Many cereal growers would be familiar with the term 'breakdown of resistance'. This is said to happen when disease-resistant cultivar becomes susceptible. In the case of rust diseases at least, the cultivar becomes susceptible because of a genetic change in the rust pathogen.
Such changes in the pathogen are thought to result from spontaneous mutations, and give rise to new rusts (pathotypes) which can attack a previously resistant cultivar.
Attempts to produce rust-resistant oat cultivars are often thwarted by the occurrence of new rust pathotypes. Abundant wild oats support a large oat rust population, providing ample opportunity for new pathotypes to develop. Surveys conducted in 1998 detected some 35 pathotypes of oat leaf rust across Australia.
Three notable recent examples of leaf rust resistance in oats being overcome involve the cultivars Cleanleaf, Warrego, Graza 68 and Moola. Cleanleaf, released in 1992, became susceptible to leaf rust in 1995. A pathotype with the ability to attack Warrego was detected for the first time in 1998, the year it was released.
Of further concern was the detection of a new pathotype in June 1999, which can attack the cultivars Moola and Graza 68, both released in 1997 and both possessing the same resistance gene. In all three cases, the new pathotypes appear to have developed by spontaneous mutations in existing pathotypes.
While most current oat cultivars are susceptible to the prevailing pathotypes of oat leaf rust, greenhouse testing has confirmed that the cultivars Bettong and Barcoo are resistant to the new pathotypes. The testing further suggests that these two cultivars possess different resistance genes.
This knowledge allows us to predict that the development of a new pathotype capable of attacking one would not threaten the resistance of the other.
The apparent ease with which the oat leaf rust pathogen has overcome recently deployed resistance genes is a clear indication that future attempts to breed resistant cultivars will need to avoid the release of cultivars with single effective resistance genes. Future research will target the identification of effective resistance genes so that breeders can try to combine two or more in new cultivars.
* Dr Park is at the University of Sydney Plant Breeding Institute, Cobbitty