Leaf rust is considered the world's most damaging wheat rust disease.

A recent publication in the journal Nature Ecology and Evolution estimates that diseases reduce global wheat production by about 21 per cent.

The study used an expert-based assessment of crop health, documenting losses caused by some 137 pathogens and pests associated not just with wheat but also with rice, maize, potato and soybeans.

In wheat, 31 pests and pathogens were identified as problematic. These included 17 fungi, nine invertebrates, four viruses and one phytoplasma.

In Australian growing conditions, stem rust can cause total crop failure and stripe rust infection can cause up to 50 per cent losses.

Of these 31 pests and pathogens, the study found that just four accounted for half of all wheat cropping losses. Leaf rust was the most damaging, followed by Fusarium head blight (scab), septoria tritici blotch and stripe (yellow) rust.

Putting these losses into perspective, we published a study with colleagues in 2015 that estimated stripe rust caused 5.47 million tonnes (US$979 million) in global annual losses.

In Australian growing conditions, stem rust can cause total crop failure and stripe rust infection can cause up to 50 per cent losses.

However, in-crop losses due to leaf rust tend to be lower, with one Australian study documenting 30 per cent yield losses.

Although its yield penalty is lower than other rust diseases, leaf rust has long been considered the most damaging wheat rust overall because it is the most common rust and develops under a wider range of environmental conditions.

The Nature Ecology and Evolution study found that wheat crop losses due to pests and diseases were lower in regions that produced food surpluses.

Significant progress

While wheat diseases continue to impact production in Australia, significant progress in controlling pathogens has been made using in-built disease resistance genetics.

The major ongoing challenge, however, is that these pathogens are a moving target, continually evolving to overcome the genetic resistance incorporated into new cultivars by plant breeders.

The leaf rust pathogen is no exception, with the emergence of a new pathotype (strain) in South Australia in 2017, which is now present in all Australian wheat-growing regions.

Just how this new pathotype developed is not clear, but it appears likely to have arisen locally - possibly via asexual hybridisation.

If this proves to be the case, it will be only the second documented example in the world of a new pathotype of the wheat leaf rust pathogen arising in this way. The first occurred in the early 1990s when I found a hybrid pathotype specialised to hybrid wheats in northern NSW.

The variability of rusts and their rapid spread across the Australian continent reinforces the importance of regular and nationally coordinated monitoring of these pathogens.

As always, growers are encouraged to monitor crops closely for rust this season and to send freshly collected samples, in paper only, to the Australian Cereal Rust Survey, University of Sydney, Australian Rust Survey, Reply Paid 88076, Narellan, NSW 2567.

GRDC Research Codes 9175448, 9175952, 9176057

More information: Robert Park, 02 9351 8806, robert.park@sydney.edu.au