To overcome their host, yellow leaf spot (YLS) and septoria nodorum blotch (SNB) must find a way to work together, with devastating consequences
Like contestants on Survivor, two of wheat’s most damaging fungal pathogens have been caught in a strategic alliance, with farmers paying the price. CCDM researchers have discovered that the pathogens behind two related foliar diseases don’t just coexist on wheat leaves; they form opportunistic partnerships. The work shows Pyrenophora tritici-repentis (YLS) and Parastagonospora nodorum (SNB) can shatter host resistance by working together.
In a battle for survival, alliances are never equal, with research revealing that timing is everything. When the YLS pathogen establishes first, it increases the damage done by SNB, allowing both pathogens to thrive and overwhelm host resistance in elite cultivars like Scepter. Flip the arrival order however, and it’s a different game entirely. When the SNB pathogen arrives first it betrays YLS by inhibiting its growth to vote it off the island.
This breakthrough is the result of a five-year research effort led by scientists at the Centre for Crop Disease Management (CCDM), in collaboration with the Department of Primary Industries and Regional Development (DPIRD) and Australian Grain Technologies (AGT), supported by the Grains Research and Development Corporation (GRDC).
The team set out to investigate how SNB and YLS pathogens interact when they infect the same plant. With mixed infections regularly reported in the field, researchers wanted to explore the dynamics that often appear in paddock, ensuring we are looking at the reality of what is happening on the ground.
CCDM Research Assistant Leon Lenzo using digital PCR technology to investigate co‑infection dynamics in wheat diseases.
CCDM Research Assistant Leon Lenzo using digital PCR technology to investigate co infection dynamics in wheat diseases.
Digging Deeper with Digital PCR
In 2022, CCDM scientists ran trials in the growing season at AGT’s long-term disease nursery in Northam and trial plots at the DPIRD nursery in South Perth. These trials tested how infection order influences disease severity using cultivars with different resistance levels.
To understand the plant biology of the infected samples, researchers turned to molecular tools, developing a new PCR testing method. The digital PCR (dPCR) accurately measured how much disease was in the plants and showed how it led to tissue damage.
CCDM researcher Leon Lenzo stated the new dPCR method was an essential development for this study.
“Studying disease symptoms such as lesion sizes works well to estimate the pathogen presence when you have controlled conditions and only a single disease infection. However, using digital PCR we could reliably distinguish between these two common diseases, and get accurate quantifications on their relative presence within the crop,” he said.
Researchers believe tools like this could also be valuable for industry, supporting more accurate disease identification as new technologies become more accessible. For growers, this opens the door to better diagnostics that complement variety choice, rotation and other on‑farm management decisions.
The team’s key findings included:
- Of the naturally infected varieties sampled, about two in three plants contained both pathogens. Co-infected samples showed greater disease levels and more pathogen biomass than when either disease occurred on its own, particularly in moderately resistant varieties.
- Visual inspection is not sufficient for diagnosis for these diseases. YLS and SNB symptoms look very similar, and you can’t reliably tell if one pathogen or both are present just by eye.
- Environmental factors such as weather stress and natural aging can also change how wheat plants look, making disease symptoms harder to interpret and reinforcing the need for molecular tools to accurately measure infection levels.
These findings are backed up by DPIRD research scientist Jason Bradley, who can relate the results back to what he’s seen and heard from the field.
“These diseases have always been grouped as a complex since field identification is notoriously difficult. It’s invaluable to know what pathogens we are dealing with thanks to the dPCR technology,” said Jason.
Watch the video for more information on this finding.
What It Means for Management
While resistance ratings still focus on a single disease, current management recommendations aim to preserve this resistance long term. DPIRD plant pathologist Geoff Thomas said effective management rests on two key principles: variety selection and rotation.
“Most of our varieties aren’t too bad for YLS currently, which is a big advantage, especially now that we know this disease can help facilitate SNB infection,” Geoff said.
“Variety choice is the first step, and rotation is the real winner.”
“If you can avoid or reduce proximity to infected stubble, you reduce your risk and delay the onset of yellow spot infection.”
For growers in higher rainfall zones or running wheat-on-wheat programs, choosing varieties with stronger resistance can reduce or even remove the need for fungicide intervention.
Growers and consultants can also use DPIRD’s YellowSpotWM decision support tool, which combines paddock-specific inputs with current resistance ratings to predict disease severity, yield loss and the economic return of different management options for this disease, in WAthe tool accounts for the likelihood of YLS/SNB co-infection.
Yellow leaf spot and septoria nodorum blotch are unlikely to disappear, but informed breeding, surveillance and stewardship can keep the industry ahead.
The Final Vote on disease ratings
Like Survivor contestants, YLS and SNB don’t just battle the host. They size each other up, form alliances, and exploit timing to gain the upper hand. Sometimes they work together to beat the challenge of host resistance. Other times they use sabotage and subterfuge to force their competitors off the island.
In this game of survival, understanding the rules may be the key to keeping wheat standing long after the votes are cast. For decades, wheat disease ratings have focused on single pathogens, but by uncovering these hidden alliances and betrayals, CCDM researchers can provide more information to help create stronger management strategies in the future.
“If your plant is resistant to just one of these pathogens, it actually might need to be resistant to both before you can give it that high disease rating” Leon said.
“Thus, we highly recommend that disease resistance testing of commercial wheat cultivars from here on consider rating for the SNB-YLS complex over the current single disease resistance rating system in wheat-growing regions where both pathogens are endemic” said CCDM Associate Professor Kar-Chan Tan.
Geoff said the findings help explain why disease outcomes in the paddock might not always match expectations based on single pathogen ratings.
“This work shows how these pathogens interact and how infection order influences disease expression and resistance,” he said. “Every time we release a new variety, insights like this help improve resistance rankings and add real value for growers.”
For breeders, the research reinforces the need to design varieties that can withstand not just individual diseases, but the complex interactions that occur under field conditions. AGT breeder Dr. Dion Bennett said the work adds an important layer of realism to breeding decisions.
“This research improves our understanding of how yellow spot and septoria nodorum blotch interact,” he said.
“As breeders, it helps us think more carefully about how we select for resistance, knowing that these diseases don’t act alone in the paddock.”
Dr. Bennett believes this deeper understanding will ultimately help growers rely more on genetic resistance and less on fungicides to manage disease pressure.
