Downy mildew can cause wide-ranging destruction to horticultural crops, and this fungal disease flourishes in cooler, wet conditions. In this column, Syngenta Technical Services Lead – Horticulture and Viticulture Dr Belinda (Brandy) Rawnsley discusses preventative measures growers can implement to limit the development of downy mildew in their crops.

The forecast of above average rainfall events has increased the likelihood of diseases in onion crops across Australia.

Wet, cool weather can increase the risk of downy mildew infection. This disease can occur relatively quickly and spread rapidly. Spores are easily dispersed by wind and splashed by rain or irrigation.

Downy mildew disease symptoms will initially show as pale, oval shaped spots. Masses of grey fungi are easily visible and cause the leaf to yellow and die.

This disease can cause devastating yield loss from leaf death, poor quality bulbs and reduced bulb development.

There are cultural practices growers can implement, such as timing of irrigation and weed control, to reduce the risk of downy mildew. For example, onions should not be watered early in the morning when fungal spores are released. This is particularly the case for low lying sections of paddocks and on plants wounded by hail, wind or herbicides where environmental and physical conditions are conducive to fungal infection.

Management advice

Preventative fungicide programs can stop development of downy mildew, particularly when wet weather is forecast.

To achieve good preventative control of downy mildew, application timing is critical. Spray prior to a rain event to protect the leaves from infection and prior to the first signs of disease. Schedule sprays at 7-14 days intervals (always follow label directions), using the shorter spray timing if rain persists.

For onions, using angled nozzles that spray forward and backwards can improve coverage on upright leaves. This ensures both sides of the leaf are protected.

Systemic fungicides, such as ORONDIS® FLEXI, penetrate the plant to protect old and new leaf growth. In comparison, contact fungicides (e.g. mancozeb) do not enter plant tissue. For this reason, coverage when using contact fungicides is critical to obtain good disease control.

Managing downy mildew in onion crops requires specific strategies to minimise fungicide resistance developing. When wet conditions do favour disease, it is important to use fungicides preventatively and rotate fungicide groups. ORONDIS FLEXI is formulated with two modes of action (Group 49 and Group 11) for built-in resistance management.

Other products may require tank mixing to ensure the fungus is killed by several modes of action. Having activity on other key diseases, ORONDIS FLEXI also provides suppression of white rot in bulb vegetables, including onions.

Plants infected by downy mildew can be more susceptible to secondary fungal infections, such as Stemphylium and Alternaria (purple blotch). As they are weak pathogens, these fungi occur on damaged leaves, and can further reduce yield and quality.

Curative fungicides are available for downy mildew control. They are best used immediately after an infection period and their efficacy can be reduced if application is delayed

To best minimise the risk of downy mildew this season, implement a good preventative fungicide program and use the weather forecast to get your sprays on your crop in good time.

Syngenta Technical Services Lead – Horticulture and Viticulture Dr Belinda (Brandy) Rawnsley.

Find out more

For more information or to ask a question, please contact your local Syngenta Territory Manager, the Syngenta Advice Line on 1800 067 108, visit the Syngenta website or email Vegetables Australia: Please note that your questions may be published.

The R&D content for this article has been provided to Vegetables Australia to educate Australian vegetable growers about the most relevant and practical information on crop protection technologies and their on-farm applications.

This article has appeared in Vegetables Australia – Summer 2020/21. To read the full publication, please click here.