In subtropical Australia, oats are grown as a major winter forage crop with approximately 300 000 ha of oats grown in Queensland in 1988. Compared to wheat and barley, oats have a greater ability to regenerate after grazing and are therefore preferred. However, oats are susceptible to a number of rust diseases. In subtropical Australia, leaf or crown rust caused by Puccinia coronata f. sp. Avenae (Pca) is one of the major diseases affecting oats. When oats are planted early in the season (February, March, April), build up of inoculum can occur early on. This inoculum is then present for a long period of time (May to October) during which conditions ideal for rust epidemics occur.
The grazing potential of an oat crop may be reduced by up to 50% as a result of an epidemic as was the case in 1988 when much of southeast Queensland experienced a mild wet winter. Although cultivars resistant to Pca are available, races of the pathogen capable of infecting these cultivars have appeared relatively quickly (ie. Within 5 years). The nature of resistance in these cultivars has generally been complete ie. Allowing little or no rust development. However, cultivars expressing slow-rusting (partial or incomplete resistance) may express a more durable resistance in the field due, in part, to the reduction in the selection pressure on the pathogen.
Following the terminology of Parlevliet (1979), partial resistance, the term adopted for this report, is " a form of incomplete resistance in which spore production is reduced, even though the host plants are susceptible to infection". By reducing the numbers of spores produced, inoculum build up will be slowed and the rate of an epidemic will be reduced (compared to that of a highly susceptible cultivar). In reducing the rate of the epidemic, a crop may be harvested or grazed before much damage has been done. It is for this reason and the premise of greater durability of the resistance that endeavours have been made to identify slow-rusting or partially resistant cultivars.