The sclerotinia management challenge

Photo of Moora grower Andrew Nixon

Moora grower Andrew Nixon has been working with researchers to try to develop more effective management approaches to controlling sclerotinia in canola.

Moora grower Andrew Nixon is looking at a variety of ways to manage the long-term impact of the fungal disease sclerotinia on his canola production

Three years ago, the fungal disease sclerotinia wiped out 60 per cent of the total yield potential from some of Moora grower Andrew Nixon’s canola crops.

Snapshot

Growers: Andrew and Tonia Nixon
Properties: Nunnook, Mount Yule, Mallory and Springdale
Location: south-east Moora, Western Australia
Farm size: 10,000 hectares
Average yearly rainfall: 450 to 500 millimetres (long-term average)
Soil types: medium to heavy loam
Soil pH: 5.5 to 6.5 

With an average yearly rainfall of between 450 and 500 millimetres, combined with occasional warm late-winter temperatures, canola crops in the Moora district of Western Australia have offered rich pickings for the disease and growers have had a huge task to keep it at bay.

In 2016, with 500mm falling before the end of August, and with a massive canopy creating a humid environment within the crop, the conditions were perfect for the Sclerotinia sclerotiorum fungus to breed, causing an outbreak of the sclerotinia stem rot disease.

These almost-perfect growing conditions forced growers to consider all options to tackle the disease long term.

Andrew says that once the disease takes hold of the plant it is almost impossible to eradicate. “Given the wet conditions we had, combined with some warmer winter temperatures during flowering of between 18ºC to 20ºC which caused high canopy humidity, the sclerotinia was definitely worse than ever before,” Andrew says.

With his cropping program accounting for 95 per cent of business profits, and with 30 per cent of his total farming business reliant on canola production, protecting the crop against this devastating disease has become a priority.

Andrew believes early intervention is crucial, but says with fungicide applications being little more than a preventive measure, it is important to monitor crops closely as soon as flowering occurs. He says in some years, monitoring should occur even earlier than flowering if there is high disease pressure in crops, particularly since the sclerotinia infections can enter through the roots of canola.

Fungicides that contain prothioconazole (Prosaro®), iprodione or procymidone as active constituents comprise some of the only registered chemicals available. “The spray is like a protective coating and if you don’t get the coating on before the plant is infected, you can’t turn back the infection,” Andrew says.

“So it’s about watching the weather and getting the fungicide onto the plant before that period of ideal weather occurs for the disease to spread and infect plants.”

Treatment tactics

Photo showing sclerotinia stem rot in Andrew Nixon's canola in 2016

Sclerotinia stem rot in Andrew Nixon's canola last year.

PHOTO: Evan Collis

Andrew believes using high water rates from ground application rigs is critical to get under the thick canopy and protect the plant stems. “It’s hard to get canopy penetration from an aerial spraying technique,” he says.

He discovered different levels of the disease on two separate parts of the farm, so two fungicide strategies were applied.

On one part of the farm, as a trial he sprayed 400mL per hectare of Prosaro® at 30 per cent flowering and then undertook a second pass at 60 per cent flowering stage. (The Prosaro®  label specifies the full range of flowering time options.)

On the second part of the farm he was able to use just one pass at the 50 per cent flowering stage. “These strategies come at a high cost to profitability, but once the disease takes hold in the plant, there is no cure,” he says.

With two passes costing more than $80 per hectare, attempting to manage the disease for the long term using agronomic and rotational tactics becomes extremely tempting. “Canola is such a significant part of our cropping program, so we are looking at various strategies to minimise the risk of the disease impacting on our total yield in the long term.”

Photo of canola showing symptoms of sclerotinia

One strategy the Nixons are considering is to widen row spacings (44 centimetres instead of 22cm) to reduce the thickness of the crop canopy, thereby reducing humid conditions in the crop. “There is no clear research data on this strategy yet, so putting this in place in 2017 would be more of an experiment than anything, but it has worked in a similar way with barley,” Andrew says.

Another strategy he is considering is cutting the canola stubble as short as possible, then following up with a stubble burn to kill off the fungi. Changing rotations will also be part of the long-term management plan to reduce the disease’s impact. “Instead of a tight one-in-two-year rotation, we might look at canola every three or four years to reduce our disease exposure.

“Interestingly we’ve been finding that the sclerotinia seems to accelerate after an application of UAN (liquid fertiliser) so now we always make sure we apply the UAN with a fungicide.”

