A confluence of factors has created a higher risk of the fungal pathogen Fusarium causing Fusarium crown rot (FCR) and potentially Fusarium head blight (FHB) infections in this season’s winter cereals.

GRDC grower relations manager Graeme Sandral says FCR is the “big sleeper” for 2023.

“If rainfall is average or below average this year, FCR will most likely hit hard,” he says.

The Bureau of Meteorology’s modelling indicates that when La Nina breaks it can often swing hard in the opposite direction. This can result in drier-than-average conditions.

Crown rot effects are greatest in periods of moisture and/or heat stress. In addition, crown rot inoculum has been building over the past three years following big cereal crops with high stubble loads.

What are Fusarium crown rot and head blight?

Fusarium crown rot (FCR) is caused predominantly by the fungal pathogen Fusarium pseudograminearum and is characterised by brown basal infection of tillers.

Fusarium head blight (FHB) is usually caused by the fungus Fusarium graminearum and likely comes from airborne spores produced on maize or sorghum stubble or some known grass weed hosts.

However, the crown rot fungus Fusarium pseudograminearum may cause FHB in wet years when rain splash distributes the fungus from lower stem nodes into heads during flowering or early grain fill.

Both become apparent after flowering, in different ways. FHB requires prolonged wet weather during flowering and grain fill. This compares to FCR, which expresses itself as whiteheads following periods of moisture and/or heat stress.

Preparation

New South Wales Department of Primary Industries (DPI) senior plant pathologist Dr Steven Simpfendorfer says being prepared is important and suggests considering the following elements.

Seed source

“Think about seed sources for this season’s winter cereal crops. This includes considering Fusarium infection level, germination and crop vigour,” Dr Simpfendorfer says.

Test grain well in advance of sowing if any level of FHB was noticed in the seed crop in 2022 or white grains are evident in the retained seed, he says.

Sowing Fusarium-infected grain results in seedling blight (death) and can introduce FCR infection into surviving plants. Grain with moderate (greater than five per cent) to high (greater than 10 per cent) Fusarium infection usually has lower germination and vigour. This is because the fungal pathogen replaces the seed contents with its own mycelium.

If seeds with low levels of Fusarium infection (less than five per cent) are being sown, he says a fungicide seed treatment should be considered.

Cleaner seed should be sourced if Fusarium grain infection levels are greater than five per cent.

Paddock planning

Dr Simpfendorfer also says each paddock’s Fusarium risk should be taken into account. “Test with PREDICTA® B or NSW DPI stubble testing and then triage those paddocks at a low risk that will be planted to barley, bread wheat or durum.”

Durum should not be considered in moderate-risk paddocks, while high-risk paddocks are ideally suited to non-host broadleaf crops such as canola or a pulse.

Preparing also means ensuring the green bridge (volunteer wheat) is controlled well in advance of sowing for stripe rust management. “This is because a late herbicide spray-out of volunteers and grass weeds can increase Fusarium inoculum levels at sowing.”

In paddocks with cereal stubble and a moderate level of Fusarium infection, he suggests thinking about how risks may be reduced.

“That may mean not cultivating, as that spreads stubble-borne inoculum into the infection zones below ground close to sowing. Also, think about inter-row sowing to reduce contact with inoculum confined to intact standing cereal rows. Aggressive strategies such as a late stubble burn to reduce above-ground inoculum levels, although not normally considered, may be required given the elevated FCR risk this season.

“However, this seriously needs to be balanced against other benefits related to stubble retention and a high probability of restoring stubble loads this season through successfully growing a cereal crop in burnt paddocks. We do not want to see a repeat of 2017 where failed cereal crops left paddocks bare until 2020 in some regions.”

Dr Steven Simpfendorfer says being prepared for potentially higher cereal pathogen risks this season is important. Photo: Nicole Baxter

If raised black structures (perithecia) of Fusarium can be seen around nodes on sorghum, maize stubble or winter cereal stubble, this presents an elevated FHB risk from Fusarium graminearum.

Perithecia contain small spores (ascospores) that are airborne and can more readily infect heads at flowering, causing FHB. These structures are not produced with Fusarium pseudograminearum, which relies on larger macroconidia produced around nodes on FCR-infected tillers to rain splash into heads during flowering to cause FHB infections.

Sowing winter cereals into or adjacent to paddocks with visible perithecia represents an elevated risk of FHB, which should be considered in regard to rotation, cereal choice, stubble management and planning for fungicide application at early flowering.

Within this planning stage, it is also important to consider the cut height of 2022’s cereal stubble if it was infected with Fusarium to limit inoculum redistribution.

This is particularly important if a chickpea break crop is going to be planted. The Fusarium fungus can potentially grow vertically up the standing wheat stubble during the season to above the harvest height of the 2023 chickpea crop. If this occurs, it can result in Fusarium-infected wheat stubble being spread from the back of the header across a paddock during 2023’s chickpea harvest.

Sowing

Dr Simpfendorfer says sowing at the start of the recommended window suited to the local area will also potentially help limit FCR disease expression later in the season.

