Cereal rust response ratings how do growers know what's good and what's not? by Dr Robert F. Park, University of Sydney Plant Breeding Institute Cobbitty

How well briefed are you on the rust-resistance status of your cereal crop?

DECIDING WHAT cereal variety to grow can be difficult and may include a consideration of rust resistance. This type of information has generally been circulated by state departments in annually updated lists of recommended varieties.

The information on these lists is generated by various specialist groups and, in the case of rust response, it often comes from testing conducted by the Australian Cereal Rust Control Program (ACRCP) based at Cobbitty, NSW.

Accurate data on variety performance are important. This in turn takes a commitment by all parties involved in the development and/or release of cereal varieties in Australia to submit their material for appraisal by programs such as the ACRCP.

In the absence of rigorous testing, varieties that are susceptible to rust may slip through the recommendation system, threatening not only individual crops, but also crops in neighbouring regions, placing rust-resistance breeding at risk of further mutation change from the ensuing large pathogen population.

To determine how a given variety will respond to a rust disease, ACRCP staff conduct tests at seedling growth stages in the greenhouse, and at adult plant growth stages in artificially infected rust nurseries in the field. The isolation of Cobbitty from commercial cereal-growing regions means that the chance of rust spreading from field nurseries is minimal.

A variety with resistance that is effective at all growth stages will usually have a good mst-resistance rating. However, these resistances can be vulnerable to mutational changes in the rust pathogens, and it is for this reason that the ACRCP monitors Australian cereal rust populations. A recent example of this was the development of virulence for the leaf rust resistance gene Lr24 in eastern Australia. The new pathotype resulted in an immediate and significant change in the rust response of many varieties that carried the resistance gene. (See also p16.)

Varieties that are not highly resistant can support some rust development and, in these cases, growers may at times consider using fungicides.

Making an accurate connection between expected rust response and the potential economic benefits of fungicide applications is usually difficult and requires additional field-testing to quantify yield losses associated with different levels of rust infection.