC) collected from wheat, barley and maize in France in 2011 was determined using a microsphere-based multilocus genotyping assay. F. graminearum was predominant on all three hosts, but three isolates of F. cortaderiae and two isolates representing F. graminearum × F. boothii hybrids were also identified from maize. The 15-ADON trichothecene chemotype predominated on all three hosts, representing 94.7 %, 87.8 % and 85.4 % of the strains on barley (N?=?19), wheat (N?=?90), and maize (N?=?185), respectively. However, the NIV chemotype was found in 12.2 % of the wheat isolates and in 14.6 % of the maize isolates. Only a single FGSC isolate from this study, originating from barley, was found to have the 3-ADON chemotype. Regional differences could be observed in the distribution of the 15-ADON and NIV chemotypes, with the NIV producing-isolates being present at higher frequency (21.2 %) in the South of France compared to the rest of the country (4.4 %). Such information is critical because of the increased concern associated with NIV contamination of cereals. In addition, these results are needed to develop management strategies for FHB and FER in France and to improve understanding of the distribution and significance of FGSC diversity in Europe and worldwide.     

Genetic diversity and pathogenicity of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> isolated from wilted Welsh onion in Japan

Thirty isolates of Fusarium oxysporum from wilted Welsh onion plants were examined for their diversity in nucleotide sequences of the ribosomal DNA (rDNA) intergenic spacer (IGS) region and for pathogenicity with regard to five Welsh onion cultivars. Phylogenetic analysis based on the IGS sequences revealed polyphyletic origins of the isolates and a relationship between phylogeny and pathogenicity; low virulence isolates differed genetically from those with high and moderate virulence. Mating type analysis revealed that all F. oxysporum isolates were MAT1-1 idiomorphs, suggesting that the pathogens may be clonal in the fields examined.     

Diversity in genetic structure and chemotype composition of <Emphasis Type="Italic">Fusarium graminearum</Emphasis> sensu stricto populations causing wheat head blight in individual fields in Germany

Fusarium head blight (FHB) is one of the most destructive diseases of wheat. Twelve small commercial wheat fields (size 1–3 hectares) were sampled in Germany for Fusarium populations at three spots per field with 10 heads each. PCR assays using generic primers confirmed 338 isolates as F.graminearum sensu stricto (s.s.) (64.9%) out of 521 Fusarium spp. that were further analyzed. Populations of F. graminearum s.s. in Germany contain three types of trichothecenes with a dominancy of 15-acetyldeoxynivalenol chemotype (92%) followed by 3-acetyldeoxynivalenol chemotype (6.8%) and a few isolates of nivalenol chemotype (1.2%). All these isolates were genotyped using 19 microsatellite loci. The 12 populations showed a high genetic diversity within the small scale sampling areas resulting in 300 different haplotypes. Genetic diversity within populations (71.2%) was considerably higher than among populations (28.8%) as shown by analysis of molecular variance. Gene flow (Nm) between populations ranged from 0.76–3.16. Composition of haplotypes of one population followed over 2 years changed considerably. No correlation between genetic and geographical distance was found. In conclusion, populations of F. graminearum s.s. in Germany display a tremendous genetic variation on a local scale with a restricted diversity among populations.     

Genetic diversity of <Emphasis Type="Italic">Fusarium oxysporum</Emphasis> f. sp. <Emphasis Type="Italic">spinaciae</Emphasis> in Japan based on phylogenetic analyses of rDNA-IGS and <Emphasis Type="Italic">MAT1</Emphasis> sequences

Twenty-eight isolates of Fusarium oxysporum f. sp. spinaciae (FOS; the causal agent of spinach wilt) collected from Japan were assessed for mating type and subjected to phylogenetic analysis. Mating type analysis revealed all isolates to be MAT1-2, suggesting that there is no sexual recombination within the population. Phylogenetic analyses based on nucleotide sequences of the ribosomal DNA intergenic spacer (IGS) and the mating type locus (MAT1) suggested that FOS is polyphyletic. The cluster analysis based on IGS showed four phylogenetic groups (S1–S4) among the isolates. Two distinct lineages, S1 and S3, included FOS isolates both of the vegetative compatibility group (VCG) types, 0330 and 0331, demonstrating that VCG differentiation in FOS may not necessarily reflect the phylogenetic relationships based on IGS and MAT1-2-1.     

