Onion gray mold

Systematic position and type of pathogen. Botrytis aclada is a fungal phytopathogen belonging to the kingdom Fungi, phylum Ascomycota, and genus Botrytis. It is a highly specialized pathogen that targets Allium species. The fungus produces a mycelium characterized by septate hyphae and conidiophores that bear masses of gray spores, along with sclerotia, which are dormant survival structures that enable the fungus to overwinter in soil or crop debris.

Diseases and susceptible crops. This pathogen is the primary cause of neck rot in bulb onions (Allium cepa), and it can also infect shallots and garlic. The infection typically results in the softening of the onion bulb tissue, leading to complete decay. The disease is often characterized by a fuzzy gray mold appearing on the outer scales of the bulb, especially near the neck area where the fungus enters the plant.

Biology and life cycle. The life cycle of Botrytis aclada involves the germination of sclerotia in the soil during spring, leading to the production of conidia. These spores are dispersed by wind, water splashes, and insects. Infection usually occurs in the field when spores land on wounded tissue, such as damaged necks or mechanical injuries caused by tools. The fungus may remain latent in the bulb tissue during the growing season, showing severe symptoms only after the crop has been harvested and stored.

Conditions for development and impact. The development of the fungus is favored by high humidity (often exceeding 85%) and moderate temperatures ranging from +15°C to +20°C. Botrytis aclada is highly destructive, capable of causing significant losses in storage facilities. If management practices are ignored, it can destroy over 50% of an onion harvest, making it a major economic concern for large-scale vegetable producers.

Protection and control measures. Management of Botrytis aclada focuses on cultural practices since curative treatments are ineffective once the storage rot begins:

• Crop rotation to reduce inoculum build-up in the soil.
• Planting healthy, pathogen-free seeds or sets.
• Ensuring proper maturity of onions before harvest to allow the neck to dry and seal naturally.
• Rapid and thorough drying of harvested bulbs using forced ventilation.
• Strict culling of damaged or diseased bulbs before storing to prevent secondary infections.