Identification of a major locus conferring resistance to powdery mildew (Erysiphe polygoni DC) in mungbean (Vigna radiata L. Wilczek) by QTL analysis

A major locus conferring resistance to the causal organism of powdery mildew, Erysiphe polygoni DC,, in mungbean (Vigna radiata L. Wilczek) was identified using QTL analysis with a population of 147 recombinant inbred individuals. The population was derived from a cross between 'Berken', a highly susceptible variety, and ATF 3640, a highly resistant line. To test for response to powdery mildew, F-7 and F-8 lines were inoculated by dispersing decaying mungbean leaves with residual conidia of E. polygoni amongst the young plants to create an artificial epidemic and assayed in a glasshouse facility. To generate a linkage map, 322 RFLP clones were tested against the two parents and 51 of these were selected to screen the mapping population. The 51 probes generated 52 mapped loci, which were used to construct a linkage map spanning 350 cM of the mungbean genome over 10 linkage groups. Using these markers, a single locus was identified that explained up to a maximum of 86% of the total variation in the resistance response to the pathogen.

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.

Genetic diversity of Australian Fusarium graminearum and F-pseudograminearum

Genotypic diversity in Fusarium pseudograminearum and F. graminearum from Australia and the relationship between diversity and pathogen aggressiveness for head blight and/or crown rot of wheat were examined. Amplified fragment length polymorphism (AFLP) analysis revealed a high level of genotypic diversity within each species. Sixty-three of the 149 AFLP loci were significantly different between the two species and 70 of 72 F. pseudograminearum and 56 of 59 F. graminearum isolates had distinct haplotypes. When head blight and crown rot severity data from a recently published work on isolates representing the entire range of aggressiveness were used, only the genotypic diversity of F. pseudograminearum was significantly associated with its aggressiveness for the two diseases. Cluster analyses clearly demonstrated the polyphyletic structures that exist in both pathogen populations. The spatial diversity within F. graminearum was high within a single field, while frequent gene flow (N-m similar to 14) and a low fixation index (G(st) = 0.03) were recorded among F. pseudograminearum isolates from the adjacent states of New South Wales and Queensland. The differences in population structure between the heterothallic F. pseudograminearum (teleomorph G. coronicola) and the homothallic F. graminearum (teleomorph G. zeae) were not as pronounced as expected given their contrasting mating systems. Neither species was panmictic or strictly clonal. This points to sexual recombination in F. pseudograminearum, suggesting that ascospores of G. coronicola may also play a role in its biology and epidemiology.