CHANGES IN the rust diseases attacking Australia's wheat crops have left a "gaping hole" in our defences and researchers are once again playing catch-up according to Hugh Wallwork, leader of the fungal pathology group in the South Australian Research and Development Institute.
Development of resistant wheat varieties in the mid-1990s led researchers to believe they were "pretty much on top of things" in the tight against stripe, stem and leaf rust.
"Stripe rust was rarely seen in crops, stem rust had not been a problem since 1984, and while leaf rust was still a danger, the susceptible variety Excalibur was being phased out and we were well on the way to minimising the damage from that disease as well," Dr Wallwork says.
But, in 1999, everything changed.
"First, we found a new strain of leaf rust that was attacking Krichauff and a large number of related breeding lines that were looking positive," he says. "Then there emerged a new strain of stripe rust that was very aggressive on Stylet, Trident and Camm - and another large batch of high-yielding breeding lines became history.
"To make matters worse, we then heard of a new strain of leaf rust in WA that was also aggressive on Stylet and related lines."
Dr Wallwork says researchers now have to play a long-term game of catch-up on rust, developing resistant varieties that will deliver some security for growers.
"Firstly, by not releasing Stylet, we have taken the critical decision to try to stem the tide of new rust strains by keeping the level of disease inoculum down," he says. "Secondly, we are quickly promoting breeding lines that provide resistance which is still effective.
"Thirdly, we're seeking new sources of resistance which will help diversify the resistance we currently have in case of future changes in rust populations. This strategy draws heavily on the expertise of the Australian Rust Control Program at the University of Sydney and is funded by the GRDC and the South Australian Grain Industry Trust."
Dr Wallwork says researchers cannot guarantee if new resistant cultivars will prove to be any more robust than those already lost.
"Some of them may not do better," he says. "There are two strategies, however, that will be critical to improving our position.
"In one, we are using specific sources of partial resistance to leaf and stripe rust which, when combined, appear from historical and overseas data to provide a durable level of resistance.
"Work in the CRC for Molecular Plant Breeding has identified the genetic basis of this resistance and provided molecular markers to help us select for resistance in breeding programs.
"Second, we must convince growers that with their cooperation we can succeed, providing they play their part by not growing susceptible varieties and putting the new resistance genes at risk."