Onion Pests and Diseases, Symptoms, Controls

Onion Pests and Diseases:

Today, let us talk about the most common Onion Pests and Diseases, their symptoms, and their controls.

There are many diseases that affect onion plants. Onion plant diseases affect due to warm, moist weather, and all most all diseases have similar symptoms, that has spots and lesions on leaves and bulbs, and affected parts look like they are water-soaked browning foliage and toppling.

Read this: Organic Onion Growing Information.

Fungal Diseases:

Black Mold

Symptoms of this fungal disease:

Black discoloration on the neck before harvesting; lesions on outer scales; black streaks on the outer dry scales; the entire bulb surface will turn black and shriveling.

Fungus is the main cause of this disease. Wash hands thoroughly after contacting affected parts of the plants.


The seeds should be treated with an appropriate fungicide prior to planting that can reduce rot in mature bulbs; don’t bruise bulbs during and after harvest; Ideal temperature for storing below 15°C (59°F).

Botrytis leaf blight:


You can observe small white lesions with light green halos which may expand gradually and can cause leaf blighting

High humidity and warm temperatures are the root cause of this disease; this fungus survives on piles of crop debris or in the soil; older leaves are highly susceptible to blight than younger leaves


Appropriate spacing between the plants (at least 30 cm between plants) will promote good air circulation and quick drying of foliage: Irrigating the plants at regular periods will control the diseases to some extent.

Use appropriate organic fungicide sprays when plants show early symptoms of the disease.

Downy mildew:


You can observe pale spots or elongated patches on leaves; along with gray-purple fuzzy growth on leaf surface; leaves turning pale and then yellow; the tips of the leaves start to die. Cool temperatures and wet foliage are the root cause of this disease.


Selected well-treated seeds; crop rotation; well-drained soil and appropriate spacing can control the disease to a certain extent.

Fusarium basal plate rot:


  • Curving, yellow or necrotic leaves.
  • The tips of the leaves will wilt and moves downward.
  • The bulbs infected will turn brown and watery with rot spreading from the stem plates.
  • Discoloration of stems.

The disease can occur in moderate to high temperatures


Crop rotation for at least 4 years and planting diseases resistant varieties.

Fusarium damping-off:


Seeds will rot and are covered with mold; the tips of the roots will get discolored root and turn pink, yellow, red, or black; the seedlings will get swollen and they wilt and die

Moist and wet soils will survive this fungal disease.


  • Select the disease-free seed varieties.
  • Treat seeds with fungicide.
  • And rotate crops with cereals or grasses.
  • Steam treatment or fumigation of soil can prevent this disease.

Pink Root:


The roots turn to light pink roots and will turn to darken and turn purple when disease progress; roots turn transparent and water-soaked; plant turns weak.

The seedling affected by pink root will die; Matured plants affected by pink root become stunted and produce undersized, shriveled bulbs.


This disease is most severe when the onion crop more than 3 years a rotation of 3-6 years is highly recommended; plant disease-resistant varieties; solarization and fumigation will reduce bacterial and viral infections in the soil.

Purple blotch:


You can observe small water-soaked lesions on leaves or stalk with white centers; large lesions may girdle leaf, spoil the tissue between the lesions and the leaf tip; severely infected foliage may die.

Wet foliage and high humidity will favor the diseases.


  • Implement Crop rotations.
  • Avoid wet foliage and use well-draining soil.
  • Timely irrigation will keep plants healthy and away from diseases.
  • Organic fungicides will control the disease. 



You can observe small white flecks on leaves and stems which turn circular or elongated orange pustules; severe infestations will spoil the leaves and stems completely.

High humidity levels and low rainfall levels will favor the disease.


Plant only disease-free seed and plant in well-draining fertile soil; weed control measure should be implemented; Application of protective organic fungicides.



The stems will turn dark, thickened, and bend downwards; blisters are formed on the base of the older plants; the lesion will mature and get covered in black powdery fungal masses; the growth rate of the plants will get reduced; plants lead to death within 3-4 weeks.

Smut will remain in the soil for many years and is induced in plants through insects.


Crop rotation and use only health sets and transplants.

White rot:


Yellowing of older leaves yellowing; slow growth; fall of leaves; fluffy white growth at the base of the bulb.


Organic Fungicide treatment will control white rot: Avoid transferring soil or plant material between sites; Treating the seeds with hot water before sowing; long-term rotation.

Bacterial Diseases:

Leaf streak and bulb rot:


Water-soaked, dark green oval lesions and streaks on leaves; burns at tips of the leaves; dark spots on wrapper scales of bulbs; discoloration on the inner scales.

This disease mainly affects during winter; rainfall promotes the spread of the diseases.


Don’t fertilize the plants during winter. Use appropriate organic bactericidal sprays to control these diseases.

Dwarf Onion yellow dwarf virus:


You can observe some yellow streaks at the base of the first leaves; leaves get flattened or crinkled; the plant produces undersized bulbs; flower stalks turn yellow and twisted and produces small flower clusters and poor-quality seeds.

This viral disease is transmitted through aphids.


  • You can control aphids by applying insecticides.
  • Select the planting sets or transplants that free from viruses.
  • Growing plants from high-quality seeds.
  • Pick off the infected plants immediately.

Pythium seed rot:


Seed turn water-soaked and mushy and start to decompose; the roots turn gray and water-soaked; the seedling will collapse and die; older plants infected with this virus will be severely stunted.

High soil moisture and cool temperatures will encourage the development of diseases.


Control the soil moisture: Well-drained and fertile; treat the seeds organic fungicides before planting.

Pests in Onion:

Bulb mites:


Slower growth of the plant; bulbs rotting; you can observe cream-white pests on the stems and on bulbs.


  • Avoid successive crops of onion or garlic in the same location.
  • The fields should be cleaned properly so that residual organic matter decomposes completely.
  • Treating the seed with garlic seed cloves with hot water before planting may reduce mite populations



You can observe thin, white, winding trails on the leaves; this can cause premature leaf drop and reduced yields; adult leafminers are small black and yellow flies that lay its eggs in the leaf.


Check the transplants properly prior to planting; after harvesting removes the plant residue completely; Use organic insecticides as soon as you observe leafminers.

Onion Maggots:


Stunted or wilting seedlings are the major symptoms for onion maggots; the plant will become weak and breaks off at the soil line, the bulbs will be deformed and susceptible to storage rots.

Adult onion maggot is a grayish fly, which lays white, elongated eggs at the base of the plant.


Good sanitation is the best preventive method to control onion maggots; the affected onion bulbs should be removed. Commercial onion growers use mild granular insecticides and organic, insecticide sprays to control onion maggots. Floating row covers around the plants can protect insects from laying eggs.



Thrips will discolor and distorted the tissue of the leaves and creates scars on the leaves.

Thrips feed on the onion bulbs at the early stage.; both the onion thrips and western flower thrips can affect the plants to a great extent.


Encourage the growth of natural like predatory mite, pirate bugs, and lacewings which can control thrips; avoid planting onion near grain fields in the spring; foliage water or overhead watering of plants may reduce thrips; You can also some organic insecticides to control thrips at the early stages.

Bottom Line: If you are growing Onions on large scale, you must be aware of Onion Pests and Diseases and their control measures.

In case if you are interested in this: Onion Growing Tips, Tricks, and Secretes.


  1. What animal or pest will leave large bite marks on onion bulbs? It looks a squirrel bite, as it is about the size of a quarter or a golf ball. But the wound is round and smooth. I don’t see any mites or maggots. What else might feed on the half-grown onion?


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