Pathogenic variability and yield losses associated with rust diseases of barley and oats in South Africa

[摘要] English: This study showed less variation in the South African populations of P. hordei, P. graminis f. sp. avenae and especially P. coronata f. sp. avenae than was expected. In the case of P. hordei only two pathotypes were identified from a total of 810 isolates sampled over a six-year period. Pathotype SAPh 3231 was virulent to resistance genes Rph 1, Rph4, Rph5, Rph 10 and Rph 11, while pathotype SAPh 7231 had added virulence to Rph12. The occurrence of only one pathotype up to 1998 and the gaining of virulence to Rph 12 can be explained by the cultivars planted during this period. The cultivar Clipper dominated production until 1998 when the release of SSG 532, containing Rph 12, supplied sufficient selection pressure to allow a mutation to establish. It was found that the reaction types of barley differential sets were not only influenced by the host and pathogen genotypes, but by a complicated interaction of environmental factors. The presence of the resistant gene Rph12 was postulated in various cultivars. The study of accessory hosts identified some wild Hordeum accessions to be weak hosts for P. hordei while no evidence of the teleomorph of P. hordei could be found in South Africa. The 269 single-pustule isolates of P. eoronata f. sp. avenae revealed only one pathotype (SAPc 1) with virulence to the resistance genes Pc40, Pc45, Pc46, Pc51 and Pc54. Four greenhouse mutants were also detected. These isolates overcame Pc35, Pc48, Pc50, Pc52 and Pc64, indicating the vulnerability of these genes should they be used commercially. Although more pathotypes were expected, the presence of only one race could be explained by the cultivars grown and the differential set used. The wild oat species Avena fatua, A. byzantina, A. steriIis and A. barbata were susceptible to crown rust. The most variation in population composition was found in P. graminis f. sp. avenae with four pathotypes identified out of the 242 single-pustule isolates tested. Pathotype SAPga 1 had virulence to resistance genes Pg1, Pg2, Pg4, Pg8 and Pg15, while pathotype SAPga 2 was similar except for added virulence to Pg9. Pathotype SAPga 4, on the other hand, had the same virulence profile as SAPga 2, except for added virulence to Pga. Pathotype SAPga 3 was virulent to Pg1, Pg8 and Pg 15. All four pathotypes were virulent on A. fatua, A. byzantina and A. sterilis, while A. barbata was resistant to all four pathotypes. Losses due to these three rust species in South African barley and oat cultivars were higher than expected. The barley cultivar SSG 525 sustained losses of 58% in yield and 61% in kernel plumpness, while the percentage siftings in Sterling was increased by as much as 590%. In resistant cultivars, differences between sprayed and unsprayed plots were in most cases not significant. In oats, yield was reduced by as much as 85% in the cultivars Euro and Echidna, and hectolitre mass with 45% in Witteberg. This study clearly showed the destructive nature of these rust diseases and their impact on economically important parameters. It laid the foundation for further investigations and it should be emphasised that continual monitoring of these pathogens is necessary to provide meaningful contributions to rust control of barley and oats in South Africa.

[发布机构] University of the Free State