Aetiology and toxigenicity of Fusarium graminearum and F. pseudograminearum causing crown rot and head blight in Australia under natural and artificial infection

Globally fusarium head blight (FHB) of wheat is predominantly caused by Fusarium graminearum (FG) and crown rot (FCR) by F. pseudograminearum (FP). While both FG and FP can cause FHB in Australia, the reasons why the morphologically and culturally similar FG is not a major FCR pathogen has remained elusive. Using aetiology and toxigenicity, this study clarifies the contrasting roles of FG and FP in FCR and FHB in Australia. Naturally infected wheat from 42 sites during 2010 FHB epidemics, and wheat inoculated with either pathogen to induce FCR or FHB at three field plantings in 2011, were used to determine pathogen prevalence and deoxynivalenol (DON) content of the crown, stem base, stem top, rachis and grain. As the primary aetiological agent, FP prevalence in the crown correlated with FCR severity while FG in grain and/or the rachis correlated with FHB severity. FG was an effective colonizer of the crown and stem base but colonization was symptomless. DON content was linked to FG biomass in all tissues except the crown, where FP biomass was the main contributor. Of the 30 measures derived to analyse pathogen fitness in 2011, 10 described the superior fitness of FG for FHB; six defined FP fitness for FHB including inoculum dispersal; and eight defined FCR fitness of both FP and FG. FG had superior FHB fitness but weak saprophytic survival may have undermined its FCR fitness.     

Phenotypic and pathogenic traits of two species of the Fusarium graminearum complex possessing either 15-ADON or NIV genotype

Fusarium head blight (FHB) in Brazil is caused mainly by two members of the Fusarium graminearum species complex, each possessing either a DON/15-ADON chemotype (F. graminearum sensu stricto, Fgss) or a NIV chemotype (F. meridionale, Fmer). In this work, we aimed to characterize and compare isolates belonging to each species, obtained either from wheat or barley, in relation to phenotypic traits (mycelial growth, sporulation and germination) and pathogenicity (in vitro glume infection and in vivo central floret inoculation) to two Brazilian wheat cultivars, Guamirim (moderate susceptible) and BRS 194 (susceptible). Results showed significantly higher growth rates, greater spore production and quicker germination for the Fgss isolates compared to the Fmer isolates, which were also more sensitivity to tebuconazole than the Fgss isolates. All isolates were capable of infecting glume tissues of both varieties, with an overall higher infection frequency for Fgss than Fmer isolates when inoculated in cv. Guamirim than in cv. BRS194, which showed similar infection frequency between the species. Accordingly, in the central-floret inoculation assay, Fgss isolates were also more aggressive than Fmer isolates towards cv. Guamirim, but not towards BRS194, based on the mean area under disease progress curves. It is hypothesized that phenotypic traits and host resistance may play a role in the selection of more aggressive populations under field conditions, thus partially explaining the dominance of Fgss populations associated with FHB of wheat in Brazil.     

Mating Relationships between Isolates of Phaeosphaeria nodorum, (anamorph Stagonospora nodorum) from Geographical Locations

Mating types of Phaeosphaeria nodorum isolates (from North Africa, North America, Australia, Europe and Near East) were determined in laboratory conditions. Although both mating types were found, MAT1-1 and MAT1-2 were not evenly distributed among the isolates. Sexual compatibility pairings with standard mating type testers revealed that 56 of 101 isolates could be assigned to MAT1-1 and 5 to MAT1-2. Although the teleomorph occurred in different countries, the two mating types were observed only among isolates collected from France, Great Britain, Germany and Morocco. For Morocco, where no P. nodorum pseudothecia have been reported, this is the first report of the existence of the two opposite mating types. The remaining 40 isolates could not be designated to a specific mating type. However, in 17 crosses, pseudothecial initials or sterile pseudothecia were observed on wheat straw after two months. The implications of the predominance of one mating type are discussed.     

Mating Behaviour and Vegetative Compatibility in Spanish Populations of Botryotinia fuckeliana

Mating behaviour and vegetative compatibility were studied in Spanish populations of Botryotinia fuckeliana. Fifty-seven isolates out of the 61 tested were sexually fertile with one or more of the reference strains of known mating type (MAT1-1 or MAT1-2). Thirty-nine isolates were heterothallic, giving fertile crosses when mated with the MAT1-1 (24 isolates) or the MAT1-2 (15 isolates) reference strain. Eighteen isolates crossed successfully with both MAT1-1 and MAT1-2 reference strains, and were referred to as homothallic or MAT1-1/2. Both mating types were widespread, being represented in isolates from two regions, from the same and different greenhouses, from different hosts, and from different years of isolation. Isolates were paired on Malt Extract Agar + NaCl to evaluate vegetative compatibility. Most of the paired isolates were unable to fuse and showed a different reaction of incompatibility. Nitrate-non-utilising (nit) mutants were selected by growth on a medium amended with 30–50gl^–1 potassium chlorate. Over 600 chlorate-resistant sectors were recovered from 40 isolates at a mean frequency of 0.15–2.39 sectors per colony, but only 11% were identified as nit mutants by their thin growth with no aerial mycelium on minimal medium. However, most of these nit mutants reverted to wild type during six months of storage on chlorate-amended medium. Genetic complementation between nit mutants occurred only in two cases between mutants from the same isolate.     

