Damping off is a fungal disease that affects the seeds and seedlings of various plants, including onions. It is caused by a group of soil-borne fungi that thrive in damp conditions and attack the base of the stem, causing the seedling to wilt and die. Onion seedlings affected by damping off may exhibit stunted growth, yellowing leaves, and a mushy stem at the soil line. The disease can cause significant damage to crops and reduce yields.
Damping Off Management in Onion
What Is Damping Off in Onions?
- Damping off is a fungal disease that affects the seeds and seedlings of onions.
- Soil-borne fungi, including Rhizoctonia, Fusarium, and Pythium, cause it.
- The disease thrives in damp conditions and attacks the base of the stem, causing the seedling to wilt and die.
- Infected seedlings exhibit stunted growth, yellowing leaves, and a mushy stem at the soil line.
Onion Plant Characteristics
- Onion is a biennial plant that belongs to the Allium genus.
- It has a shallow root system and a bulbous stem that grows underground.
- The leaves of the onion plant are long, narrow, and green, with a waxy texture.
- Onions prefer well-drained soil with a pH between 6.0 to 7.5.
- They require a cool and moist climate to grow successfully.
Causes/Pathogen of Damping Off in Onion
- Damping off in onions is mainly caused by the Fusarium oxysporum fungus, common in most onion-growing areas.
- The Pythium sp. fungus has also been reported to cause damping-off in some regions.
- The disease thrives in damp soil conditions and attacks the base of the stem, causing the seedlings to wilt and die.
- The disease is more prevalent in northern and eastern India during the kharif season, causing 60-75% damage to onion crops.
- Overcrowding, over-watering, and poor soil drainage are contributing factors that increase the likelihood of the disease.
The Disease Cycle of Damping Off in Onion
- The damping-off disease cycle in onions begins with the fungal pathogen(s) in the soil.
- The pathogen(s) survive on infected crop debris and soil, the primary inoculum sources.
- The fungal spores can persist in the soil for several years, so practicing proper crop rotation and soil sterilization methods is important.
- The pathogen(s) infect the seeds and seedlings when they come into contact with contaminated soil or debris.
- The fungus then grows on the surface of the seedlings, causing the stem to weaken and collapse, resulting in damping off.
- The disease is more prevalent in damp conditions, especially during the plant’s early growth stages.
Conditions Favorable for Damping Off Spread in the Field
- Damping off in onions is more prevalent during the Kharif or rainy seasons.
- The disease can cause significant damage, up to 60-75% in some cases.
- High soil moisture, moderate temperature, and high humidity are favorable conditions for developing and spreading the disease.
- Over-watering or poor soil drainage can increase soil moisture levels and create conditions that favor fungal growth.
- Crowding of seedlings can also increase humidity levels, providing a conducive environment for fungal spores to germinate and spread.
- The disease is more common in areas with heavy rainfall or poor soil drainage.
- Maintaining proper soil moisture levels, spacing seedlings adequately, and practicing good sanitation and hygiene can help prevent the spread of damping off in onions.
Damage Symptoms of Damping Off
- Damping off in onions can result in two types of symptoms: pre-emergence damping-off and post-emergence damping-off.
- Pre-emergence damping-off causes seed and seedling rot before they emerge from the soil. Affected seeds fail to germinate, and the seedlings appear stunted or fail to emerge altogether.
- Post-emergence damping-off occurs when the pathogen attacks the seedlings’ collar region on the soil’s surface. The collar portion rots and the seedlings ultimately collapse and die.