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Harvest of onion trials in SA continue to shed light on detection and management of onion basal rot

The project Epidemiology and management of fusarium basal rot in onions (VN20006) aims to develop an integrated pest and disease management (IPDM) strategy to reduce the impact of fusarium basal rot in onions.

Infection of onion bulbs in the field by soil borne fusarium basal rot is known to result in substantial losses before harvest and in storage, however disease epidemiology was not well understood, which has limited the development of an appropriate management strategy.

In order to develop a best practice, cost-effective IPDM strategy, the project is working toward understanding of the pathogen and its epidemiology, and evaluate the use of cultural, biological and chemical controls.

Now in its final year, the project has identified Fusarium oxysporum f. sp. cepae (Foc) as the main cause of fusarium basal rot symptoms of onions in Australia.

During the 2022 and 2023 planting seasons, to understand key drivers of disease development, crops were monitored in paddocks that had a range of rotation and production practices, soil moisture conditions, and varieties. Incidence of bulb rot associated with Foc species ranged from nil to 55 per cent in monitored areas.

Testing of samples in the 2022 season at the five to seven leaf stage indicated that Foc infection can already have occurred, though may not be visually evident until late in the crop or storage. Subsequent testing this season has found some infection is already present at the one to two leaf stage, indicating control options may need to be applied at planting or early in the crop.

One key outcome of the research is that managing soil moisture to limit prolonged high or low soil moisture is required to optimise yield and reduce the risk of fusarium basal rot. Results from the first season of monitoring highlighted the impact that high and prolonged soil moisture can have on increased incidence of bulb rots caused by Foc and bacterial infection.

Trials and monitoring in the second season found that development of basal rot can also be favoured by persistent lower than optimum soil moisture through the duration of the growing season, that is at soil moisture levels at which some yield reduction occurs. In addition, while high and prolonged soil moisture supports basal rot development, continuously waterlogged conditions appear to be too wet for high levels of disease. Such conditions dramatically reduce yield and can favour bacterial rots.


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