ofSvdnev. NSW2(HI6. Ausiratia Conidial germination, appressoria! formation, penetration of epidermal walls, formation of intraeellular vesicles and growth of inlracellular hyphae in epidermal cells occurred within 12 h of inoculation. Hyphae tben grew slowly between mesophyll cells for the next 12 h. Some papillae formed beneath appressoria and mosi infected epidermal eells retained stain by 24 h after inoeulation. indicating major changes in eellular physiology. Slight differences between cuUivars in some of these events were not related to resistance.On ibe second day. intercellular hyphae emerged more extensively from the infection sites into the mesophyll of the susceptible cultivar Banks, and formed significantly larger myeelia than in the resistant cultivar BH1146 by 3-5 days from inoculation. Rapid intercellular growth then continued in the susceptible eultivar but not in tbe resistant cultivar. Necrotic lesions expanded fasier in the stjsceplible cultivar from day 3. By da\ 10. most lesions in ihis cultivar were large and light brown with a conspicuous ehlorotic margin but those in the resistant eultivar were small and dark brown with inconspicuous chlorosis.

Net type net blotch (NTNB) is an important barley disease in Australia and elsewhere, with significant yield reduction. This trait is important in selection along with other traits of quality and agronomic value. Two-hundred doubled-haploid lines were generated through anther culture from a cross between ÔPompadourÕ and ÔStirlingÕ. Quantitative trait loci (QTL) were identified against five isolates of Pyrenophora teres f. teres, which represent virulences across Australia. QTL were mapped on chromosomes 3H and 6H using simple sequence repeat (SSR) markers. The resistance locus on 6H was detected with all isolates while the 3H locus was detected with two isolates. The 6H QTL from ÔPompadourÕ contributed resistance to isolates 97NB1, 95NB100 and NB81, whereas 6H QTL from ÔStirlingÕ contributed resistance to isolates NB50 and NB52B. The 3H QTL from ÔPompa-dourÕ contributed resistance to NB50 and NB52B. Significant epistatic interactions were detected between QTL on 3H and 6H. These resistance QTL are a useful resource and identifying closely linked SSR markers with allelic combinations will facilitate in marker-assisted selection to develop NTNB resistant breeding lines.

A potential source of resistance to septoria nodorum blotch had been identified in an accession of the wild wheat, Aegilops tauschii. A cross was made between the resistant Ae. tauschii accession, AUS21712, and a susceptible accession, CPI110889, to study the genetics of resistance. The parental accessions and the F1, F3, and F4 progeny were screened in the glasshouse as seedlings. The resistant parent took significantly longer to develop symptoms, developed significantly fewer lesions, and expressed significantly lower levels of disease than the susceptible parent. The F1 mean response for disease severity indicated resistance was dominant. The genotypic ratios generated from the screening of the F3 and F4 generations were not significantly different from the genotypic ratio expected for a single gene. The efficacy of the resistance and its simple genetic control in the Ae. tauschii accession AUS21712 means that the potential exists to use this Ae. tauschii resistance gene in a bread wheat breeding program.

In controlled inoculation studies with Septoria nodorum and Pyrenophora tritici-repentis, estimates of the relative proportion of each pathogen demonstrated differences in the responses of cultivars to pathogen mixtures that were not apparent from measurements of diseased leaf areas . Under field conditions estimates of the relative proportion of S. nodorum, P tritici-repentis and S tritici varied between field screening locations in Western Australian but also between lines within locations . Lines with known resistance to P tritici-repentis and S tritici, but susceptible to S. nodorum, could not be distinguished from susceptible lines on the basis of leaf area diseased or grain weight depression when S. nodorum was present in the disease complex . Such conditions, while suitable for the selection of combined resistance to these pathogens, were unsuitable for identifying resistance to individual pathogens. As symptoms were similar, the proportion of diseased leaf area sporulating with each pathogen provided a means of measuring the variation in disease development induced on lines varying in resistance . Knowledge of the components of disease and their relative importance were essential in understanding varietal response information under mixed infections of these leaf spot pathogens .