Onion Botrytis Leaf Blight – Treating Onions With Botrytis Leaf Blight

By: Mary H. Dyer, Credentialed Garden Writer

Onion botrytis leaf blight, often known as “blast,” is a common fungal disease that afflicts onions grown around the world. The disease spreads rapidly, significantly affecting quality and yield when harvest time rolls around. Below, we have provided helpful information on onion botrytis leaf blight prevention and its control.

Symptoms of Botrytis Leaf Blight on Onions

Onions with botrytis leaf blight display whitish lesions on the leaves, usually surrounded by silver or greenish-white halos. The centers of the lesions may turn yellow and take on a sunken, water-soaked appearance. Botrytis leaf blight on onions is most common on older leaves.

Causes of Onion Botrytis Leaf Blight

Botrytis leaf blight on onions is most likely to develop as a result of heavy rainfall, extended periods of relatively cool, damp weather, or overwatering. The longer leaves remain wet, the more severe the outbreak. When foliage remains wet for at least 24 hours, the risk of developing botrytis leaf blight is high. Although it is less likely, the disease can occur when leaves are wet for only seven hours.

Temperature is also a factor. Onions are most susceptible when temperatures are between 59 and 78 F. (15-25 C.). The disease takes longer to develop when temperatures are cooler or warmer.

Leaf Blight Control of Onions

Unfortunately, no onions currently on the market are resistant to botrytis leaf blight. However, there are steps you can take to prevent or slow the disease from spreading.

Plant onions in well-drained soil. Soggy soil promotes fungal disease and rot. If possible, avoid overhead irrigation and water at the base of the plant. Water early in the day so the foliage has time to dry before temperatures drop in evening, especially if you use a sprinkler. Limit irrigation late in the season when onion tops are drying. Don’t fertilize late in the season either.

Fungicides may slow spread of onion botrytis leaf blight if applied at the first sign of disease, or when weather conditions indicate the disease is imminent. Repeat every seven to 10 days.

Keep weeds under control, especially wild onions and other alliums. Rake the area and destroy plant debris after harvest. Practice crop rotation of at least three years, with no onions, garlic, or other allium planted in that soil during the “off” years.

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The infection can occur at any growth stage and usually develop first on older leaves. Initial symptoms appear as small (1-5 mm), circular or elongated white spots on the upper leaf surface. Individual spots and later groups of spots are surrounded by a light green or silvery halo that often has a water-soaked appearance at the beginning. Over time, the number of lesions increase and the center of the older spots become sunken and straw-colored, a sign of developing necrosis. A characteristic slit that is oriented lengthwise in the lesion may appear at later stages. Leaf tips and margins soften and gradually become necrotic, resulting in blighting and dieback. In favorable conditions, the disease also affects the bulb, reducing its size and its quality. As the disease spreads further, large yellow patches of dying plants can be observed from the distance in the field.

The disease is caused by the fungus Botrytis squamosa, which survives on infected bulbs or other plant debris left in the field, or in storage facilities. When conditions are favorable, fungal spores are produced on these tissues and spread by the wind to neighboring plants, serving as a primary source of infection. Temperatures between 10 and 20 °C, high rainfall, prolonged periods of leaf wetness or high relative humidities favor the life cycle of the fungus. The symptoms may be confused with other pathologies or disorders such as drought stress, hail injury, thrips infestation or herbicide damage.


Scientific Name: Botrytis squamosa

In Ontario, botrytis leaf blight is one of the top two most common fungal diseases seen in onion crops and may cause a dramatic reduction in onion bulb growth and yield.В . В В В В


  • Small (1 to 5 mm) grayish-white oval shaped lesions
  • Spots often surrounded by slivery-white ‘halo’
  • Active lesions are grayish-white colour
  • Older lesions tend to be brownish-white and desiccated (dehydrated or dried out)
  • When numerous lesions are present the leaf tips begin to dieback eventually affecting the entire leaf

Botrytis leaf blight overwinters as sclerotia in the soil, on onion debris and in cull piles.В The sclerotia in onion fields will produce conidia (spores) which infect sprouting bulbs and other onion plants.В Sclerotia on leaf debris produce conidia and also ascospores (sexual spores) which will infect leaves of other onion plants.В The ascospores reproduce sexually resulting in possible new strains of the pathogen and potential tolerance to fungicides.В Conidia are produced repeatedly, up to four times, by the germination of the sclerotia.В В В

Period of Activity
Development of the fungal disease usually occurs after mid-June, typically when temperatures and leaf wetness are ideal for infection.В Warm (16-28 В°C), wet weather is most favorable for disease development however, warm nights and high humidity also provide ideal development conditions.В В В

Scouting Notes
To scout for botrytis leaf blight, examine the three oldest green leaves on 50 to 100 plants and then determine the number lesions per leaf.

Management Notes

  • Practice a 2-3 year crop rotation and remove cull/volunteer onions from the field
  • Plant spacings that permit better air movement and irrigation schedules that do not extend leaf wetness periods may be helpful.
  • To reduce the incidence and severity of botrytis leaf blight, remove cull piles and cull onions from field areas, rogue out volunteer onions and rotate crops.

There are two ways to determine whether or not a preventative fungicide should be applied:

  • Tomatoes: leaves are distorted, dark brown to black spots on the leaves with distinct concentric ring pattern (hence the term ‘target’ spot). You will often find the lower leaves are attacked first and in severe cases, the leaves wither, become crispy and eventually die. Brown elongated markings may also be seen on the stalk and are surrounded by a yellow halo. On the fruit, the fungus attacks near the fruit stalk and can cause rotting on one side.
  • Potatoes: leathery-looking dark brown to black spots on the leaves, which become more angular as they develop. The spots are usually 6mm wide, but they can double in size under favourable conditions (high temperatures and high humidity). The spots also have the classic concentric ring pattern and if left untreated, can spread.

The disease can be difficult to treat once established, but as soon as symptoms are noticed, apply a protective fungicide to prevent it from spreading. Destroy plants after harvest as the disease can spread to seeds.

Early blight often occurs during humid weather and is most likely to attack plants under stress. Remove dead or damaged leaves and make sure there’s good air movement around the plant. Fungal pathogens can persist on old material in the soil or on other ‘host’ plants, like nearby weeds, for up to a year. Remove diseased plants and bin or burn them.

Practice crop rotation and ensure potatoes and tomatoes plantings never follow. Choose a different spot each year (avoid planting in the same part of the garden for at least four years). If you don’t have enough space for this, consider growing tomatoes in pots and potatoes in grow bags. Look for disease-free seeds and only save seed from disease-free plants. Water the soil and feed plants regularly to keep them growing well – healthy plants can better resist pests and diseases.

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