Damping off is a common disease that affects young seedlings, particularly those started indoors or in seed trays. It is caused by a group of microscopic, soil-borne fungal pathogens that thrive in specific growing environments. This condition is responsible for the sudden failure and collapse of newly planted seeds and fragile sprouts. The disease can quickly wipe out an entire tray of plants before they establish themselves. Recognizing the signs of this infection is the first step in protecting developing plants.
How to Visually Identify Damping Off
Damping off manifests in two distinct phases, depending on when the pathogen attacks the developing plant.
Pre-Emergence Damping Off
The first is pre-emergence damping off, where the seed or germinating sprout rots beneath the soil surface before it can break through. This often results in patchy or sparse growth across a planting tray. If the affected seeds are excavated, they may appear soft, mushy, or discolored brown.
Post-Emergence Damping Off
The second, and more recognizable, phase is post-emergence damping off, which occurs after the seedling has successfully sprouted. The plant initially looks healthy, but the stem tissue near the soil line rapidly becomes compromised, appearing water-soaked, soft, and constricted. This decay at the base causes the seedling to quickly wilt, buckle, and collapse onto the soil surface.
The damage can spread quickly, moving from one collapsed seedling to its neighbors in the tray. In high-humidity conditions, a fuzzy, cobweb-like growth may become visible on the soil surface or surrounding the dead plant material. This mold-like growth is a clear indicator of active fungal infection, often appearing white, gray, or sometimes pink. Once a seedling has collapsed, it must be removed to prevent the disease from spreading further.
The Environmental Conditions That Encourage It
The organisms responsible for damping off are present in virtually all soils. They include several genera of fungi and fungus-like organisms, such as Pythium, Rhizoctonia, and Phytophthora, which are collectively known as soil-borne pathogens. These pathogens require specific environmental conditions to become active and infect young plants.
The primary factor driving these infections is excessive soil moisture, often resulting from overwatering or poor drainage. The spores of some pathogens, like Pythium, are able to swim through water-saturated media to reach and infect nearby seeds or roots.
Cool soil temperatures compound this problem, as they slow down the seedling’s growth and make it more vulnerable. Conditions of high humidity and poor air circulation also create a hospitable environment for the disease. Overcrowding seedlings reduces airflow, causing moisture to linger on the soil surface. While Pythium favors warm, wet conditions, Rhizoctonia often causes problems in cooler, moist soils. Any factor that stresses the seedling or slows its development increases the window of opportunity for these organisms to cause decay.
Immediate Steps for Control and Prevention
If damping off is identified, immediate action is necessary to prevent the disease from spreading to healthy seedlings. The first step involves removing and discarding all infected plants, along with the surrounding soil, to eliminate the source of the active fungal growth. The remaining seedlings should be isolated and moved to an area with improved conditions.
Preventative measures focus primarily on sanitation and environmental control. Always use a sterile, soilless potting mix rather than garden soil, which often harbors these pathogens. Furthermore, all trays, pots, and tools should be thoroughly cleaned and sterilized before use.
Managing the moisture level is the most effective long-term strategy. Water seedlings from the bottom by placing the tray in a shallow bath. This allows the soil to wick up moisture while keeping the surface and the stem crown relatively dry. Ensuring adequate air circulation, often by using a small fan, helps reduce surface humidity and strengthens the young plant stems.