Fusarium head blight (FHB) in Flanders: population diversity, inter-species associations and DON contamination in commercial winter wheat varieties

The presence of Fusarium spp. causing Fusarium head blight (FHB) of wheat was studied in Flanders (Belgium) in 2007 and 2008. Symptoms, deoxynivalenol content (DON), Fusarium spp. and trichothecene chemotypes were determined at seven locations on different commercial wheat varieties. Overall, significant differences in disease pressure between locations and varieties were observed within 1 year. In addition, we were able to detect consistent and significant resistance differences among the common varieties both under high disease pressure (2007) and low disease pressure (2008). The accumulation of DON was not related to the presence of F. graminearum but showed a clear correlation with rainfall during and after the period of anthesis. During the two-year survey, characterisation of 756 Fusarium samples by species-specific PCR designated F. poae and F. graminearum as the predominant species in Flanders. Furthermore, most of the ears were colonised by multiple FHB pathogens in 2007 whereas the Fusarium population was less complex in 2008. Log-linear analysis of these multiple (two- and three-way) species interactions revealed a clear correlation between F. poae and several pathogens of the FHB disease complex. Finally, chemotype analysis showed that F. culmorum and F. graminearum were respectively of the NIV chemotype and DON chemotype. 3-ADON and 15-ADON chemotypes occurred in more or less equal amounts within the F. graminearum population both in 2007 and 2008. The congruence of these results with observations throughout Europe are discussed.     

Development of three fusarium crown rot causal agents and systemic translocation of deoxynivalenol following stem base infection of soft wheat

Fusarium pseudograminearum, F. culmorum and F. graminearum are the most important fusarium crown rot (FCR) causal agents. They have the common ability to biosynthesize deoxynivalenol (DON). To elucidate the behaviour of each of the three species, a comparative study was carried out to investigate symptom progression, fungal systemic growth and translocation of DON following stem base inoculation of soft wheat. FCR symptoms were mainly localized in the inoculated area, which extended up to the second node for all inoculated species. Only the most aggressive strains caused symptoms up to the third node. Real‐time quantitative PCR showed that fungal colonization reached the third node for all the tested species, but a low percentage of plants showed colonization above the third node following inoculation with the most aggressive strains. Fungal growth was detected in symptomless tissues but none of the three species was able to colonize as far as the head tissues. However, even if the pathogens were not detected in the heads, DON was detected in head tissues of the plants inoculated with the most aggressive strains. These results demonstrate that F. pseudograminearum, F. culmorum and F. graminearum, under the same experimental conditions, follow a similar pattern of symptom progression, fungal colonization and DON translocation after stem base infection. Differences in the extent of symptoms, fungal colonization and mycotoxin distribution were mainly attributable to strain aggressiveness. These findings provide comparative information on the events following infection of the stem base of wheat by three of the most important FCR casual agents.     

Geographic location and year determine virulence,and year determines genetic change,in populations of Neopseudocercosporella capsellae

White leaf spot (Neopseudocercosporella capsellae) can be severe and problematic worldwide across both horticultural and oilseed Brassicaceae, including susceptible rapeseed. In this study, 82 isolates from 2015 and 106 isolates from across Australia in 2016 were first assessed for their virulence against three different rapeseed (Brassica napus) cultivars. For both years there were significant (P < 0.001) differences. Also, there were significant (all P < 0.001) differences between isolates in each year, and between cultivars. For 2016 isolates, there were also significant differences (P < 0.001) between isolates across three different Australian states, and a significant interaction (P < 0.001) between isolates with cultivars. Of the three Australian states, isolates from Victoria were most virulent. Among tested cultivars, cv. Scoop was most susceptible. Subsequently, phylogenetic analysis of 114 of these same 2015 and 2016 isolates showed current isolates clustered separately from the majority of 2005 N. capsellae isolates collected from Western Australia a decade earlier, confirming significant genetic change within N. capsellae populations over the past decade. However, isolate clusters showed no association with geographical location. The results suggest that phylogenetic association among 2005 and 2015–2016 N. capsellae isolates is complementary with pathogenicity variations explained by geographically different N. capsellae pathogen populations. Neopseudocercosporella capsellae populations are evolving rapidly, challenging management through host resistance at a time of increasing incidence and severity of white leaf spot disease over the past decade. The outcome is well illustrated by cv. Scoop, previously resistant to 2005 isolates but moderately susceptible to 2015 and highly susceptible to 2016 isolates.     

