Unusually high levels of leaf rust were reported in experimental plots of the wheat cultivars Janz, Krichauff and Worrakatta on the Yorke Peninsula, South Australia, in October. This was of considerable concern as these cultivars possess the leaf rust resistance gene Lr24, a gene that has provided valuable protection against leaf rust in Australia since it was first deployed in the cultivar Torres in 1983.
Glasshouse tests at the University of Sydney confirmed that a new strain (pathotype) of leaf rust was responsible. The new pathotype most likely developed from an existing common pathotype by mutation.
Crops of Krichauff with higher than expected levels of leaf rust have subsequently been detected at multiple sites across the Yorke Peninsula as well as in the lower and mid-north of SA. Previous experience would suggest that the new pathotype will already be in other parts of the eastern cereal-growing region.
In addition to Janz, Krichauff and Worrakatta, other cultivars that possess Lr24 may be affected by the new pathotype. These include Anlace, Babbler, Cunningham, Datatine, Giles, Goroke, Lang, Mira, Mulgara, Nyabing, Perouse, Petrie, Sunco, Sunelg, Sunsoft, Sunpict, Swift and Tasman.
Recent greenhouse tests of these cultivars showed that most are seedling susceptible to the new rust. They may, however, have additional adult plant resistance, and tests of adult plants with the new pathotype are required.
The National Cereal Rust Control Program at the University of Sydney has breeding lines with other sources of resistance that are effective against the new pathotype, and will back-cross other sources of resistance into important Lr24-carrying cultivars.
Destroy the green bridge
If the summer is wet, there will be increased opportunity for survival of rusts on self-sown cereals. If this eventuates, growers are advised to destroy these plants during the summer period. Wheat crops of cultivars with Lr24 sown in 2001 should be closely monitored for leaf rust and, if found, samples should be forwarded to the Australian Cereal Rust Survey, University of Sydney, Plant Breeding Institute Cobbitty, Private Bag 11, Camden NSW 2570.