THREE grower trials conducted in 2020 in the Geraldton port zone investigated fungicide management of sclerotinia in narrow leaf and albus lupins, with the results finding that foliar fungicide applied at early pod emergence significantly reduced, but did not eliminate, incidence of sclerotinia and only gave a small yield response above the untreated.
The small yield response of three to six per cent was unprofitable in narrow leaf lupins, but marginally profitable in albus lupins.
The trials were conducted by growers in commercial paddocks with the Department of Primary Industries and Regional Development (DPIRD) and Mingenew Irwin Group (MIG) conducting yield component and disease assessments.
The collaborative research was an opportunity to better understand the infection process of sclerotinia in lupins, yield and quality impacts and the capacity of foliar fungicide intervention to reduce impacts.
DPIRD research scientist Ciara Beard said sclerotinia stem rot in lupins was a sporadic disease with higher risk in crops that have a history of it, were dense, on heavy soil types, have early canopy closure and good yield potential.
"Sclerotinia development in lupins is highly dependent on there being sufficient rainfall to favour the complete disease cycle and there were several reports of the disease in lupins in WA last year," Ms Beard said.
She said damaging disease levels in lupins were difficult to predict and it was challenging to know how to manage the disease and if management would be profitable as it is not the same as in canola.
In the trials, two of the large-scale onfarm demonstrations were conducted in the one paddock at Narra Tarra, near Geraldton, where albus and narrow-leaf lupins were sown near each other on a similar date.
The third was conducted in a narrow-leaf lupin paddock at Yandanooka, near Mingenew, in a low lying, protected and loamy part of the paddock where sclerotinia had been an issue in the past.
Ms Beard said the sites had a history of sclerotinia which means sclerotia - the fruiting bodies of the fungus - would have been present in the paddock and germinated during wet weather conditions in mid-July and August, which coincided with crops flowering.
"The dense lupin crop canopies at these sites closed over early, and this provided favourable humid conditions for sclerotinia development," she said.
"Disease infection of pods on the main spike is likely the main cause of yield loss in lupin - this has implications for the optimal timing of fungicide application in lupin, which is likely to be later, flowering to early podding, than in canola.
"Disease at the narrow leaf lupin sites in 2020 appeared to be more responsive to fungicide applied at early pod emergence than the albus lupin was."
Based on that, the optimal timing of fungicide to protect the main spike pods might be different between narrow leaf and albus lupin because of the different crop architectures.
Despite foliar fungicide providing a significant reduction in disease at all three demonstrations, there were still significant levels of disease present in treated plants.
That indicates an earlier fungicide timing or multiple applications may have been more beneficial and may have provided a larger yield response and profitable outcome.
Ms Beard said application occurring at full flower on the main stem may have been beneficial in preventing some earlier infections
"The two narrow leaf lupin sites had similar disease incidence but fungicide application was more effective in reducing disease at Narra Tarra, possibly due to the higher water volume used there," she said.
"However, the small yield response from fungicide applied at early pod emergence means this timing of application was not profitable in either of the narrow leaf crops.
"It was marginally profitable in the albus crop as this crop had a higher yield response and albus fetches a higher price at delivery."
Sclerotia in seed is a disease risk for future crops if growers plan to use that seed as a seed source for the next season.
Fungicide application for sclerotinia in lupins may not guarantee a yield response, but may be worthwhile for reducing sclerote production, which in turn would reduce the need to grade seed and reduce inoculum spilling into paddocks for future infection.
At the Narra Tarra site with both lupin species sown at similar sowing dates, sclerotinia infection was observed in albus lupin several weeks before it was first seen in the narrow leaf lupin.
Ms Beard said albus lupins were still flowering in mid-September, compared to narrow leaf lupins.
"That, along with the dense tall bulky canopy, could potentially make the albus species more vulnerable to sclerotinia infection across more of the season, requiring earlier and possibly multiple fungicide applications," she said.
"This requires further research as the mechanism for infection in lupin is still to be determined, especially the role that petals play in the disease cycle."
In trials last year, albus lupins developed sclerotia in the stem while the narrow leaf lupins did not, plus albus lupins had significantly more sclerote contamination of grain than the narrow leaf lupins.
Further research is also needed to investigate the differences between the lupin species, including the sclerote burden left behind in paddock stubble and soil after each growing season and the need to grade the seed to meet delivery and sowing requirements.