From http://pss.uvm.edu/ppp/pubs/gd9.htm:
Damping-off is caused by several fungi including the water molds such as Phtophthora and Pythium. These fungi
occur in all soils, are water loving organisms, and thrive in wet or poorly drained soils. Slow-growing or weak plants
are more susceptible to damping-off than vigorous fast-growing seedlings. If the plant can grow roots faster than the
fungus can decay them, the plant will survive and be healthy.