The Invisible Threat to Onions

Recently, onion cultivation has begun to face new challenges. Quite unexpectedly, the plants begin to wither, and a detailed inspection reveals a rotted underground part that is already being eaten away by fly larvae. There can be several reasons for this situation, and they are all quite different.

Main Causes and How to Tell the Damage Apart:

1. Stem Nematode Infestation

Nematodes are microscopic roundworms, but during mass reproduction, they can be noticed with the naked eye or under a magnifying glass as tiny, thin, wriggling white or translucent threads. Their appearance leads to plant damage and the subsequent development of rot.

• What the rot looks like: The bulb tissues become soft, loose, granular, and turn a dirty gray or brown color. The scales often detach from one another (becoming "loose"), and the bulb itself feels hollow or spongy to the touch.

• Characteristic sign: The bottom (basal plate) of the bulb often cracks or degrades, and if you peel back the scales, the rotting runs in stripes or rings along the juicy tissues. The smell is usually heavy and putrid.

2. Bulb Mite (Basal/Root Mite) Infestation

The mites themselves are hard to see without a powerful magnifying glass or microscope—they are extremely small (less than 1 mm), oval, glassy-white, or yellowish. However, they leave behind characteristic tracks.

• What the rot looks like: The mite always starts its attack from the basal plate and the roots. The bottom becomes decayed, completely rots out, or turns into dust (brown powder), easily falling away from the bulb.

• Characteristic sign: The inner juicy scales become covered with brown dust (excrement and dead tissue). If you see specifically fine brown dust in the basal plate area, it is the work of the mite, and bacterial rot joined in later.

3. Primary Disease: Bacterial or Fungal Rot (Fusarium)

If a disease was the actual root cause, then the appearance of worms on the rotten onion is most likely the larvae of the onion fly or small saprophytic nematodes that simply arrived/bred there attracted by the smell of decomposition.

• Bacterial rot: If you cut the bulb open, healthy and rotten layers alternate (for example, 2–3 inner scales have completely rotted and turned into slime with a sharp, unpleasant odor, while the outer and central ones are absolutely healthy). The bulb itself softens from the neck down to the base.

• Fusarium (Basal plate rot): A fungal disease. A white, pink, or grayish mycelium (mold-like coating) appears on the basal plate. The roots die off, and the bulb rots from the bottom up, becoming watery.

Any of these causes can be introduced to the planting area along with the seedling material, which triggers an outbreak of the pest or disease and significantly worsens the phytosanitary condition of the plot.

(An onion affected by rot and the larvae feeding on it)

How to Protect Your Onions?

In any case, an affected bulb must be immediately isolated and destroyed (do not throw it into regular compost to avoid infecting the soil with pests or fungal spores).

To combat soil pests such as nematodes or bulb mites (which can be the primary cause of rotting), we recommend applying the biological products Nematocid and Imexab™ to the soil.

• Nematocid, based on the bacteria Bacillus firmus and Bacillus pumilus, actively destroys nematodes and their egg clutches, thereby sanitizing even new planting material.

  • Dosage: 20–30g / 10L of water / 100 sq. meters (1 sotka). It is applied via drip irrigation or by drenching with a watering can. The main goal is for the solution to completely wet the root system of the plant.

• Imexab™ is a biological insecto-acaricide based on a complex of natural avermectins and emamectin. It destroys mites and their eggs, allowing you to protect the onions from the pest.

  • Dosage: 20g / 10L of water / 100 sq. meters (1 sotka).

Such comprehensive pest protection will allow you to be confident in reliable defense. By destroying the pest, you minimize the possibility of rot infection.

However, if the beginning of the infestation was missed and the consequences of the pests' work are already visible, we recommend adding the biofungicide Sporomax™ T to the treatment at a rate of 10–20g / 10L of water / 100 sq. meters (1 sotka). The Trichoderma in the product's formulation actively suppresses rot pathogens and will protect even the damaged areas from pathogen penetration.