P&PDL Picture of the Week for
February 22, 2010

Seedling damping off and root rot

By Daniel S. Egel, Extension Plant Pathologist Southwest Purdue Agricultural Center, Vincennes, IN 

Cold winds howl across the landscape today, but it won’t be long before gardeners will turn their attention to starting transplants inside the house or in cold frames.  These transplants include all manner of flowers and vegetables destined for outside gardens when the weather improves.  Unfortunately, some of those transplants will become diseased and die before making it to the garden.  Damping-off and root rot are two of the most common diseases of seedlings of all types of plants.

Damping-off may cause seeds or seedlings to rot prior to emergence through the soil (pre-emergence damping-off) or cause a brown area on the stem of the seedling where the stem (technically, the hypocotyl) contacts the soil (post-emergence damping-off, Figure 1).   Affected stems may shrivel and become thin; ultimately the seedling will fall over, that is, damp-off. 

Root rots may be suspected if seedling growth is slow and leaves and seed leaves (cotyledons) become yellow (chlorotic).  Two muskmelon seedlings have been removed from the transplant tray in Figure 2.  The seedling on the right is a healthy green and the white roots have completely filled the cell of the transplant tray.  In contrast, the seedling on the left is stunted, has yellow leaves and discolored, rotted roots that have failed to fill the transplant tray cell. 

Both damping-off and root rot are caused by a number of fungi which inhabit the soil (Pythium spp. and Rhizoctonia solani are two of the most common fungi involved).  These fungi may infect seedlings alone or in combination with other fungi.  The fungi responsible for these diseases may survive for long periods in soils, soil mixes or even on contaminated tools, benches or transplant trays. 

Environmental conditions may also help to worsen damping-off and root rot.  Cool, wet soils favor damping-off and root rot.   Another factor that may aggravate damping-off is when seedlings become long and thin (etiolated).  Seedlings raised in low light conditions tend to be long and thin because they are growing toward light.  Such seedlings are often very susceptible to damping-off. 

Below find some ideas for managing damping-off and root rot of seedlings:

  • Avoid bags of soil or soilless mixes that have been broken open and store bags so that they do not come in contact with water or contaminated soil.

  • Use only new or disinfected pots and trays to grow transplants.  To disinfect used pots or trays, first clean these items so that they are free of soil and debris.  Then use a 10% bleach solution or a quaternary ammonium solution as a disinfectant and then rinse with clean water.  Always use gloves and goggles. 

  • Clean and disinfect all tools and bench tops that come into contact with soil to be used with transplants. 

  • Gardeners should avoid overwatering soils and should make an effort to keep soils warm.  Avoid soils or soilless mixes that drain poorly. Outside gardens with raised beds increase soil drainage and warmth.   

  • As much as possible, gardeners should use a light source that minimizes seedling stretch.  

Click image to enlarge

Seedling

Figure 1:  This muskmelon seedling is discolored and withered where the stem contacts the soil and will soon collapse or damp-off. 

Seedling root rot

Figure 2:  While the muskmelon seedling on the left has a healthy green color and nice white roots, the seedling on the left suffers from root rot. 

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service