Key points

• A new statistical method can separate the breeding attributes of tolerance, resistance and yield potential
• It will help pre-breeders develop germplasm in the fight against crown rot
• It is important to tease apart these traits because selecting genotypes with favourable traits for tolerance can clash with the goal of selecting for resistance and vice versa.

Although tolerance, resistance and yield potential are key attributes when breeding cereals to face the threat of crown rot (Fusarium pseudograminearum), together they can muddy the waters when selecting germplasm.

However, a new statistical method can separate these attributes. It was developed by Dr Alison Kelly, leader of the Statistics for the Australian Grains Industry (SAGI) north team, as part of GRDC’s investment in pre-breeding through the ‘Integrated Genetic Solutions to Crown Rot in Wheat’ project.

Dr Cassandra Percy, a plant pathologist with the project’s pre-breeding team, explains. “Breeding for resistance, tolerance and yield is important, but it can involve potentially competitive objectives. Selecting genotypes with favourable traits for tolerance can clash with the goal of selecting for resistance and vice versa,” she says.

“We often use the terms tolerance and resistance interchangeably, but tolerance refers to the plant’s ability to maintain yield in the presence of disease. Resistance is the plant’s ability to limit infection and spread of the pathogen. Both are important attributes and now we can use this statistical method to tease them apart and select for both in a breeding program.”

Usually, phenotyping for tolerance and resistance is done in isolation. That means methods of selecting for tolerance are undertaken independently of the genotype’s underlying resistance level. “But both tolerance and resistance are defence mechanisms used to maintain host health in the presence of a pathogen.”

Technique

Uncoupling the effect of tolerance and resistance is challenging, yet now possible with SAGI’s advanced statistical analysis technique.

It works by allowing for genotypes to be selected first for yield retention under inoculation treatments, independently of yield potential. It then allows for genotypes for tolerance to be selected independently of their resistance status.

“By analysing results through the new model, we can derive and select for three traits of interest – high yield potential, resistance and tolerance. This statistical method pulls apart these attributes,” Dr Percy says.

All are equally important, she says. “Resistance, which reduces the amount of the pathogen, contributes to yield retention and is important in managing the disease. Tolerance traits allow us to use germplasm that could face levels of the disease and still yield well. Obviously, the ultimate aim is the ‘complete package’ and this will bring us a step closer.”

Practical example

The technique was used to examine the commercial wheat varieties Suntop (PBR) and Sunguard (PBR).

Suntop is used as the commercial standard to tolerance. It has yield potential and limited resistance. This compares to Sunguard, which is used as the commercial standard for resistance, and has competitive yields and some tolerance.

Dr Percy says the analysis supported the commercial status. However, for the first time it allowed the pre-breeding team to demonstrate that increasing resistance can lead to reduced yield loss or yield retention; that crown rot-tolerant germplasm exists; and that resistance and tolerance are independent traits and can be selected independently in a breeding program.

“The practical results of this analysis allows selection of pre-breeding and breeding lines with high yield potential, higher levels of tolerance than Suntop and higher levels of resistance than Sunguard.”

GRDC investment in supporting breeders through pre-breeding is important, she says.

“The biometry team at the Queensland Department of Agriculture and Fisheries has really allowed us to move ahead in leaps and bounds. They have developed statistical methods that create new ways of looking at data and these methods will be transferable to other diseases.”

Caused by the fungus Fusarium pseudograminearum, crown rot is a significant disease in winter cereals such as wheat, barley, durum, triticale and oats, costing the nation’s grains industry an estimated $404 million in lost yield annually in wheat alone.

More information: Dr Cassandra Percy, 07 4631 1241, cassy.percy@usq.edu.au