“What are the long-range weather predictions for spring and how much plant-available water is in the soil profile? If the season is predicted to be hot and dry, then consider target yield potential at sowing, especially if there is a Fusarium risk. That is, think about plant population and nitrogen inputs to help limit the severity of crop stress during grain filling. This will in turn reduce the extent of yield loss within plants infected with FCR.”

Spraying

If there is an elevated risk of FHB in 2023 with predictions of extended rainfall periods during flowering and grain fill, be prepared. “Have fungicide (prothioconazole and tebuconazole) on hand for a flowering application. And think about access to a ground rig at the critical time for a flowering application. Make sure it is set up properly for an FHB spray.”

Research has shown that spraying at flowering (GS61) is more effective in limiting FHB and has more yield benefits than spraying seven days before flowering.

Spray coverage also needs to be considered, with overseas research demonstrating its importance in FHB control. Twin nozzles (forward and backward facing) angled to cover both sides of a wheat head and high volumes of water (at least 100 litres per hectare) are critical to efficacy. However, at best this still provides about 80 per cent control.

In contrast, aerial applications provide poorer head coverage. At best they provide about 40 to 50 per cent control. “Some agronomists who used this application method in 2022 are questioning if the efficacy is even this high after their experience.”

Dr Simpfendorfer says while fungicides such as Prosaro® is usually only applied to durum wheat in parts of northern NSW that have dealt with FHB since 1999, this may change. “Application to bread wheat has never previously been deemed economical. Given this year’s experience, it is likely to be reconsidered, but where do we draw the line?”

Why all the concern about Fusarium crown rot this coming season?

Graeme Sandral and newly appointed GRDC grower relations manager (north) Rebecca Raymond say there are many factors, built up over the past few years, that are indicating higher-than-usual disease risks.

“Fusarium crown rot inoculum has been building over the past three years following big cereal crops. In 2022 it was particularly bad and manifested in FHB head infections, which are not typical,” Mr Sandral says.

These head infections create white grain. A high proportion of this is blown out the back of the header at harvest, he says.

Ms Raymond says that failed cereal crops from flooding events also need to be managed and could lead to more FCR inoculum build-up as Fusarium can continue to grow up the infected tillers with moisture.

She says that low chickpea prices have also pushed more cereal-on-cereal crop sequences, which favour the build-up of inoculum within paddocks. At the same time, a 50 per cent chance of a drier-than-average outlook for 2023 could create the perfect storm for FCR impacts.

“Fusarium crown rot can lead to a root system and stem bases being limited in their capacity to translocate nutrients and water to the shoots. In wet years the plant isn’t stressed and can handle the infection better, but during drier years the plant can really struggle.”

NSW Department of Primary Industries researcher Dr Steven Simpfendorfer says Fusarium has been the pathogen detected at the highest concentrations in random crop surveys across NSW from 2019 to 2022.

“In 2019, Fusarium levels in the base of cereal plants were higher in southern NSW crops, but by 2021 this had shifted to higher levels generally in northern NSW and southern Queensland.”

However, levels have not declined in the south – rather they have simply exploded with consecutive wet seasons in the north.

How did we get here?

The answer to this lies in how the previous few seasons have played out.

In 2019, cereal crops in NSW were coming off a relatively low FCR risk level.

“The extended drought conditions during 2017 and 2018 had considerably reduced cereal stubble cover in most paddocks,” Dr Simpfendorfer says.

To restore ground cover post-drought, a significant planting of cereal crops occurred in 2020. “This consequently carried through to elevated levels of cereal-on-cereal within rotations in 2021.”

Crown rot build-up is favoured in wet conditions, which 2020, 2021 and 2022 delivered, but will make itself known when the crop is moisture and/or temperature stressed during grain-filling.

The mild grain-filling conditions over the past three seasons across most of the northern region have masked the expression of FCR.

Cereal survey maps, however, clearly paint a concerning picture. They show that significant levels of Fusarium crown rot infection and subsequent inoculum loads have built up in paddocks over this period.

Dr Simpfendorfer says it is also important to know which species of Fusarium you may be dealing with if you experienced FHB in 2022.

“Knowing the Fusarium species of head blight – or even if it’s white grain disease (WGD) – is important in determining the likely inoculum source and management going forward.”

Both diseases also have implications for late-season harvesting and seed retention decisions for 2023 sowings.

More information: Steven Simpfendorfer, 0439 581 672, steven.simpfendorfer@dpi.nsw.gov.au

FHB and WGD diagnosis and species identification

Send 15 heads with symptoms in paper envelopes with variety and location to Tamworth laboratories (below). No charge as funded by the GRDC project.

Testing of grain infection levels

Send 200 to 250 grams of seed in plastic zip-lock bags with variety and location to Tamworth laboratories (below). No charge as funded by the GRDC project.

Send head or grain samples

Steven Simpfendorfer
NSW DPI
4 Marsden Park Rd
Tamworth NSW 2340

Pre-sowing paddock FCR/FHB risk

PREDICTA® B soil/stubble testing available through SARDI. Sampling Protocol PREDICTA® B Northern Regions PDF

Or alternatively contact Steven Simpfendorfer (0439 581 672) about stubble plating.