Cereal Rust corner by Dr Robert F. Park, University of Sydney, Plant Breeding Institute Cobbitty, Australian Cereal Rust Control Program: Alert: leaf rust in Camm wheat Are we losing the rust resistance arms race?

Unlike many soil-borne cereal disorders, the rusts produce massive quantities of spores that can migrate over large distances. A nationally coordinated approach and the release and cultivation of resistant cultivars only are vital for sustainable control.

SAMPLES OF leaf rust collected recently from CammPBR logo wheat in WA have been shown to be a new pathotype with the ability to overcome the resistance of this cultivar. The new leaf rust pathotype is a mutant derivative of the pathotype that has been common in WA since 1990.

CammPBR logo was developed as a rust-resistant wheat by the incorporation of a resistance that is often referred to as the VPM (ventricosum/persicum/Marne) resistance. It is a triple rust resistance that provides protection to not only leaf rust (gene Lr37), but also stem rust (Sr38) and stripe rust (Yrl7).

Unfortunately, all three resistances are no longer effective in all regions of Australia. Late last year, a new pathotype of the stem rust pathogen with virulence for Sr38 was detected in WA, and virulence for the stripe rust component, Yr17, was first detected in eastern Australia in 1999.

Other at-risk varieties

Other cultivars that also possess the VPM resistance include Trident, Bowie, Sunbri, Sunlin and Sunstate#. The adult plant responses of these cultivars to the new leaf rust pathotype are currently unknown, but nurseries established in WA this year will hopefully provide some indication. The loss of the three VPM rust resistance genes, along with the leaf rust resistance gene Lr24 in 2000, raises the question of whether we are losing the rust resistance breeding race in wheat. Many current Australian wheat cultivars have resistance to rust that is still effective despite these breakdowns, and ongoing work by the Australian Cereal Rust Control Program at the University of Sydney is continuing the search for and incorporation of new sources of resistance.

The control of rust diseases by resistance is sustainable only if the hard-won gains through resistance breeding are managed responsibly. Rust diseases are social diseases; unlike soil-borne diseases like take-all, and nematodes, the rust pathogens can spread rapidly between all Australian cereal-growing regions, and the actions of one grower can have considerable implications for neighbouring properties.

Fungicides, while an important tool in combating these diseases, do not provide complete protection. A policy of releasing and growing only rust-resistant cultivars reduces and stabilises the rust population, decreases the threat to individual growers and minimises the chance that new mutant pathotypes develop.