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Disease outlook and managing increased risk

Plant pathologist Dr Grant Hollaway provided a cereal disease briefing at the 2024 GRDC Research Updates.
Photo: Brad Collis

Wet conditions through early summer brought a significant risk of cereal disease in 2024. Providing a cereal disease outlook at the GRDC Adelaide and Bendigo research updates, plant pathologist Dr Grant Hollaway from Astute Ag said there was risk of yield losses from diseases including Septoria tritici blotch in wheat, net form of net blotch (NFNB) in barley and rust pathogens in both cereals.

But proactive disease management combining variety and paddock selection with strategic fungicide application when required could provide effective disease management. Removal of volunteer plants (or the green bridge) that might have emerged over summer was also critical for the control of rust, Dr Hollaway said.

The rust threat

With most of south-eastern Australia receiving decile 10 levels of rainfall across December and January, the scene was set for high risk of stripe rust in wheat, Dr Hollaway said. This is because rust can only survive on living plants, and the early rainfall provided the opportunity for the pathogen to carry over on volunteer cereals growing over summer and infect 2024 crops.

“Fortunately, our understanding of the benefits of removing the green bridge for soil water conservation has led to less of a carryover now than in the past,” he said. “But there are still paddocks green with volunteers growing. So, the message is that our rust risk – and particularly stripe rust risk – is high.”

disease infographic

How fungicide resistance evolves. Source: Modified from CropLife Australia’s Fungicide Resource Management Fact Sheet

Hollaway said barley grass stripe rust (BGYR) was also a growing concern. First detected in Victoria in 1998, it was a minor issue for many years, but in 2021 a new variant resulted in a large increase in virulence to a number of varieties, he said. Glasshouse studies undertaken by the University of Sydney have shown it also has significant fungicide resistance.

“When we have a pathogen that is increasing in virulence and bringing with it a reduced sensitivity to fungicides, that is of real concern,” he said. “So, it’s really important to monitor and if you are seeing stripe rust on barley grass or barley crops make sure you send samples to the University of Sydney (for testing).”

Australian growers, he said, were fortunate to be served by the University of Sydney’s Australian Cereal Rust survey supported by GRDC. Testing hundreds of samples each year, it provides a comprehensive picture of the distribution and spread of cereal rusts and their pathotypes.

This information also supports breeders in the development of resistant varieties and informs ratings for a particular variety’s resistance to rust pathogens. These ratings are regularly updated in disease guides such as the GRDC National Variety Trials Disease Ratings helping growers to select appropriate cultivars to shore up defences against rust and range of other diseases.

Septoria and NFNB

Other disease risks for growers to keep on their radar in 2024 included NFNB in barley, and Septoria in wheat, particularly if conditions are wet, Dr Hollaway said.

GRDC-supported trials by Agriculture Victoria in the Wimmera and Mallee have highlighted how sensitive Septoria is to the environment, he said. In the relatively dry 2021 season, for example, there was no progression of Septoria. However, in the wet 2022, yield losses of up to 35 per cent were seen in susceptible varieties in the Wimmera.

Dr Hollaway said Septoria had become a significant disease because many wheat varieties grown on a large scale were highly susceptible to it.

Similarly, 2023 Agriculture Victoria trials involving NFNB highlighted the disease’s impact on highly susceptible varieties. They found that when exposed to NFNB, there was less infection and no significant yield losses in the moderately susceptible variety Titan AX , compared to yield losses of 18 per cent in the highly susceptible RGT Planet .

Variety choice is key

These results demonstrate the importance of avoiding highly susceptible (or sucker) varieties in an integrated disease management plan. “We can talk about a whole range of disease strategies, but the one we know that works time and time again is avoiding sucker varieties,” Dr Hollaway said. “If we don’t grow sucker varieties a lot of our disease issues aren’t there.”

Powdery mildew, for example, is now a problem because highly susceptible varieties such as Scepter, RockStar, LRPB Trojan, Vixen, Corack and Wallup were introduced. “But if we replace those highly susceptible varieties with partially resistant varieties, we will go back to ongoing disease control,” he said.

The efficacy of this approach is demonstrated through past experience. Cereal cyst nematode, a serious threat to wheat in the 1980s and 1990s, is largely unknown now because susceptible varieties were replaced with resistant strains. Yellow leaf spot too, prevalent 15 years ago, is now rarely detected in Victoria, he said.

“Today you struggle to see yellow leaf spot in the paddocks, and that's because we’ve taken the suckers out of the system,” he said.

A range of options

Contrary to common assumption, he said, strongly resistant varieties such as those rated R, RMR or MR were not always required to achieve sustainable levels of disease control. Protection from yield loss could be achieved with moderate susceptibility (MS) and, in some cases, in crops rated moderately susceptible to susceptible (MSS), he said.

disease graphic

Prevalence of Yellow Leaf Spot (YLS green) and Septoria (STB orange) in Wimmera and Mallee. The decrease of Yellow Leaf Spot and increase of Septoria aligns with susceptible varieties to each disease being removed or introduced. Wheat varieties such as Rockstar, Scepter and Vixen are resistant (MRMS) to Yellow Leaf Spot but susceptible (S) to Septoria.

In the case of Septoria, for example, because of its interaction with the environment, moderately susceptible and susceptible (MSS) varieties could be grown in low-rainfall areas with some confidence. In medium-rainfall areas, growers should target varieties with moderate susceptibility (MS) or better resistance.

“Genetics works. We just need to deploy it,” he said.  “Fortunately, our breeders do a huge amount of work at getting resistance into varieties. And avoiding susceptible (S) and very susceptible (VS) varieties does provide ongoing disease control.”

Avoiding S and VS varieties was important, because they can result in direct yield loss, and they produce a high inoculum load that increases reliance on fungicides and promotes fungicide resistance, he said.

Slowing fungicide resistance

With fungicide resistance now common in cereal pathogens, adopting disease management strategies to provide effective protection and help slow resistance were important, he said.

Along with variety and paddock selection (such as avoiding planting susceptible crops in high-risk zones) this included other proactive non-chemical solutions such as avoiding planting into infected stubble, rotating crops, delaying sowing and grazing early.

Fungicide resistance could also be slowed by avoiding unnecessary fungicide use and ensuring products with different modes of action were mixed and rotated. Group 7 SDHI and Group 11 Qol fungicide actives should not be used more than once in a growing season, even if applied on seed or as fertiliser, he said.

“It’s important to note that crops can tolerate low disease levels, so only use fungicides when necessary and don’t overuse them,” he said.

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