Downy mildew on onion leaves: symptoms, causes, and control

Downy mildew is a fungal disease that affects onion leaves, resulting in the appearance of downy spots and a decline in plant growth. This article from the “WORLD OF PLANTS” website reviews how to prevent and effectively control downy mildew on onion leaves.

Causes of downy mildew on onion leaves

• Disease name: Downy mildew on onion leaves
• Scientific name: Peronospora destructor
• Type of disease: fungal
• Disease family: Peronosporaceae

Powdery mildew infects onion leaves and flower inflorescences through the Peronospora destructor fungus. The disease usually spreads in late spring and summer and can attract other pathogens such as white mold fungi.

Symptoms of downy mildew on onion leaves

The initial symptoms of the infection begin to appear on the leaves, where pale yellow spots appear, then a violet or gray fuzzy growth appears on the leaves and stems, then they turn pale green or yellow. As the infection progresses, the leaves die at the edges and their color changes to brown. If humid conditions are present, white mold then develops. Purple later on the affected parts of the leaf.

Development cycle of downy mildew on onion leaves

The fungus spends the winter dormant on plant remains, and with appropriate conditions, it completes its life cycle within 15 days. It produces spores in the spring that spread the disease to newly planted plants through irrigation or rain water, and is also carried by the wind over long distances.

Suitable conditions for the spread of downy mildew on onion leaves

The fungus needs cool and humid weather conditions to form spores and release them in the field. High temperatures and dry conditions slow down the development of the fungus. It needs water to complete the infection on onion leaves, as it requires a continuous period of wetness.

Losses resulting from the spread of downy mildew on onion leaves

Powdery mildew is not considered a serious disease, as it leads to a reduction in the size of the leaves, which reduces the quality and quantity of production. However, if the infection is neglected, the leaves may die.

Controlling downy mildew on onion leaves

1. Organic control: remove infected plants and dispose of them away from the field, avoid replanting onions in the same place for five years, and avoid overhead irrigation.
2. Chemical control: Use fungicides containing cephalosporin, and avoid using the same active ingredient so that the fungus does not acquire resistance to it.

Preventive measures for downy mildew on onion leaves

1. Leave sufficient distance between plants to improve air flow and avoid the spread of infection.
2. Planting disease-resistant varieties.
3. Sterilize agricultural tools before use.
4. Installing a suitable irrigation and drainage system to reduce soil moisture.
5. Avoid excessive nitrogen fertilization.
6. Getting rid of harmful weeds and plant remains in the field.

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Sources:

Onion downy mildew – The Royal Horticultural Society 
onion disease: Downy mildew – pdf