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Tasmanian Institute of Agriculture

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Published: 28 Sep 2018

Suzie Jones

Onion white rot is a destructive fungal disease that’s common throughout Tasmania’s coastal production areas and spreading rapidly.

Research by the Tasmanian Institute of Agriculture (TIA), funded by Hort Innovation, is investigating how to reduce crop losses by identifying the key factors that influence disease development.

For the last two growing seasons, Plant Pathologist Dr Suzie Jones has been measuring root development in six commercial onion fields and in planter bag trials at TIA’s Forthside Vegetable Research Facility.

“Understanding how onion roots develop is important because the roots initiate fungal infection and drive disease spread,” she said.

The fungus is called Sclerotium cepivorum, and affects not just onions but also garlic, leek, chives, and spring onions.

“The fungus grows along the roots, and moves up and into the bulb causing the bulb to rot. As the plant tissues decay, the fungus forms small black ‘survival structures’ called sclerotia,” said Dr Jones.

Sclerotia resemble poppy seeds and can lie dormant in the soil for as long as 20 years, and perhaps longer.

Previous research in Tasmania has shown the threshold for severe disease to be as low as 40 sclerotia per kilogram of soil, which means the risk of white rot can be difficult to detect.

When a new onion crop is planted in infected soil, the cycle begins again, because chemicals given off by the growing roots trigger the germination of the sclerotia.

“Our field trials have shown that more than 80 percent of the biomass of onion roots is in the top 10cm of soil,” Dr Jones said.

“The planter bag trials showed that disease symptoms are highest when the fungus is in the top 10cm of soil too.”

Chairman of the Tasmanian Onion Agronomy Group, Mr Tim Groom of Wynyon Pty Ltd, said this is new and important information for growers and industry advisers.

“It indicates that the fungicide needs to be applied in the top 10cm of the soil. Prior to this research, we were just guessing,” he said.

“Now we know that to better control onion white rot we should target that part of the root zone by irrigating fungicides into the soil.”

Fungicides do not eradicate sclerotia, but they can reduce the rate of fungal growth and the amount of crop affected.

Dr Jones said the research also found that soil temperature seems to have an important influence on disease outbreaks.

Commercial onion crops in Tasmania are planted from May until September. The bag trials planted in May and July had much higher disease incidence than those planted in September.

“We think the lower disease incidence in the September plantings may be partly explained by higher soil temperatures,” Dr Jones said.

“This suggests that fields considered to be of moderate or high disease risk should be planted later in the season to minimise the chances of an outbreak.

“While hot weather conditions and soil temperatures above 23 °C are believed to kill the fungus, it can persist deeper in the soil where it is cooler, so late plantings may also benefit from a late fungicide application to prevent infection reaching the bulbs.”

“It would be good to do further work on a model for predicting disease risk that includes the effects of environmental conditions like temperature on the germination of sclerotia and fungal growth under field conditions.”

Throughout this project, Dr Jones has been in discussion with Dr Fred Crowe, a world-leading US authority on white rot in onions and garlic.

He has researched organic alternatives to fungicides, such as treating soil with garlic extracts to ‘trick’ the sclerotia into germinating early and then dying off, which allows crops to be planted after the soil has been ‘cleaned’.

Dr Crowe will be speaking at the Forthside Vegetable Research Facility Open Day on Wednesday 10 October. This family event is free to attend, and offers the opportunity to see agronomy trials run by TIA and ask questions of researchers.

The Open Day runs from 10am to 6pm at 124 Forthside Road, Forth, and will end with a spit roast dinner.  More information and RSVP online.

This article appeared in Tasmanian Country on 28 September 2018