Selection of pea lines for resistance to pathotypes of Ascochyta pinodes, A. pisi and Phoma medicaginis var. pinodella

The pathogenicity of 45 isolates of Ascochyta pinodes, Ascochyta pisi and Phoma medicaginis var. pinodella collected in South Australia has been examined on selected pea lines. Twenty-six isolates of A. pinodes were differentiated into 15 pathotypes, 15 isolates of A. pisi were differentiated into 13 pathotypes, and four isolates of P. medicaginis var. pinodella into one pathotype. Adequate sources of resistance were identified against all the pathotypes excepting pathotypes 1 of A. pinodes and A. pisi. The necessity to breed for broadly based resistance to Ascochyta species is discussed.

Inheritance of scald resistance in barley. I. Resistance of genes of group A barley cultivars

The genetic basis of resistance and susceptibility of barley cultivars to Rhynchosporium secalis (Oud.) Davis was determined from F2 and F3 progenies of crosses among five resistant and four susceptible cultivars. The resistant cultivars Psaknon, Atlas 46, Atlas 57, Hudson and Turk were found to share a common gene in addition to other genes for resistance. No race of the pathogen found in Western Australia was able to overcome the resistance conferred by the common gene. The effectiveness of the identified resistance genes to two isolates of R. secalis was studied under glasshouse conditions in winter and summer, and in the field during winter.

Inheritance of scald resistance in barley. II.* Resistance genes of group B barley cultivars

The genetic basis of resistance to Rhynchosporium secalis (Oud.) Davis of barley cultivars La Mesita, West China and Sakigake differentiated as group B were determined from F2, F3 and BC1 progenies. The resistance genes identified are situated at different loci from each other. The effectiveness of the identified resistance genes to two isolates of Rhynchosporium secalis was studied under glasshouse conditions in winter and in summer, and also in the field. The inheritance of resistance genes of group B cultivars was found to be greatly influenced by environmental factors. *Part I, Aust. J. Agric. Res., 26: 243 (1975).

Host range and physiologic specialization in Rhynchosporium secalis

The pathogenic variability of isolates of R. secalis collected in Western Australia has been examined on different host genera of the Gramineae and on selected barley cultivars. Depending on the host-isolate combination and the conditions of the test, evidence has been obtained of inter- and intra-isolate variability in both host reaction and isolate pathogenicity. This complicates definitive interpretation of the results, militates against identification of conventional 'races' of the pathogen and shows that R. secalis does not exhibit strict host specialization. Hosts which consistently express resistance or susceptibility under different environmental conditions, and isolates which express their pathogenic characteristics consistently, have been identified. The need for more precise genetic studies and adequate sampling of genetic diversity is emphasized.

Factors influencing infection, colonization and symptom expression in barley by Rhynchosporium secalis

Germ tube growth and mycelium establishment of Rhynchosporium secalis, the causal agent of scald in barley, were studied in seven resistant and four susceptible barley hosts of known genotype. Although no visible symptoms were apparent, none of the resistant hosts were free from mycelium establishment in leaf tissue. The resistant hosts could be divided into two categories according to the rate of germ tube growth. The effect of host age, duration of incubation period, temperature, spore concentration, and pathogen age on symptom expression in the 11 genotypes was examined. Excluding the effects of temperature, the resistant genotypes could be grouped into (a) those expressing their resistance consistently, e.g. Psaknon, Atlas 46, Atlas 57, Hudson and Turk, and (b) those expressing their resistance inconsistently, e.g. West China and La Mesita. High temperature caused breakdown of resistance in Psaknon, Turk, Hudson and West China. The pathogenicity of one isolate (UWA307) was found to be impaired by high temperature. Implications of the above findings are discussed in relation to the disease situation in Western Australia.