Onion rot is a frustrating issue for gardeners, often turning a promising harvest into a soft, mushy, and foul-smelling disappointment. Decay typically begins while the bulb is developing underground, manifesting as sudden yellowing and wilting of the foliage, followed by the plant’s collapse. Understanding why onions succumb to rot requires looking at the environmental conditions, pathogens, and pests that compromise the plant’s defenses. Rot is a chain reaction where one factor weakens the bulb, allowing another to cause destructive decay.
Environmental Stressors That Encourage Rot
The initial step toward bulb rot often begins with non-living, or abiotic, factors that weaken the onion plant’s natural resistance. Poor soil drainage is the most significant environmental trigger, as waterlogged conditions quickly deprive the roots of necessary oxygen. Saturated soil creates an anaerobic environment that stresses the root system, making it susceptible to infection by soil-borne pathogens.
Soil compaction exacerbates this problem by preventing water from moving away from the bulb and inhibiting healthy root growth. Planting onions too deep or too shallow can interfere with proper bulb formation and neck drying, creating a moist entry point for bacteria and fungi. Over-application of nitrogen fertilizer is another factor, promoting lush, soft top growth at the expense of a strong, firm bulb structure, which leaves tissues vulnerable to disease.
Pathogens Responsible for Onion Rot
Once a bulb is stressed, specific biological diseases, both fungal and bacterial, cause active decay. White Rot, caused by the fungus Sclerotium cepivorum, is destructive because it can survive in the soil for up to 20 years. This fungus is identifiable by a white, fluffy growth at the base of the bulb, which later develops small, black, poppy-seed-sized resting structures called sclerotia.
Pink Root, caused by the fungus Setophoma terrestris, is characterized by the roots turning pink, then purple, and eventually decaying. While this pathogen does not typically rot the bulb itself, the loss of the root system severely stunts the plant and makes it vulnerable to other diseases. Basal Plate Rot, or Fusarium rot (Fusarium oxysporum f. sp. cepae), starts where the roots attach, causing the basal plate to become soft and water-soaked. This fungal decay advances upward into the bulb scales, leading to wilted, yellowed top growth as the plant’s vascular system becomes plugged.
Bacterial soft rots are major contributors to decay and are responsible for the foul odor associated with rotting onions. These bacterial pathogens, such as Pectobacterium species, enter the bulb through wounds near the neck or base. They cause the internal scales to become water-soaked and pale yellow, quickly progressing to a soft, mushy breakdown of the entire bulb. Warm, wet conditions favor the spread of these bacterial infections, which can be easily splashed from the soil surface onto the plant.
Pests That Damage Bulbs and Invite Infection
The activity of certain insect pests often provides the physical pathway for rot-causing pathogens to enter the onion. The Onion Maggot (Delia antiqua) is a major culprit, as its small larvae tunnel into the roots and the base of the developing bulb. This feeding creates open wounds that act as entry points for secondary bacterial and fungal pathogens that cause visible rot.
The maggot’s tunneling also helps spread existing pathogens from infected plant material to healthy bulbs. Microscopic worms called nematodes can also damage the root system, weakening the plant and creating wounds for pathogens like Fusarium to invade. In these cases, the rot is frequently a secondary symptom, with the pest being the initial agent that breaks the skin barrier.
Remediation and Future Crop Management
Upon discovering a rotting onion, carefully remove the affected bulb and any surrounding soil immediately to reduce the spread of soil-borne pathogens. Improving soil drainage is a rapid remediation step for the current season, which may involve temporarily reducing irrigation or creating surface trenches to draw water away from the planting area. Managing rot is primarily a long-term strategy focused on prevention.
Future management must center on mandatory crop rotation, avoiding planting any Allium species (onions, garlic, leeks) in the same soil for at least three to four years, as many fungal pathogens are highly persistent.
Preventative Measures
- Practice sanitation by cleaning tools and footwear before moving between garden beds to avoid transporting sclerotia or bacteria.
- If rot has been severe, conduct soil testing to identify specific pathogen levels.
- Use soil solarization—clear plastic sheeting to heat the soil—to reduce the quantity of certain resting fungal spores.
- Select onion varieties that exhibit resistance to common diseases.
- Avoid excessive nitrogen applications to ensure stronger, healthier bulbs next season.