White rot forecast model for Tasmania’s onions

Field Onions

Suzie Jones, Tasmanian Institute of Agriculture

The Tasmanian Institute of Agriculture (TIA) is conducting important research to help growers better predict and control white rot disease in onions.

TIA has been leading research trials over the past two years to measure onion root growth, disease development and environmental conditions, including soil temperature, soil moisture and crop growth.

This information will be used to determine the conditions that precede infection and to develop a risk prediction tool to help growers better control the disease. For example, the information will help onion producers identify when disease is likely to be active in the soil so that fungicides can be applied during periods likely to provide the most effective control.

Onion white rot is a destructive fungal disease that reduces the yield of commercial onion crops. This soil borne disease is caused by Sclerotium cepivorum and affects Allium species including garlic, leek, chives and spring onion.

The fungus infects plant roots and progresses up the roots to the base of the onion bulb. Infection can spread from the roots of one plant to another, resulting in patches of infected plants.

Early above-ground symptoms are white fluffy mould on the stem plate (the junction between the bulb and the roots) and on the lower part of the bulb near the soil surface.

Infection then moves up and into the bulb causing the bulb to rot. As the plant tissues decay, the fungus forms small black reproductive structures (sclerotia). These structures resemble poppy seeds and can lie dormant in the soil for up to 20 years. Sclerotia germination is stimulated by volatile compounds given off by allium roots. When a new onion crop is planted in infested soil, the infection cycle can begin again.

Fungicides are the basis of an integrated disease management approach that includes crop rotations and avoidance of fields known to have had the disease. Fungicides do not affect or eradicate sclerotia, but they can reduce the rate of fungal growth, spread of infection and the subsequent level of affected plants.

Control of the disease at this early stage not only helps to improve saleable yield, but also reduces the chance of sclerotia forming and contributing to disease in subsequent seasons.

Onion white rot infection occurs on plant roots before the above-ground symptoms are visible. This means effective fungicides need to ideally be applied before above-ground symptoms are seen.

Onion bag trial

Onion root growth data will be collected from six commercial fields for three planting periods: May, July and September.

Planter bag trials are being established at the Tasmanian Institute of Agriculture Vegetable Research Facility at Forthside.

To provide additional data on conditions that promote infection, soil temperature and moisture will also be monitored at an additional three fields (one for each planting period) that are considered to be at risk of developing onion white rot infection.

The trial sampling period commenced in July and will continue until February 2018.

The project is funded by Hort Innovation, using the onion industry levy and funds from the Australian Government.

This article originally appeared in the Advocate on 28 September 2017.

Published on: 03 Oct 2017 9:38am