Watchful eyes are vital for early warning on rust

By Professor Robert F. Park and Dr Harbans Bariana, University of Sydney Plant Breeding Institute, Cobbitty

Monitoring cereal rust pathogen populations to determine which cultivars show evidence of rust infection, which strains are present, and where the strains occur, is a key element of rust resistance breeding efforts in many parts of the world.

In Australia, the University of Sydney has maintained a watchful eye on cereal rust populations every year since 1919. These surveys have revealed that our local rust pathogens sometimes overcome resistance genes by random mutation (for example, development of virulence for Lr24 in leaf rust), and that occasionally new rust isolates are introduced from outside Australia (for example, new stripe rust in WA in 2002).

The surveys have also shown how rust isolates spread over long distances.

In the June/July issue of Ground Cover, we discussed 10 instances over the period 1919- 2002 where exotic rust isolates are believed to have been introduced into Australia, and how cereal breeders had responded by incorporating new sources of resistance in their germplasm. Since then, work at the Plant Breeding Institute has provided strong evidence of an 11th incursion, a leaf rust isolate that can infect wheat cultivar MackellarPlant Breeder Rights icon, first detected in Victoria in 2004.

Because wheat cultivars can be susceptible to rust at early growth stages and become resistant as they grow (adult plant resistance), it is not yet clear just how the yield of MackellarPlant Breeder Rights icon will be affected by this new rust, if at all. Field trials are under way to assess the adult plant response of a range of Australian cultivars to this new wheat leaf rust variant.

By notifying the agricultural community of the occurrence of this new rust strain and its association with MackellarPlant Breeder Rights icon, the Australian Cereal Rust Control Program has effectively issued an early warning to advisers and others to monitor crops of MackellarPlant Breeder Rights icon for signs of leaf rust infection. This has confirmed the presence of the new strain at Cooma in southern NSW and three sites in Victoria (Inverleigh, Lake Bolac and Ararat).

Although clearly beneficial, notifying the community at large of the occurrence of rust on a cultivar may cause undue concern regarding the vulnerability of the cultivar to yield loss due to rust infection. As already indicated, the full impact of this new strain on MackellarA wheat is not yet known.

Another recent example was the unconfirmed report of stripe rust on EGA GregoryPlant Breeder Rights icon at Cowra in our last Ground Cover article. This cultivar is currently considered to have excellent resistance to stripe rust. Preliminary tests on samples from Cowra are suggesting no change has occurred in the stripe rust pathogen, and hence there is no strong reason at this point to suspect that the rust response of this cultivar has changed. EGA GregoryPlant Breeder Rights icon and other Batavia derivatives remain highly resistant in our artificially inoculated trials at Cobbitty. While our tests on survey samples are continuing, we encourage industry personnel to remain vigilant with all currently resistant cultivars for evidence of stripe rust infection, to help us detect any new changes in stripe rust pathogen as early as possible.

The early warnings on the occurrence of rusts that our surveys provide are important, not only in alerting growers to resistance breakdowns but also soliciting help to resolve situations where rust has been reported on a particular cultivar. There is, however, a need for information issued to be kept in context, so that the wrong message is not conveyed to the broader agricultural community.

Rust samples, ideally comprising several infected leaves, should be forwarded to the address below. Please note that we continue to receive samples in plastic bags, notably express paid packages. Rust will not survive under these conditions; please use paper envelopes only.

Australian Cereal Rust Survey Plant Breeding Institute Private Bag 11 Camden NSW 2570

GRDC Research Code US315
For more information: Robert Park, 02 9351 8806, robertp@camden.usyd.edu.au

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