The epidemiology of stem rust (Puccinia graminis Pers.:Pers.) of barley (Hordeum vulgare L.) in north-eastern Australia has been studied. Progress of stem rust epidemics was examined in Australian barley cultivars and differences in the rates of epidemic development between cultivars were found. Epidemics were shown to develop rapidly in some Australian cultivars (Galleon and Corvette) and cause severe reductions in both grain yield and quality. Although some barley cultivars (Grimmett and Prior) possessed a degree of resistance to P. graminis f. sp. tritici Eriks. & E. Henn., none of the barleys examined demonstrated complete resistance. Yield losses were largely attributed to the negative effect of stem rust on grain weight and size. Grain quality was also severely affected by stem rust through reduced grain size and high levels of screenings (grain less than 2.2 mm
diameter). Initial inoculum levels have been shown to be important in determining disease severity and incidence for individual epidemics; however, severe epidemics can develop in susceptible cultivars even under mild inoculum pressure. These findings showed that the cultivation of highly susceptible barley cultivars coupled with stem rust epidemics in triticale and wheat crops, and favourable seasons, resulted in the stem rust epidemics observed in barley crops in the northern part of the eastern Australian barley area in several years over the past decade.
The response of seedling plants to infection with P. graminis was examined. Seedling reactions were characterised by a mesothetic response to all the biotypes of P. graminis used. Differences in the receptivity of barley genotypes to P. graminis were shown and the ranking of the commercial cultivars examined was correlated with the ranking of adult plants in the field.
There was evidence indicating that receptivity in barley is a race specific phenomenon and that isolates of P. graminis (scabri) were more pathogenic on barley than the pathotypes of either P. graminis f. sp. tritici or P. graminis f. sp. secalis used in this study.
The management of stem rust through the use of host resistance was examined. To locate potential sources of resistance to stem rust, germplasm collections were screened as adult plants in field nurseries. Selected lines were further evaluated as seedlings in controlled environment facilities. Three sources of resistance, Q21861, Q21928 and Q21972, were identified and their resistance appeared to be at least as effective as that of Chevron, which possesses the Rpg1 factor for resistance.
To incorporate the resistances of Q21861, Q21928 and Q21972 through breeding,
a screening procedure that is both effective and easy to apply is required. Seedling screening in barley has previously been hampered because the mesothetic response of seedlings makes it difficult to distinguish resistant and susceptible genotypes. In this study the resistance of the three 'Q lines' could be detected by the reduced receptivity of seedlings under controlled environment conditions, allowing the development of seedling tests for early generation screening.
Genetic studies with the lines Q21861, Q21928 and Q21972 indicated that the resistance to P. graminis f. sp. tritici of the accession Q21861 was conferred by a single partially dominant gene different from Rpg1. The resistance of the accessions Q21928 and Q21972 was found to be conferred by Rpg1. There was also evidence for the presence of a single partially dominant gene conferring resistance in Grimmett, independent
of Rpg1 and the factor in Q21861. Evaluations were made on seedling F1 and F2 progenies in controlled environment facilities and confirmed with F3 families of adult plants in the field.