Crown rot - what is coming in the breeding pipeline

Take home message

Breeding lines with improved resistance/tolerance to crown rot in adapted backgrounds identified in pre-breeding programs. These are being provided to commercial breeding companies.

Complete resistance/tolerance to crown rot however is unlikely to be achieved. Resistance is only part of the solution, and management will always have a role in the control of this disease.

Background

Crown rot caused by the fungus Fusarium pseudograminearum is a major constraint to winter cereal production in the northern grains region. This disease causes significant yield loss in highly susceptible varieties. While management practices can offer some strategies for growers to minimise losses due to crown rot, varietal selection remains a critical component of the integrated disease management strategy.

This being said, there are no high yielding well adapted wheat varieties available with acceptable resistance or tolerance to crown rot currently available. Recent improvements have been made with releases of Spitfire (2010) and Sunguard (2012), although these varieties are still rated at moderately susceptible. Indeed, the industry standard for crown rot resistance remains Sunco, released in 1986 with a moderately susceptible rating to crown rot. To address this, the GRDC has invested significantly in pre-breeding programs to identify sources of resistance and tolerance to crown rot, and make them available for breeding companies to exploit.

What are the challenges for crown rot pre-breeding?

Progress in breeding for resistance and tolerance to crown rot has been slow. There are a number of reasons for this, including the complex genetic control of a plants response to crown rot. Resistance and tolerance traits are controlled by many minor genes each of small effect, meaning combining these together in a single genotype is challenging. Further, there is little understanding of the mechanisms of resistance and tolerance, and the activity of genes to make a line more resistant or tolerant. This leads to difficulty in understanding whether combining multiple genes for resistance will achieve additive gains in a plants response under crown rot by combining multiple favourable physiological traits. Broadly useful physiological traits such as water use efficiency, drought tolerance and maturity also play a role in breeding for tolerance to this disease.

Further hampering the development of adapted germplasm with enhanced crown rot resistance and tolerance is the poor agronomic characteristics of the source material. While there are a number of key sources of resistance and tolerance to this disease, they are largely associated with poor agronomic traits including late maturity, height, rust susceptibility, low yield, poor grain quality and poor threshability. Many of these sources have been widely used by commercial breeding companies without success due to these poor agronomic traits, resulting in the need for pre-breeding to combine desirable agronomic traits with improved resistance and tolerance as an initial step in improving crown rot traits in released varieties.

The role for pre-breeding for crown rot resistance and tolerance is therefore to identify sources of resistance from landraces, international and local collections and from synthetic wheats, and to improve these breeding lines agronomically while maintaining the superior resistance and tolerance traits. Given that even the best sources of resistance only provide partial resistance or tolerance, multiple sources of resistance and tolerance are combined, resulting in breeding lines with an enhanced crown rot response which can then be made available to breeding companies.

Key to the success of this pre-breeding program is the delivery of germplasm that is already well adapted. This will ensure that this material is agronomically competitive within commercial breeding programs, increasing the likelihood that crown rot resistance and tolerance traits will reach Australian wheat growers in a timely manner.

How is this achieved?

Significant effort has been invested in glasshouse seedling and adult plant field screening for crown rot resistance, measuring the degree of stem browning, a proxy for the amount of fungus in the stem. This allows breeders to select material with enhanced resistance to the pathogen, which is able to restrict the invasion of the pathogen. While there is no complete resistance to this disease, partial resistance is available in a number of sources, which are being combined in this program.

Ultimately however, the most relevant measure of a genotypes value commercially is yield, and in this case yield loss owing to crown rot. For breeding lines with enhanced resistance or tolerance to crown rot to be successful within commercial breeding programs, they must be yield competitively when compared with existing elite germplasm. For this reason, strong emphasis has been placed on the yield competitiveness of advanced germplasm through repeated yield and yield loss testing.   

How far have we come?

Resistance and tolerance trials have been conducted since 2012, assessing the performance of breeding lines under crown rot conditions. The results of these trials have been combined in a multi environment trial (MET) analysis to assess the performance of individual lines across seasons and environments (Figures 1 and 2).

From this analysis, approximately 50 lines have been identified with yield under crown rot significantly higher than Gregory, of which 40 lines were equivalent to or exceeded Suntop (Figure 1). This analysis also revealed 26 lines with resistance levels exceeding the industry standard Sunco, including 16 lines with resistance not significantly different to the most resistant source 2-49 (Figure 2). This is a significant achievement as while the resistance levels in 2-49 are high, this source is notoriously challenging to use in commercial breeding programs due to exceptionally poor agronomic performance. These improved sources represent a new pipeline for breeding companies to access resistance and tolerance to crown rot in agronomically advanced backgrounds.

Where are we going?

While there are a number of lines which have combined high yield under crown rot conditions with high levels of crown rot resistance, generally the most resistant lines are genetically distinct from those with greatest levels of tolerance (indicated by increased yield under crown rot). This provides an opportunity for further pyramiding of multiple sources of resistance and tolerance sources together to provide highly resistant germplasm to commercial breeding companies in yield competitive backgrounds.

Significant gains have been made in recent years with the development of germplasm with enhanced resistance in agronomically well adapted backgrounds. The most advanced material has been provided to breeding companies, with new improved sources provided each year. This combined with existing breeding and selection by commercial breeding programs, will assist in providing growers with varieties with improved crown rot resistance and tolerance in the future.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC. The authors would like to thank them for their continued support.

Contact details

Dr Philip Davies
Plant Breeding Institute, University of Sydney
12656 Newell Hwy, Narrabri
Ph: 02 6799 2244
Fx: 02 6799 2239
Email: philip.davies@sydney.edu.au