Social diseases require a public solution
Serious wheat rust epidemics are sporadic and some people may question why we seek Australia-wide resistance. The cereal rust fungi are obligate pathogens requiring green plants of their host species (in some cases also other cereals or grasses) for year-round survival.
The spores they produce can be windborne across the country, even to New Zealand. Severe rust levels also increase the likelihood that mutants (mutations occur at low frequencies for all genes in all species) will occur and will increase on previously resistant varieties, negating years of hard work by cereal breeders.
The combination of these features of year-round survival and airborne dispersal implies the need for community effort in rust control. This year a wheat rust alert was posted for the whole WA grainbelt and many in the industry, both within WA and further east, are very concerned, especially if moist conditions continue to prevail.
Set minimum disease resistance standards?
Many also believe there should be protocols specifying minimum disease resistance standards (MDRS) for variety release and recommendation in order to minimise the cultivation of rust-susceptible and very susceptible wheats, that is, varieties likely to experience crop loss and to produce the largest numbers of spores.
The operation of an objective MDRS scheme will require the cooperation and goodwill of all concerned including the National Cereal Rust Control Program (NCRCP), located at the University of Sydney's Plant Breeding Institute, Cobbitty, to assist breeding, programs and to determine actual levels of rust response.
The NCRCP Steering Committee consisting of state and public breeder and pathologist representatives sets and oversees the actual standards, which may vary across the country depending on risk assessment.
Also, the National Rust Forum Working Group, with representation from the above groups as well as GRDC, variety commercialisers and Grains Council, aims to ensure acceptability and fairness of the scheme.
Losses to leaf rust in WA in 1999 were estimated at $20 million. Had the most susceptible of the varieties not been present, these losses could have been much less, with the associated reduction in greater rust spread and resistance-breaking mutants.
While better or cheaper chemicals are now available for cereal rust control, there are still significant costs involved. Moreover, applications may be excessively delayed or uneconomic under some circumstances.
* Professor Bob Mcintosh is Director of Rust Research and Professor of Cereal Genetics and Cytogenetics at the University of Sydney, Plant Breeding Institute, Cobbitty.