Stem rust resistance in South African wheat and triticale

Abstract not available

The rapid distribution of Ug99 and its variants emphasized the vulnerability of global cereal production to stem rust. Due to the evolution and mutation of Puccinia graminis f. sp. tritici (Pgt) pathotypes in South Africa (SA) there is a constant need to discover new sources of resistance, understand the genetic base of presently deployed sources in cereal cultivars, and to manipulate the future deployment of resistant sources through a more sustainable approach. The current study includes various fundamental aspects for effective management of stem rust in SA. Assessment of wheat varieties elucidated a decline in seedling and adult plant stem rust resistance from 2009 to 2011. Previous studies confirmed the presence of the resistance genes Sr24 and Sr31 in various South African varieties and the sudden decrease in adult plant resistance (APR) observed from 2009 to 2011 after the appearance of Pgt pathotype UVPgt60 (PTKST) corroborated the presence of these genes in certain South African varieties. Since 2011, the withdrawal of susceptible cultivars and selection for resistance to this pathotype, resulted in an annual increase in the number of entries with seedling and APR. In the past conventional methods of phenotyping wheat and triticale have proven to be difficult and time consuming and multi-pathotype tests were only feasible on seedlings in the greenhouse. The development of a dependable screening system for assessing APR for stem rust in the greenhouse provides an additional instrument to assess for stem rust resistance and allows for simultaneous multipathotype screening. The validation of the technique emphasized that it is unlikely that greenhouse phenotyping will replace field testing, especially for additively inherited resistance genes that may yield more reliable results under field conditions. The effective management of stem rust requires race-monitoring, characterization of resistance sources and deployment thereof. A single rust species, such as Pgt, can be variable in its ability to attack different genera of host plants such as wheat and triticale. Although there is some understanding of rust resistance in wheat, little SUMMARY 226 is known about resistance in triticale. Inheritance studies were undertaken to determine the genetic base of certain South African cultivars through seedling analysis and field work to detect APR. Breeding emphasis should be on the stacking of resistance genes as observed for Duzi, SST 047, Steenbras and Tankwa. However, the presence of single genes detected in Tobie and Kiewiet explains the short-lived resistance to Pgt in both triticale cultivars. The additional seedling resistance gene expressed in Line SrNin needs further clarification. To add to our understanding of the host-pathogen interaction of triticale, histological methods such as microscopy were used to elucidate the infection structures and subsequent colonization process of rust pathogens in different cereal hosts. Previously no comprehensive information existed for the infection process of Pgt in triticale and scanning electron microscopy results showed no evident morphological differences in pre-penetration fungal behavior when compared with wheat. Results further revealed that colony size and biomass growth are equally applicable to triticale when the expression of resistance is studied. In conclusion, the utilization and development of different resistance screening methods, a valued study to elucidate host genetics as well as the use of histological and microscopic methods to study early resistance responses broaden our knowledge and understanding of stem rust resistance in South African wheat and triticale cultivars.