Trichothecene mycotoxin genotypes of Fusarium graminearum sensu stricto and Fusarium meridionale in wheat from southern Brazil

A total of 82 fungal isolates was obtained from wheat kernel samples affected by fusarium head blight collected from 20 locations in southern Brazil. Polymerase chain reaction (PCR) assays were used to characterize trichothecene mycotoxin genotypes [deoxynivalenol (DON), nivalenol (NIV) and two acetylated derivatives of DON]. To identify isolates that producing DON and NIV, portions of the Tri13 gene were amplified. To identify 3-acetyl-deoxynivalenol (3-ADON) and 15-acetyl-deoxynivalenol (15-ADON) genotypes, portions of Tri3 and Tri12 were amplified. Nearly all of the isolates studied (76/82) were of the DON/15-ADON genotype. Six of the isolates were of the NIV genotype. The DON/3-ADON genotype was not observed. Portions of three genes were sequenced from representative isolates of the NIV and DON/15-ADON genotypes and compared with sequences from curated reference isolates of Fusarium in GenBank. blast queries for individual gene sequences and pairwise comparisons of percentage identity and percentage divergence based on 1676 bp of concatenated DNA sequence suggested that the isolates representing the DON/15-ADON genotype were Fusarium graminearum sensu stricto and the isolates representing the NIV genotype were Fusarium meridionale . This is the first detailed report of trichothecene mycotoxin genotypes of F. graminearum and F. meridionale in Brazil.     

Characterization of Fusarium asiaticum and F. graminearum isolates from gramineous weeds in the proximity of rice fields in Korea

Fusarium graminearum species complexes (FGSCs), such as Fusarium asiaticum and F. graminearum, are important pathogens that cause Fusarium head blight (FHB) in several cereal crops worldwide. In this study, we collected 342 gramineous weed samples in the proximity of rice fields from May to June 2018 in Korea. Among the 500 Fusarium isolates from the weed samples, 13 species of Fusarium were identified, and F. asiaticum (41.2%), F. avenaceum (18.0%), F. acuminatum (16.4%) and F. graminearum (14.8%) were the most frequently isolated. The trichothecene genotype analysis showed that 206 F. asiaticum strains consisted of the nivalenol (NIV) genotype (n = 195, 94.7%) and 3-acetyldeoxynivalenol (3ADON) genotype (n = 11, 5.3%), whereas 74 F. graminearum strains consisted of the 15-acetyldeoxynivalenol (15ADON) genotype (n = 58, 78.4%) and 3ADON genotype (n = 16, 21.6%). Geographical differences were observed in the FGSC and trichothecene genotype compositions, which appeared host-dependent between the southern provinces and mid-eastern provinces. The aggressiveness assessment of FHB showed that the 3ADON chemotype was most aggressive followed by the 15ADON and NIV chemotypes in wheat, while the NIV chemotype was most aggressive followed by the 3ADON and 15ADON chemotypes in rice. The F. asiaticum strains grew slowly and produced fewer conidia and perithecia than the F. graminearum strains, regardless of their chemotypes. The results of this study suggest that F. asiaticum with the NIV chemotype has a host preference for rice, and FHB-causing pathogens can be harboured in gramineous weeds, which play a role in the dispersal of FHB pathogens to rice and other cereal crops.     

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.     

Development of PCR assays to Tri7 and Tri13 trichothecene biosynthetic genes, and characterisation of chemotypes of Fusarium graminearum, Fusarium culmorum and Fusarium cerealis

Fusarium graminearum, Fusarium culmorum and Fusarium cerealis are major causal agents of Fusarium Head Blight (scab) which is a disease of global significance in all cereal growing areas. These fungi produce trichothecene mycotoxins, principally nivalenol (NIV) and deoxynivalenol (DON). Genes Tri13 and Tri7 from the trichothecene biosynthetic gene cluster convert DON to NIV (Tri13) and NIV to 4-acetyl-NIV (Tri7). We have developed positive–negative PCR assays based on these two genes, which accurately indicate a DON or NIV chemotype in F. graminearum, F. culmorum and F. cerealis. These assays are useful in assessing the risk of trichothecene contamination, and can be informative in epidemiological studies. All NIV chemotype isolates studied have functional copies of both Tri13 and Tri7, and all DON-producing isolates have both genes disrupted or deleted. We have identified several mutations in these genes, which are conserved across F. graminearum lineage, RAPD and SCAR groupings and between the three species. There appears to be evidence of inter-species hybridisation within the trichothecene biosynthetic gene cluster.