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Monitoring is key to prevent leaf rust losses

Leaf rust in wheat.
Photo: Bob Freebairn

It was recently estimated that pests and diseases cut global wheat production by 21.5 per cent. While these reductions were due to 31 pests and pathogens, just four accounted for half of all losses – with the most damaging being leaf rust, followed by Fusarium head blight/scab, then Septoria tritici blotch, then stripe rust.

Despite causing less-spectacular crop damage than diseases such as stripe rust, leaf rust has long been considered the most damaging rust pathogen of wheat overall because it is the most common and it can develop under a broader range of environmental conditions. Yield losses exceeding 30 per cent have been documented in Australia.

Knowing how common Lr24-virulent pathotypes are, and where they occur, is important to ensure growers of varieties with this gene minimise their risk of losses due to leaf rust.

Although historically more frequent in northern New South Wales and Queensland, leaf rust of wheat has been well-controlled in this region for at least 30 years by the ongoing release of rust resistant cultivars.

Sporadic problems with leaf rust occurred in Western Australia from 1990-2000, while the disease was of concern in South Australia and Victoria in 1999. Localised outbreaks also occurred in irrigated crops of the cultivar Triller in the Murrumbidgee Irrigation Area during 1998 and 1999.

At the time of writing, so far this year leaf rust of wheat has been reported in southern Queensland and three sites in NSW, at low frequencies. Two well-separated detections of virulence for Lr24, from Queensland and southern NSW, are of concern, and we ask that crops with this resistance gene be monitored for leaf rust.

Resistance gene

The leaf rust resistance gene Lr24 has been used extensively in Australia. It was introduced to Australia by staff at the University of Sydney’s Plant Breeding Institute and was first deployed in 1983 in the cultivar Torres.

Since then, more than 60 wheat cultivars have been released with this gene (for example, Bremer , Chief CL Plus , Cutlass , Elmore CL Plus , Impress CL Plus , LRPB Gazelle , LRPB Lancer , LRPB Oryx , LRPB Parakeet , Sunchaser , Naparoo , Sunguard and Supreme ).

Virulence for Lr24 was first detected in 2000, 17 years after the release of Torres. A second pathotype with virulence for this gene was detected in 2013. Virulence for Lr24 only occurs in eastern Australia. However, unlike virulence for the stripe rust resistance gene Yr17, virulence for Lr24 has not become dominant in eastern Australia and its frequency has fluctuated over the years.

Wheat varieties with the resistance gene Lr24 should remain resistant to leaf rust unless one or both pathotypes virulent for this gene increase in frequency and, for this reason, we ask that growers of varieties carrying this resistance gene keep a watching brief and forward samples of leaf rust if the disease is found.

Knowing how common Lr24-virulent pathotypes are, and where they occur, is important to ensure growers of varieties with this gene minimise their risk of losses due to leaf rust.

Rust responses of wheat varieties can be found in our Cereal Rust Report 17(3). The distribution of cereal rust pathotypes can be monitored by viewing our regularly updated map. Both of these resources are available at our (University of Sydney) website.

Please forward freshly collected rust samples in paper only to the Australian Cereal Rust Survey, University of Sydney, Australian Rust Survey, Reply Paid 88076, Narellan, NSW, 2567.

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