Damping off is a common and frustrating disease that can quickly devastate a tray of young plants. This affliction is caused by several soil-borne fungi and fungus-like organisms, primarily species of Pythium, Rhizoctonia, and Fusarium. These pathogens thrive in the overly cool, wet, and stagnant conditions often found in indoor seed-starting setups. The disease is known for its rapid progression, often appearing overnight to kill healthy seedlings near the soil line.
Recognizing the Signs
Damping off manifests in two forms, both resulting in the loss of seeds or sprouts. The first, known as pre-emergence damping off, occurs when pathogens attack the seed or germinating sprout before it can break the soil surface. Gardeners often attribute this failure to poor seed viability or low germination rates. When these seeds are dug up, they are soft, dark, and mushy, indicating fungal decay.
The second form is post-emergence damping off, which strikes after the young plant has emerged. A healthy sprout will suddenly wilt and collapse at the base, appearing pinched or shriveled right where the stem meets the soil line. This tissue is weakened and decayed by the fungi, causing the plant to topple over. In some cases, a whitish fungal growth may be visible around the infected stem or on the soil surface.
Stopping an Active Outbreak
Once damping off is identified, immediate action is required to save healthy plants. Quickly isolate and remove all infected seedlings, including the surrounding soil, as pathogens spread rapidly through the growing medium. Dispose of infected materials completely; do not compost them, as this prevents the disease from re-entering the growing area.
Next, shock the environment to make it inhospitable to the fungi. Immediately reduce the humidity by removing any clear plastic domes or covers from the seed trays. Introduce strong air circulation using a small fan to gently blow air across the soil surface, which helps dry the top layer and strengthen surviving stems.
Adjusting your watering routine can also halt the progression of the disease. Stop all bottom watering and allow the surface of the growing mix to dry out completely between waterings, only providing enough moisture to keep the rest of the soil lightly damp. Watering should be done in the morning to ensure the soil surface dries before evening.
To protect remaining seedlings, apply a light dusting of cinnamon powder across the surface of the growing medium. Cinnamon contains natural antifungal properties that can help slow the spread of the pathogens in the top layer of soil. Alternatively, a dilute solution of cooled chamomile tea can be used as a one-time drench due to its mild antimicrobial qualities.
Ensuring Future Success
The most effective control method is adopting preventative measures before planting. Starting with sterile materials is the most important preventative step, as the disease is typically introduced on uncleaned equipment or non-sterile soil. Sanitize all seed trays, pots, and tools by soaking them in a solution of one part household bleach to ten parts water for at least 30 minutes, followed by a thorough rinsing.
The choice of growing medium is equally important, as garden soil is too dense and almost certainly contains fungal spores. Always use a fresh, sterile, and lightweight seed-starting mix that is formulated for rapid drainage. While some gardeners sterilize their own potting mix by baking it, using a commercially prepared mix ensures a pathogen-free environment without eliminating beneficial soil microbes.
Proper watering discipline prevents the overly saturated conditions that allow Pythium and other pathogens to flourish. Water the seedlings with warm or room-temperature water, as cold water can stress the tender roots and slow growth, increasing their vulnerability. The preferred method is watering from the bottom to allow the mix to absorb moisture through capillary action, which keeps the soil surface and the delicate stem area dry.
Maintaining optimal environmental conditions discourages fungal growth. Aim for a consistently warm soil temperature, ideally between 70 to 75 degrees Fahrenheit, which promotes quick germination and rapid seedling growth past the most vulnerable stage. Excellent air circulation is mandatory, and a gentle fan should run continuously to prevent stagnant, humid air pockets from forming around the seedlings.
Sufficient light encourages vigorous growth, allowing plants to mature quickly and develop stem tissue resistant to the disease. Finally, ensure adequate spacing between seeds and trays to further promote air movement and light penetration. Overcrowding creates a humid microclimate that favors the disease, so thin the seedlings early if they are planted too densely.