S. Simpfendorfer

Identity and pathogenicity of Fusarium spp. isolated from wheat fields in Queensland and northern New South Wales

To establish the identity of Fusarium species associated with head blight (FHB) and crown rot (CR) of wheat, samples were collected from wheat paddocks with different cropping history in southern Queensland and northern New South Wales during 2001. CR was more widespread but FHB was only evident in northern NSW and often occurred with CR in the same paddock. Twenty different Fusarium spp. were identified from monoconidial isolates originating from different plant parts by using morphology and species-specific PCR assays. Fusarium pseudograminearum constituted 48% of all isolates and was more frequently obtained from the crown, whereas Fusarium graminearum made up 28% of all isolates and came mostly from the head. All 17 Fusarium species tested caused FHB and all 10 tested caused CR in plant infection assays, with significant (P < 0.001) difference in aggressiveness among species and among isolates within species for both diseases. Overall, isolates from stubble and crown were more aggressive for CR, whereas isolates from the flag leaf node were more aggressive for FHB. Isolates that were highly aggressive in causing CR were those originating from paddocks with wheat following wheat, whereas those from fields with wheat following maize or sorghum were highly aggressive for FHB. Although 20% of isolates caused severe to highly severe FHB and CR, there was no significant (P < 0.32) correlation between aggressiveness for FHB and CR. Given the ability of F. graminearum to colonise crowns in the field and to cause severe CR in bioassays, it is unclear why this pathogen is not more widely distributed in Australia.

Pathogen population structure and epidemiology are keys to wheat crown rot and Fusarium head blight management

This paper summarises the key findings from recent research on the population genetics and epidemiology of Fusarium pathogens causing head blight and crown rot of wheat in Australia and how this information has enabled the screening and selection of wheat germplasm with improved resistance to Fusarium. By relating new findings to the current state of knowledge, the paper serves as a timely and critical review of the international literature. In Australia, both Fusarium pseudograminearum and F. graminearum can cause both crown rot and Fusarium head blight under artificial inoculation. However, the former species is more widespread and is predominantly associated with crown rot whereas F. graminearum is mainly associated with Fusarium head blight, with limited geographical distribution in and around the Liverpool Plains in northern New South Wales. Studies of population structure and genetics have revealed that both species are genotypically diverse with similar levels of genetic recombination despite Gibberella zeae, the teleomorph of F. graminearum, being homothallic and G. coronicola, the teleomorph of F. pseudograminearum, being heterothallic. A high-throughput and reliable crown rot bioassay has been developed and used to screen over 1500 wheat germplasms to select 17 lines with putative crown rot resistance. Key differences in pathogen biology and epidemiology between Australia and the USA have emerged from other recent collaborative studies, which show that macroconidia constitute the bulk of aerial Fusarium head blight inoculum in Australia, whereas ascospores are the dominant primary inoculum for Fusarium head blight worldwide. The limited spread of splash-dispersed macroconidia of F. graminearum probably explains the restricted geographical distribution of this species in Australia. Other research collaboration has compared the aggressiveness, mycotoxin production and genotypic polymorphisms of the pathogen population from Australia and the USA. These and other differences in pathogen adaptation emphasise that research outcomes from elsewhere must be tested for relevance before applying them to Australian farming systems.

Ascosporic and conidial inoculum of Gibberella zeae play different roles in Fusarium head blight and crown rot of wheat in Australia and the USA

Air-borne spores of the Fusarium head blight (FHB) pathogenwere trapped at field sites in both Australia and the USA using susceptible wheat plants to establish whether ascospores or conidia were the dominant FHB inoculum. Although both ascospores and conidia were recovered from flowering spikes during a 28-day period between anthesis and grain filling, ascospores were the dominant spore type at Fargo, North Dakota in the USA. Conversely in Australia, conidia were the dominant spore type trapped during 16 and 21-day periods of sampling over 2 years at two sites in northern New South Wales. The effectiveness of Gibberella zeae ascospores and conidia as inoculum for FHB and crown rot (CR)was compared in plant infection assays using 10 isolates each from the USA and Australia. Overall, ascospores were less effective in causing FHB and CR than conidia, although the difference in FHB severity between the two spore types was small. Ascospores of isolates from the USA caused significantly less severe CR on three North American cultivars and some isolates with highly aggressive conidial inoculum were weakly aggressive with ascosporic inoculum. In contrast, both spore types of the Australian isolates caused severe CR and there was no significant difference between the spore types for CR severity on three North American cultivars. Studies such as these on pathogen biology and epidemiology are essential to examine whether the information gained in one geographical location can be effectively used to manage the pathogen in another location.