Identification and Control of Downy Mildew on Onions and Shallots

Downy mildew is a fungal disease caused by oomycetes, primarily Peronospora destructor , that can seriously affect onion and shallot crops. This article will help you identify signs of contamination and implement effective methods to control this pathogen.

Summary

• Identification of Mildew Contamination
• Methods of Controlling Mildew
• Prevention of Contamination
• Detailed Processing Steps
• FAQ
• Economic Impact
• The most resistant shallot varieties
• The most resistant onion varieties

Identification of Mildew Contamination

Mildew contamination can manifest itself in different ways on onions and shallots:

• Onions: Early signs include yellow spots on the leaves, which later turn brown. Leaves may also develop a gray or purple mold under humid conditions.
• Shallots: Symptoms are similar to those seen on onions, with yellow spots on the leaves that become necrotic, and the development of mold in humid conditions.

Methods of Controlling Mildew

To combat mildew, several methods can be implemented:

• Cultural hygiene: Remove and destroy infected plant debris to reduce sources of inoculum.
• Chemical treatment: Use appropriate fungicides according to local recommendations. Be sure to follow the manufacturer's instructions for maximum effectiveness.
• Crop rotation: Avoid growing alliums (onion, shallot) in the same soil for several consecutive years to reduce pathogen buildup.

Prevention of Contamination

Prevention is essential to minimize the risks of contamination by mildew:

• Aeration and drainage: Ensure good air circulation and adequate drainage to reduce excessive moisture, which promotes mildew growth.
• Plant Spacing: Plant bulbils with sufficient spacing to avoid crowding and improve air circulation.
• Humidity Control: Avoid overwatering and evening watering to limit prolonged moisture on leaves and bulbs.
• Use resistant varieties: When possible, choose disease-resistant varieties of onions and shallots.

Detailed Processing Steps

Here is a step-by-step guide to treating mildew with fungicides:

1. Identify infected areas and remove affected plant debris.
2. Choose a fungicide recommended for downy mildew, such as those containing mancozeb, chlorothalonil, or copper.
3. Mix the fungicide according to the manufacturer's instructions.
4. Apply the fungicide evenly to the plants, concentrating on infected areas.
5. Repeat treatment according to the recommended schedule, generally every 7 to 10 days during periods of high humidity.

FAQ

Here are some frequently asked questions about mildew:

• When is the best time to apply fungicides? It is generally recommended to apply fungicides during periods of high humidity or rain, when the risk of contamination is highest.
• Are biological treatments effective against mildew? Some biological treatments, such as plant extracts or beneficial microorganisms, can help reduce mildew infections, but they are often less effective than chemical fungicides.

Economic Impact

Downy mildew can have a significant economic impact on onion and shallot crops, resulting in yield losses and reduced product quality. Proactive management of this disease is essential to minimize these losses and ensure profitable production.

The most mildew-resistant shallot varieties

• Melkior Shallot
• Yellow Moon Shallot
• Biztro Shallot

The most resistant onion varieties to mildew

• Setton Onion
• Red Baron Onion
• Romy Onion