by WHP Boshoff and ZA Pretorius
Rust diseases are common on cereal and grain crops in South Africa and continue to pose a significant threat. Despite their regular occurrence and economic importance, there is no coordinated approach to control rust pathogens through resistance breeding on most crops. Generally, a rust resistance breeding strategy should consist of regular race surveys to determine the number of races and their virulence profiles for the respective rust pathogens. Further to this, genetic studies to characterize current and to identify new sources of rust resistance, and to establish a germplasm screening and improvement support programme to breeders should be encouraged.
Currently wheat is the only crop in SA for which annual rust surveys are carried out. The data is used to determine cultivar response and the results are communicated to breeders and producers annually. Researchers at the University of the Free State have recently conducted surveys to determine the number of physiologic races and their respective virulence profiles for the oat crown- and stem rust pathogens as well as for the barley leaf- and the sunflower rust pathogens. Similar studies are planned for dry bean- and maize rust. Once rust race data, including a collection of isolates representative of the different races, is available, breeders can determine the rust response of their breeding lines. This can be achieved under controlled conditions or through establishing field screening nurseries, under conducive conditions for rust development, using selected rust races with known virulence profiles. Planting and rating of rust differential lines for the respective crops when establishing field trials in areas prone to rust occurrence should be encouraged. This will help breeders to interpret rust ratings captured on their breeding material between trial sites as well as over seasons. Applying reliable rust assessment schemes in combination with repeated ratings over time should allow for the identification of germplasm with the more preferred race non-specific sources of resistance and implementation of responsible gene stewardship. The application of molecular markers, especially high-throughput markers resulting from resistance gene cloning, should assist breeders in building more complex and diverse rust resistance gene “pyramids” into new cultivars. After all, the seed is considered the most important input in agriculture and breeders should allow producers the choice of growing disease-resistant crops. This can be achieved through applying basic research principles and with the inputs from scientists in other disciplines including plant pathologists.
The rust research programme of the University of the Free State is internationally recognised as a centre of knowledge for studies carried out on the genetic relationship of several rust fungi of agricultural crops, their occurrence, virulence profiles and their impact on commercial cultivars and breeding lines.
Photo of a wheat rust nursery planted with different genotypes in 1 m observation rows. Head rows were planted with a stripe rust susceptible entry and inoculated with spores of a selected race six weeks after planting. After the initial infection was confirmed, epidemic development was allowed through secondary infection. Fertiliser and supplemental irrigation were applied to ensure optimum plant and disease development.