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.