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I have heard that several newly planted roofs are experiencing damping-off and rot this Spring.  Damping-off is a condition that causes the plant to lose vigor and die, in some cases almost overnight.  This is caused by several fungi.  Plants are vulnerable to attack by these fungi during periods of unfavorable growing conditions.  Species of Pythium, Sclerotinia, and Phytophthora are more likely to cause damping-off in cool, wet soils; whereas species of Rhizoctonia, Fusarium and Sclerotium rolfsii may cause damping-off under warmer and drier conditions. This type of injury often damages the leaves and upper stem before the roots are affected.  True damping-off may be confused with plant injury caused by planting too deep, high soluble salts, drowning in wet soil, desiccation in dry soil, and death of plants from excessive heat or cold.  Rhizoctonia and Pythium do not have an airborne stage. The spread of both fungi depends primarily on the mechanical transfer of resting spores in infested soil. As with most fungi, the spores are spread by the splash of rain water or infected tools.

Symptoms

Typical symptoms of damping-off are rotting stems at or near the soil line and root decay.  You generally notice brown stems at or just above the soil line.  Infected plants are shriveled, brown, collapsed or stunted. Moldy fungal growth may be seen on affected plants at the soil line. Phytopthera, Rhizoctonia, Fusarium, Sclerotinia, and Sclerotium generally caused damping-off by killing the plant at the soil line. Pythium attacks below the soil line, often at root tips.

 Control

The best control of damping-off is to avoid it altogether. Once damping-off has started in a plant bed, it may be difficult to control.  Proper soil treatment with heat or chemicals to reduce the level of fungi that cause damping-off is very important.  The compost added to the growing mix needs to be properly "Composted" to rid it of any pathogens.  The compost added to the growing mix needs to be from a trusted source.  The composting time needs to be a minimum of four months with proper aeration, then covering while it cures for four weeks.  Proper temperatures of 150F or above must be maintained for eight weeks for sterilization.  Chemical control is an effective tool you can use once damping-off is suspected.  Check with your University's Agricultural Extension Service for their recomendations. 

Summary

Damping-off can be avoided if you take into account that a Greenroof planting is just as suceptable to issues that most gardeners face on the ground.  We need to remember that plants aren't just another layer on the roof, they are but living organisms.