Onion rot refers to various diseases affecting onions, causing their decay and rendering them unusable. A significant type of onion rot is white rot, a destructive fungal disease.
Understanding Onion White Rot
Onion white rot is a serious disease that primarily affects plants in the Allium family, notably bulb onions, garlic, and leeks. The culprit is the soil-borne fungus Stromatinia cepivora (syn. Sclerotium cepivorum). This fungus can survive in the soil for extended periods, making it a persistent threat.
Key Characteristics of White Rot:
- Causative Agent: Stromatinia cepivora (Sclerotium cepivorum) fungus.
- Affected Plants: Primarily bulb onions, garlic, and leeks (alliums).
- Persistence: The fungus can remain viable in the soil for many years.
- Symptoms: Typically appear from mid-summer to early autumn.
Symptoms of Onion Rot
While the specific symptoms can vary depending on the type of rot, common signs include:
- Yellowing and wilting of leaves: Often starting from the tips.
- Stunted growth: Affected plants may not develop properly.
- Water-soaked lesions: Appearing on the bulbs and roots.
- White, fluffy fungal growth: Often visible on the base of the bulb and in the surrounding soil (particularly in white rot).
- Small, black sclerotia: These are resting structures of the fungus, resembling poppy seeds, and can be found on the bulb or in the soil.
- Bulb decay: The onion bulb becomes soft, mushy, and rotten.
Prevention and Control of White Rot
Due to the long-term viability of the Stromatinia cepivora fungus, complete eradication is difficult. Control strategies focus on prevention and minimizing the impact of the disease:
- Crop rotation: Avoid planting alliums in the same area for several years (5-10 years is recommended) to reduce fungal buildup in the soil.
- Soil solarization: Covering the soil with clear plastic during the hottest months can help to kill the fungus.
- Sanitation: Remove and destroy any infected plants immediately to prevent the spread of the disease.
- Disease-free transplants: Use only healthy, disease-free onion sets or transplants.
- Resistant varieties: While completely resistant varieties are limited, some show better tolerance to white rot.
- Fungicides: Some fungicides can help to suppress the disease, but their effectiveness can vary.
- Biological Control: Some studies show that beneficial microbes may help reduce white rot incidence.
In summary, onion rot is a disease complex, with white rot being a particularly damaging type caused by the fungus Stromatinia cepivora, leading to decay and loss of onion crops. Effective management relies on preventative measures and integrated control strategies.
