Rust and the never-ending search for resistance

Key points

  • Breeding for resistance is considered the best long-term strategy for controlling rust in wheat
  • An industry approach to growing effective multiple-gene-resistant varieties reduces the risk of new virulent rust strains developing
  • Wheat rusts are a global issue and Australia's involvement (with GRDC funding) in international programs is an essential part of local research.

Stem and stripe rust are capable of destroying a whole wheat crop; and leaf rust has been known to halve yields.

This was the grim reminder when Dr Ray Hare, a retired New South Wales Department of Primary Industries wheat breeder and current board member of the NSW Wheat Research Foundation, spoke at the University of Sydney Plant Breeding Institute (PBI) Narrabri field day in late 2013.

Since its incorporation 45 years ago under the management of the University of Sydney, the PBI has had a major focus on breeding rust-resistant wheats for Australia. Rust has been an ongoing problem for wheat production in many regions of Australia, especially in central and northern NSW, and Queensland.

The development of resistant varieties has played a major part in countering this and improving wheat growing reliability.

Photo of man in hat

Dr Ray Hare checking wheat varieties for rust after speaking at the recent University of Sydney Plant Breeding Institute Narrabri field day. Dr Hare stressed to growers and industry leaders the need for the Australian wheat industry to continue to strive towards growing predominantly rust-resistant varieties.

PHOTOS: Bob Freebairn

It was not until the mid-1950s, when sufficient and suitable rust-resistant varieties were released (mainly by the University of Sydney from their Narrabri program), that wheat growing became relatively viable in much of the current northern GRDC region.

The development of more rust-resistant varieties has also contributed significantly to slowing the ceaseless emergence of new rust strains.

In his address, Dr Hare stressed that while fungicides can reduce the impact of rust diseases, by far the best strategy is to develop and grow resistant varieties.

He noted that total dependence on fungicide control is constantly hindered by issues such as adequate supply of product in a major outbreak year, poor application conditions such as wet weather at critical periods and availability of application equipment such as aircraft if other disease issues are present.

Adequate control of wheat rusts also depends on regular crop monitoring and the need for precise timing of fungicide applications.

Dr Hare said that while fungicide protocols were well understood, sometimes more than one fungicide treatment was required and each rust type has different timing needs.

He also added the issue of rust developing tolerance to fungicides, similar to the herbicide resistance issue with weeds.

Photo of wheat showing leaf rust symptoms

Leaf rust in wheat. Leaf, stem and stripe rust are all capable of badly damaging wheat yield and quality. Breeding for resistance is the best long-term control strategy.

Dr Hare also used his talk to remind listeners that wheat rust is not just an Australian issue; rust spores have been shown to be able to be blown over to Australia from Africa.

He said this made it essential for local researchers to be involved closely with international breeding and assessment programs. 

An example of the value of this investment has been the pre-emptive breeding in Australia of varieties able to withstand the potentially devastating central-east African stem rust Ug99.

Many Australian varieties now have resistance to Ug99 should it ever make its way to Australia.

Photo of wheat

A susceptible wheat variety with a severe leaf rust infection, which can reduce crop yields by 50 per cent or more.

Dr Hare also re-emphasised growers’ responsibility to avoid, as far as possible, growing rust-susceptible varieties because these increase the risk of new rust strains developing, which can attack the hard-earned resistant varieties.

He explained that wheat breeders endeavour to ‘pyramid’ resistant genes into new varieties by combining as many as possible in a single variety. The greater the number of effective rust-resistant genes in a variety, the more likely its resistance will last.

In addition, breeders endeavour to have a diversity of resistance gene combinations in new varieties so that if one variety becomes susceptible, others will maintain their resistance. This prevents a crop, or a large proportion of it, from being lost. 

Dr Hare pushed the case for the Australian wheat industry to strengthen its push for the near total cultivation of fully resistant varieties.

He made the point that while varieties remain resistant and do not require frequent replacement with new resistant varieties, more of the breeding effort and investment can be directed to improving other important varietal traits.

There is not an endless supply of effective resistance genes. Dr Hare said that the effectiveness of these genes needs to be protected through wise deployment in varieties. Resistance genes need to be seen as an essential genetic resource that must be carefully protected.

More information:

Dr Ray Hare, 02 6765 6542,

ramdhare1@hotmail.com

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