The current outbreak of stripe rust is a reminder that South Australian growers need to mix up their wheat varieties and avoid growing large areas of those susceptible to rusts, says SARDI senior crop pathologist Dr Hugh Wallwork.

Reports of stripe rust have been received from almost all areas of the State. Dr Wallwork said that because the level of stripe rust was so uniform around SA, it was likely the spores blew into the State from New South Wales where the rust started developing much earlier.

Speaking at Grains Research and Development Corporation (GRDC) Updates held at Cummins and Wudinna last month, Dr Wallwork said susceptible varieties included some that were widely grown and yielded well on Eyre Peninsula and in other parts of the State. These included H45, Westonia, Krichauff and Wyalkatchem*, which were susceptible to stripe rust. Wyalkatchem* and Yitp*i were also susceptible to stem rust while Krichauff was susceptible to leaf rust.

He said large areas of susceptible varieties created greater potential for the fungus to mutate, thus weakening any resistance the crop plants may have to these diseases. Susceptible varieties also increased the chance that the inoculum would survive on volunteers over summer.

"We have had quite serious mutations in stem, stripe and leaf rusts in the past five years," Dr Wallwork said.

He said the strain of stripe rust that emerged in 2003 and became known as the West Australian strain overcame genes for resistance to the disease in H45, Krichauff and, to a lesser extent, many varieties commonly grown in the eastern states.

"In 2004 it seems to have survived over summer on volunteers around Penong on the far West Coast and near Naracoorte in the South-East," Dr Wallwork said. "It only needs to survive on one plant to cause a problem.

"The WA strain has led to two varieties currently being grown in SA being reclassified as 'very susceptible'. They are H45 and Westonia and these varieties should no longer be grown."

Dr Wallwork said a new strain of leaf rust emerged in 2000 that infected Krichauff, Janz and other varieties.

He said where the choice of varieties was limited, growers should give priority to those which had leaf and stripe rust resistance before stem rust, except in coastal sites where stem rust was a particular risk.

Dr Wallwork said Yitpi and Wyalkatchem were being widely grown because they yielded well, but he said they should not be grown en masse because they were rated as susceptible to stem rust. He said growers should not rely solely on fungicides to control a rust outbreak.

"By the time you see a hotspot, the spores have already been blown around," he said. "And fungicides won't be used to control the disease on summer volunteers." Dr Wallwork said growers needed to think about the long-term disease outlook when choosing varieties to sow, rather than merely focusing on yields from the next harvest.

He said the "sucker" varieties, which were rated as "very susceptible'" to a particular disease, should be avoided at all costs.

"The difference between varieties rated as 'susceptible' and those rated as 'very susceptible' is quite significant," he said. "It could represent a two to five-fold difference in the level of spore production and thus greatly influence both the speed of epidemic development and also the chances of new strains of fungi emerging that could further reduce the choice of varieties."

Dr Wallwork is working on two related projects that are supported by growers and the Australian Government through the GRDC. The first is aimed at finding new ways to control fungal diseases in cereal crops, while the Australian Winter Cereal Molecular Marking program is helping wheat breeders to fast track the development of new varieties with greater disease resistance.

* Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994.