Wednesday, July 6, 2022

Rusts never sleep

New Zealand is about 10 years behind many parts of the world’s wheat-growing regions when it comes to understanding the strains of rusts affecting the nation’s cereal crops. This is according to Nick Pyke, CEO of the Foundation for Arable Research (FAR), who says quarantine restrictions put a halt to rust samples being sent to Australia for analysis in 2000.

This is despite the fact that many years of surveying rust pathogens in Australia and NZ indicated rusts move freely between the two countries, carried on wind currents.

To help give the arable industry a rough idea of what rust strains are present in this country, Professor Robert Park, director of the Australian Cereal Rust Control Programme at the University of Sydney, has been visiting NZ and analysing rust samples submitted by farmers.

He says wheat is affected by three types of rust, namely leaf rust, strip rust, and stem rust, with new pathogen strains appearing at different places around the globe.

As an example of how easily rusts spread, a new leaf rust strain was discovered in Victoria in 1984. Within 10 years it was present in all the wheat-growing areas of Australia and NZ.

Park says occasionally rusts from other parts of the world will appear in Australia, as was the case with strip rust. There was no strip rust in Australia until 1979, when it first detected in Victoria.

A second incursion in Western Australia in 2002 was found to have originated in North America. Within one year this rust was found to be in Eastern Australia. Park’s work with FAR and Plant & Food Research has now shown that this strain has arrived in NZ.

With stem rust of wheat, two examples are known in which isolates are believed to have been transported to Australia from either southern or central Africa on high-altitude winds, though this is considered a rare event.

Wind currents will carry rust across Australia and NZ, but boots and clothing are also a common source of contamination, highlighting the need to be vigilant about cleaning first before crossing borders.

In controlling rusts, Park emphasised the importance of managing the fallow period between crops and destroying the “green bridge” which enables rusts to perpetuate.

“Rust needs a living plant – it can’t survive on stubble so it’s very important to destroy the green bridge.”

He points out that the problem is exacerbated in NZ where the period between crops is so short.

“So it’s important to remove or stop these things swinging from one crop to the next.” 

Robert Park says that in order to understand resistance to rust it is important to know the genetic background of cereal cultivars.

The genetic solution

Professor Robert Park says there are many genes in the wheat genome that confer resistance to rust, but rusts are variable and can overcome these resistance genes.

Furthermore there are different levels of resistance and where resistance is not going to give complete protection, the decision over whether to spray or not can be difficult.

He says genes can be divided into two categories: All-stage resistance and adult plant-stage resistance (APR).

The trouble with all-stage genes, says Park, is that they are likely to be overcome by pathogens.

APR is conferred by a combination of minor genes, so while the plant may have some disease, the level of resistance is higher.

“These genes seem to be more durable and rust finds them more difficult to overcome.”

They also appear to protect the plant from more than one disease – for example, leaf rust, strip rust and powdery mildew.

APR genes appear to be switched on at a specific growth stage; this stage depends on the genes involved but it is often around tiller to node formation.

The level of protection will also vary depending on the environment and inoculums mode.

Resistance breakdown 

A new race of strip rust has been identified in Europe which attacks the variety Claire, which is also unusually susceptible to rust in NZ but not in Australia.

Professor Robert Park, director of the Australian Cereal Rust Control Programme at the University of Sydney, explains that this is not due to resistance breakdown, but rather the pathogen changes to overcome resistance (he believes these are likely to be independent events in each country).

In the case of Claire the pathogen has changed, either due to mutation or the incursion of an external rust pathogen.

Claire carries both all-stage and APR genes. It appears the pathogen has knocked out the all-stage resistance, and the residual APR genes are still offering some protection.

“We need to understand the pathogen and the host to explain what happened to determine the full implications for the wheat industry.”

Another example is where the leaf rust pathogen changed to overcome resistance is in Conquest.

“When this variety was released it was resistant to leaf rust but this has changed,” Park says.

“I suspect Conquest carried all-stage resistant genes only and the pathogen acquired the ability to overcome these genes.”

In order to understand resistance, Park believes there needs to be some understanding of the genetic background of different wheat varieties.

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