Graingrowers are being urged to stay calm in the face of Australia's most widespread wheat rust outbreaks in two decades.

And to think twice about fungicide spray programs which could cost more to apply than they save and be uneconomic at this stage of the season, when most crops are past the flowering stage.

Leading rust pathologist Col Wellings also wants growers to wait for the results of major pathogen testing before deciding what varieties to retain and store as next year's seed.

He says scientists should "have a better handle"  within a fortnight on varietal susceptibility to the various rust strains.

Dr Wellings is a senior research scientist on secondment from NSW Agriculture to the University of Sydney, where he is an associate professor and member of the cereal rust research team at the university's Plant Breeding Institute (PBI), Cobbitty.

The Cobbitty team, led by Professor Robert Park, is responsible for monitoring rust across Australia and for identifying, characterising and incorporating new sources of rust resistance in cereal crops.

In 2002 Dr Harbans Bariana's germplasm group  rust-tested some 60,000 cereal lines for Australian cereal breeders at Cobbitty.

" The stripe rust outbreaks across so much of the grain belt this year are probably the most severe we've seen since the 1983-1984 epidemic, when the industry was coming out of the 1982 drought," DrWellings said.

"This season has delivered the same conditions of  good crop growth, with lots of moisture and nutrition and dense canopies making them vulnerable to various rust strains.

" Growers saw a lot of stripe rust in the early stages of this year's crop, with the disease reported in early to mid-August in northern NSW and late August in central and southern NSW.

"More recently there have been numerous reports of head infection, particularly in the Murray Valley and Riverina irrigation areas.

"Good results can be achieved with  fungicide sprays when crops are still in the foliar stage but, once crops pass the flowering stage, fungicides are unlikely to be economic and they won't control  infection in the plant head.

"Most crops other than late planted ones are likely to be past the flowering stage.

Dr Wellings said seasonal conditions had also contributed to the unusually high levels of rust, with relatively mild winter temperatures allowing the disease to develop  more rapidly than usual.

"A short period of unusually warm to hot conditions in early September failed to check disease development and a return to cool to cold and moist weather allowed stripe rust to continue to develop through Spring.

There had also been considerable change in strains (or races) of stripe rust, with the most notable being the first appearance in eastern Australia of the strain  134E16A+  identified in the first stripe rust epidemic in Western Australia in 2002  and a second strain, 106E137A-.

"The occurrence of strain 134E16A+ in eastern Australia is expected to change the stripe rust response ratings of at least some cultivars, with the higher than expected levels of disease on some cultivars possibly a response to it," Dr Wellings said.

"We've developed the WA strain (134E164A+) in plots at PBI Cobbitty  but, while initial results suggest it may overcome some of the adult plant resistances to stripe rust, we must stress that this is speculative at this stage."