The lower part of the stem softens and becomes thinner than the parts of the seedling above it. The seedlings then fall over on the soil and die. The most obvious symptoms of damping-off are failure of seedlings to emerge, and collapse and falling over of young seedlings. During extended periods of wet weather, damping-off fungi can invade the fleshy and often most valuable parts of older plants, such as the actual cucumber, melon or tomato, especially when these parts are in contact with the soil. A cottony fungus growth results, and the infected parts become soft, watery and rotten.
Several factors influence the extent of seedling damping-off.
The disease, when caused by Pythium or Phytophthora, is most prevalent when soil moisture is high. As seedlings mature, they become more resistant to attack by damping-off fungi. At low soil temperatures, damping-off increases because seeds or seedlings are exposed to the fungi for longer periods of time. Ensure adequate lighting because it results in sturdy seedlings that are more resistant to damping-off. High plant density favors damping-off because of reduced light penetration, greater competition for nutrients and greater retention of moisture.