High rust levels threaten resistance
The greatest concern to cereal growers of rust epidemics is that of yield losses, which in the case of stem rust can be up to 100 per cent.
Another important concern, and often not fully appreciated, is the threat of large rust populations on effective resistances, which can be overcome by mutations in the rust pathogens. Mutations occur at random and result in a change in the genetic makeup of the rust isolate. Given that mutations are estimated to occur at the rate of about 1 per 10,000,000,000 rust spores, more rust means more mutants. Although the odds sound very unlikely, it has been estimated this number of spores would be produced by an acre of wheat suffering 10 per cent rust infection. Hence, mutant pathotypes can arise with only moderate levels of rust.
The cultivation of stem rust-resistant wheats in many Australian cereal-growing regions over the past 20 to 30 years has reduced stem rust populations to the point where very few new mutant pathotypes are detected, leading to sustained control. An exception to this was the loss of the Cook stem rust resistance in the early 1980s, which resulted from an increase in stem rust populations caused by the cultivation of Oxley.
Update on Lr24
The recent demise of the leaf rust resistance gene Lr24 by mutant pathotype of leaf rust provides another example. It was first detected in South Australia in a region with moderate levels of leaf rust, and whilst it has not yet caused significant yield losses, an immediate and important consequence was the loss of an important source of leaf rust resistance.
Results obtained since the last Cereal Rust Corner (see Ground Cover, Summer 2001) have established that the new pathotype had spread to southern NSW by the end of season 2000 (Coleambally, Wagga Wagga, Temora, Cowra), and that whilst most cultivars with Lr24 are moderately susceptible to susceptible to the new pathotype, Giles, Petrie and Sunsoft 98 displayed good levels of resistance.
In some parts of the world, rust diseases are controlled primarily by chemicals. Whilst not always economical in Australia, increasing yields and reduced chemical prices have made this a more attractive means of rust control in some situations. However, any reduction in the emphasis of releasing resistant cultivars will jeopardise the benefits currently enjoyed from the long-term rust control achieved through breeding. Sustained control of these pathogens is best achieved by reducing the rust populations, by growing resistant cultivars and by destroying out-of-season cereals (the 'green bridge').