Damping-off is the destruction of young seedlings by soil organisms. There are two types. Pre-emergence damping-off rots the sprouting seed before it breaks through the soil; it is recognized by bare spaces in what should be uniform rows. Such a poor stand may be due to poor viability of seed, but more often it is due to soil fungi functioning in cold, wet soils when germination is slow. Post-emergence damping-off is the rotting or wilting of seedlings soon after they emerge from the soil. Succulent stems have a water-soaked, then necrotic and sunken, zone at ground level; the little herbaceous plants fall over on the ground or, in woody seedlings, wilt and remain upright. Root decay follows. This type of damping-off is most common in greenhouses or outdoors in warm humid weather and where seedlings are too crowded. Tree seedlings in nursery rows are subject to this type of damping-off, and so are perennial flowers started in late summer for the next year.