Pre-breeding for rust resistance is done in both spring and winter wheat backgrounds as these diseases occur in all the wheat production regions of South Africa. Often, resistance to more than one kind of rust is needed in the same line. Pre-breeders make use of molecular markers where possible to confirm the presence of resistance gene is the germplams being developed. This is, however, always confirmed by seedling evaluations in the glasshouse or adult plant evaluations under field conditions. By stacking different genes with resistance against the same rust together in one line a more durable resistance can be obtained with a lower risk of the resistance being overcome by new pathotypes.
Stem rust
Stem rust caused by Puccinia graminis f. sp. tritici is arguably the most economically important disease of bread wheat, barley and triticale worldwide. It is also sometimes called black rust. For many years the incidence of stem rust in South Africa has been very low, primarily due to the deployment of resistant cultivars in farmer's fields. This may be changing due to the ability of rust pathogens to overcome host resistance. Internationally, a rust pathotype named Ug99 (Race TTKSK) is causing concern. It has been shown to overcome a number of important resistance genes notably Sr31, Sr24 and Sr36 and has spread via winds from East Africa where it originated to new localities thousands of kilometres away. It is estimated that approximately 80-90% of global wheat cultivars are susceptible to this pathotype.
Leaf rust
Leaf rust of wheat and triticale (Puccinia triticina) can cause extensive yield losses, both in quantity and quality when environmental conditions are favourable for its development. It is generally more severe on autumn-grown wheat in the winter rainfall regions of the Cape Province, but moist conditions and elevated temperatures during spring can promote leaf rust in the Free State production region. Irrigated spring wheat throughout South Africa can also be subjected to severe leaf rust epidemics under favourable conditions. As with stem rust, the development and release of leaf rust resistant cultivars in South Africa in the past has minimised the risk posed by this disease to producers and assisted in making them internationally more competitive.
Stripe rust
Stripe rust of wheat and barley caused by Puccinia striiformis f. sp. tritici was first noted in South Africa in August 1996, in the Western Cape, South Africa. It now occurs throughout the country, but is more prevalent in cool and wet conditions such as the eastern Free State and the Western Cape. Severe yield losses occur when this funus infects the awns.
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