Onion white rot is a highly destructive fungal disease of commercial onion crops. The disease is widespread across Tasmania’s coastal production areas and is rapidly spreading to less intensively cropped areas.

This project sought to better understand the disease and its control, including the conditions that precede high-risk infection periods and how growers can optimise timings of fungicide applications to control white rot. It ran from 2015 to 2018.  While the researchers initially hoped to develop a forecast model for white rot infection periods in Tasmania, due to variability within disease incidence across seasons a fully operational model was not able to be developed – however the project still identified major risk factors that will be of benefit to Tasmanian onion growers.

The results showed that:

• Onion white rot incidence was not directly associated with root development, suggesting that factors other than root growth influence disease outbreaks
• Disease risk is primarily determined by the prevalence of inoculum in the top 100mm of soil, where onion root biomass is the highest
• Disease risk is related to temperature, with incidence decreasing at temperatures above 20°C
• Higher soil surface temperatures may kill pathogens in the critical top 50mm of soil
• Disease risk in Tasmania is lowest in late plantings
• Pathogens present at depth may survive higher soil surface temperatures and result in late infections.

The project made the following recommendations for growers:

• Fungicide applications should generally target the top 100mm of soil – in the project’s studies, this is where more than 80 per cent of onion roots were found and disease risk was the highest.
• Fields with a moderate to high risk of onion white rot are best planted later in the season, however this does not negate the need for fungicide control.
• Late fungicide applications are recommended to prevent late infections. The project results indicated that the white rot fungus may be killed in the top 50mm of soil during hot weather conditions but survives at lower, cooler soil depth. This fungus can remain active and progress up towards the bulb if soil temperatures are lower towards the end of the season and before harvest.

Read the fact sheet “Managing onion white rot in Tasmania” produced by the project

Read the article ‘Development of an onion white rot forecast model for Tasmania’, produced during the project’s trial stage, on page 32 of the 2016 Onions Australia magazine

Watch this video to hear researcher Dr Suzie Jones from the Tasmanian Institute of Agriculture talk about the project and its findings