He says that on paddocks where there has not been a tight rotation previously, maintaining the one-in-four rotational intensity will help with tackling the disease.

GRDC-funded trials

Department of Agriculture and Food, WA, plant pathologist Dr Ravjit Khangura has been running GRDC-funded sclerotinia canola trials on the Nixon’s property to investigate, among numerous preventive strategies, the effectiveness of different chemical applications to determine the most cost-effective treatment.

While she says disease pressure on the trials in recent years, particularly in WA’s northern agricultural region, has been low, conditions in 2016 were expected to produce significant results from which to guide improved management options into the future.

Infection explained

Sclerotinia sclerotiorum fungus germinates in autumn if conditions are favourable, forming tiny mushroom-like structures called apothecia that are about five millimetres in diameter. Generally, these apothecia produce millions of spores that then synchronise with flowering times and land on the petals of the flowers, which then, in turn, infect the leaves and the axils of the branches, where the infection is initiated, and cut the nutrients and water to the flowering plant.

Often the plant will lodge because the stem becomes soft and unable to stand up. DAFWA plant pathologist Dr Ravjit Khangura says infections in 2016 were seen to occur through lower leaves that had been touching the ground.

“These infections occur even before the commencement of flowering. The fungus growing through the lower leaves enters into the stem base and can kill the young plant,” she says. “These ground infections were quite widespread in WA in 2016.”

Dr Khangura says, if left untreated, yields can suffer by up to one tonne per hectare in severely affected crops.

Diseases of canola and their management – the back pocket guide

She is investigating the effectiveness of six different fungicides, all with varying active ingredients, which if proven successful should provide growers with alternatives to the registered chemicals currently available.

Dr Khangura believes one of the most important strategies for growers wishing to tackle the disease is longer rotations, to rid the soil of the inoculum. “Tight canola rotations may have contributed to the rapid build-up of sclerotinia inoculum in paddocks. We used to see this disease in the late 1990s, but infections weren’t very high – but from 2007 onwards the disease outbreaks have been on a much larger scale.

“The fungus can survive in soils for five or six years and it will attack all broadleaf crops, including lentils, chickpeas, faba beans and lupins,” Dr Khangura says.

Another part of her trial is to investigate the effectiveness of wide row spacing for canola crops, something that Andrew is keen to apply on his property.

“I’ve been trying for a number of years to look at the effectiveness of wide row spacings but in recent years we haven’t had the disease pressure to make our results viable,” Dr Khangura says.

“What we have found is that wide rows have no adverse impact on the end yield result of the crop when compared with narrow spacings, so hopefully the high disease pressure seen in 2016 will provide us with results that growers can apply on-farm.”

Another strategy she says that could be considered is burying the fungal sclerotes 10cm or deeper in the soil to prevent their germination, possibly through mouldboard ploughing or spading.

She says growers can also determine the extent of disease infiltration by closely examining their harvested seed: “The sclerotes, which look like small mouse droppings, persist in the soil and can be easily detectable in harvested canola,” she says.

In fact, in WA there is a threshold of 0.5 per cent sclerotes in deliverable loads. This is why Dr Khangura reiterates the importance of sowing canola free of sclerotes to avoid introducing the disease into sclerotinia-free paddocks.

Sclerotinia across australia

Sclerotinia has traditionally been most prevalent in the higher-rainfall grain-growing regions of NSW and Victoria’s north-east and western districts. However, in recent years the disease has become more widespread and is now also a concern for growers in south-east South Australia and across the northern, southern and central wheatbelt regions of Western Australia. While the disease is most common in canola, it can also be found in sunflowers, soybeans, lupins, chickpeas, lentils, faba beans and field peas.

According to the GRDC ‘Sclerotinia Stem Rot in Canola’ Fact Sheet, canola crops most at risk are those: in a high-rainfall area, especially if the crop has been sown early at high seeding rates; in, or adjacent to, a paddock that has had a history of sclerotinia in the past four years; in low-lying parts of the landscape such as valley floors, which stay cooler and wetter for longer than nearby hill slopes; and in intensive rotation with other broadleaf crop species including summer crops of sunflowers and soybeans.

Sclerotinia Stem Rot in Canola – fact sheet

 

More information:

Dr Ravjit Khangura, plant pathologist, DAFWA,
08 9368 3374,
ravjit.khanhura@agric.wa.gov.au

GRDC Project Code UM00051

Region South, West