No abstract

Fusarium head blight (FHB) is an important disease of wheat worldwide. Soissons is one of the most resistant varieties grown in UK. The current study was undertaken to identify QTL for FHB resistance in Soissons and to determine whether the semi-dwarfing alleles Rht-B1b and Rht-D1b have a similar influence on susceptibility to FHB. A Soissons (Rht-B1b; Rht-D1a) x Orvantis (Rht-B1a; Rht-D1b) doubled haploid (DH) population was assessed for FHB resistance in three trials. Soissons contributed a single, stable major FHB QTL linked to the Rht-D1 locus. In contrast, the Rht-B1b allele (contributed by Soissons) conferred no negative effect on FHB resistance, even conferring a very minor positive effect in one trial. The influence of the Rht-B1b and Rht-D1b alleles on FHB resistance was further investigated using both Mercia and Maris Huntsman near-isogenic lines. Under high disease pressure both Rht-B1b and Rht-D1b significantly decreased Type 1 resistance (resistance to initial infection). However, whilst Rht-D1b has no effect on Type 2 resistance (resistance to spread of the fungus within the spike), Rht-B1b significantly increased Type 2 resistance. Our study demonstrates that the choice of semi-dwarfing gene used in plant breeding programmes may be a significant consideration where resistance to FHB is an important breeding target.

Fusarium graminearum , one of the causal agents of fusarium head blight (FHB) of wheat in the UK, can be broadly divided into two chemotypes based on the production of the 8-ketotrichothecenes deoxynivalenol (DON) and nivalenol (NIV). DON-producing isolates can be further distinguished on the basis of the predominant acetyl DON derivative that they produce; 3-acetyl DON (3-AcDON) or 15-acetyl DON (15-AcDON). Functional Tri13 and Tri7 are required for the production of NIV and 4-acetyl NIV, respectively, whereas, in isolates that produce only DON and its acetylated derivatives, these genes are nonfunctional or absent. Infections caused by F. graminearum are becoming more frequent in the UK; however, it is unknown whether these represent more than one chemotype. In this study, polymerasae chain reaction (PCR) assays specific for functional and nonfunctional/deleted versions of Tri7 and Tri13 were used to characterize 101 single-spore isolates of Fusarium graminearum as DON or NIV chemotypes. Primer sets developed to Tri3 were used to classify DON chemotypes further by the acetyl derivative produced (3-AcDON or 15-AcDON). Isolates were collected from 65 fields located around England and Wales between 1997 and 2002. All three chemotypes were identified from the F. graminearum population of England and Wales, with 15-AcDON chemotypes predominating overall. All isolates characterized as 3-or 15-AcDON chemotypes had nonfunctional versions of both genes. Where multiple isolates were collected from a field, mixed-chemotype populations were identified. Variation in the number of 11-bp repeats in Tri7 was detected among 3-and 15-AcDON chemotypes. Seventy-two of the 76 DON chemotypes (95%) were classified as 15-AcDON producers and the remaining four isolates (5%) as 3-AcDON producers. In all four isolates with a 3-AcDON chemotype, Tri7 was deleted from the trichothecene gene cluster. There was no evidence of regional variation between 3-AcDON, 15-AcDON or NIV chemotypes within the F. graminearum population.

Functioning Tri13 and Tri7 genes are required for the production of nivalenol and 4-acetyl nivalenol, respectively, in Fusarium species producing type B trichothecenes. Mutations have been identified in isolates which are able to produce deoxynivalenol (DON) but unable to convert this to nivalenol (NIV). In such isolates of Fusarium culmorum , the Tri7 gene is deleted entirely. PCR assays specific for functional and nonfunctional/deleted versions of Tri7 and Tri13 were used to determine the ability of 153 single spore isolates of F. culmorum to produce the 8-ketotrichothecenes deoxynivalenol and nivalenol. The isolates were collected from 76 different locations across England and Wales between 1994 and 2002. Four isolates were also obtained from one field in Scotland. Both DON and NIV chemotypes of F. culmorum were identified, with DON chemotypes predominating overall. In addition, all DON chemotypes were shown to produce 3-acetyl DON using primer sets developed to Tri3 . From fields where more than one F. culmorum isolate was obtained, isolates were not exclusively of a single chemotype. Differences in the distribution of DON and NIV chemotypes were identified, with a greater proportion of NIV chemotypes present in the south and west of England and Wales, whereas a greater proportion of DON chemotypes were found in the north and east of England. Seasonal differences in the ratio of DON:NIV chemotypes were indicated. However, these were related to seasonal variation in the distribution of F. culmorum .

Accelerating international trade and climate change make pathogen spread an increasing concern. Hymenoscyphus fraxineus, the causal agent of ash dieback, is a fungal pathogen that has been moving across continents and hosts from Asian to European ash. Most European common ash trees (Fraxinus excelsior) are highly susceptible to H. fraxineus, although a minority (~5%) have partial resistance to dieback. Here, we assemble and annotate a H. fraxineus draft genome which approaches chromosome scale. Pathogen genetic diversity across Europe and in Japan, reveals a strong bottleneck in Europe, though a signal of adaptive diversity remains in key host interaction genes. We find that the European population was founded by two divergent haploid individuals. Divergence between these haplotypes represents the ancestral polymorphism within a large source population. Subsequent introduction from this source would greatly increase adaptive potential of the pathogen. Thus, further introgression of H. fraxineus into Europe represents a potential threat and Europe-wide biological security measures are needed to manage this disease.