Managing soil borne diseases of onions (VN13003)

What was it all about?

Running from 2013 to 2016, this project had a focus on improving the management of onion stunt, a problematic soil borne disease that reduces the size and quality of onion bulbs and is caused by fungus Rhizoctonia solani AG8. The effects of root-lesion nematode Pratylenchus neglectus and pink-root-causing Setophoma terrrestris were also investigated.

The research culminated in the production of the Best practice guide for onion stunt.

Activities of the project included field work in 43 paddocks in South Australia over two growing seasons. A key part of the work was the evaluation of sampling strategies for the pathogens, to ultimately enable growers to determine the risk of onion stunt before planting (with most decisions to reduce the impact of soil borne diseases needing to be made and implemented before crop planting).

The researchers noted that measuring Rhizoctonia solani AG8 DNA levels in the soil prior to planting provided a useful indication of the risk of onion stunt, with sampling prior to or early in the preparation of ground for sowing providing the best estimate of disease risk. The established protocol involves collecting four separate soil samples from along four transects around an onion pivot, and testing them individually to assess disease risk.

Prediction of disease risk was improved by also including the pre-planting numbers of Pratylenchus neglectus, which was associated with large areas of reduced plant growth and loss of yield in paddocks in the absence of Rhizoctonia solani AG8.

Setophoma terrestris was also associated with low productivity, especially in paddocks of red onions.

The knowledge developed lays the foundations for the delivery of a testing service to enable onion growers to identify the risk of onion stunt before planting, so they can implement appropriate management strategies to reduce losses.

The researchers also investigated cost-effective management strategies for onion stunt. Trials were conducted to assess the impacts of rotation crops, nurse crops and other pre-plant practices. For example, at sites where a high risk of onion stunt existed, the effectiveness of breaking up the Rhizoctonia solani AG8 fungal network in the soil by intensive cultivation and deep ripping to reduce disease risk was confirmed.

The importance of the choice of cereal nurse crop and the timing of nurse crop removal, was also demonstrated in relation to competition with the onion seedlings and the build-up of Rhizoctonia solani AG8 at this critical growth stage.

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Access the Best practice guide for onion stunt