Damping-off (young seedlings dying)
Damping-off generally refers to sudden plant death in the seedling stage due to the attack of fungi. These fungi are soil borne and are stimulated to grow and infect the seed or seedling by nutrients released from a germinating seed.
However, seedlings may be injured or killed by something other than fungi, for example, toxic materials in the soil, excess or deficient soil moisture, seed defects, temperature extremes, toxic gases in the air, etc. A correct diagnosis is the key to effective control measures.
Damping-off disease of seedlings is widely distributed and is a problem on a worldwide basis. It occurs in most soils, temperate and tropical climates, and in greenhouses. The disease affects seeds and seedlings of various crops. The amount of damage the disease causes to seedlings depends on the fungus, soil moisture, and temperature. Normally, however, cool wet soils favor development of the disease. Seedlings in seedbeds often are completely destroyed by damping-off, or they die after transplanting. Frequently, germinating seeds are killed by damping-off fungi before they emerge from the ground, which accounts for poor stands in many crops.
Older plants are seldom killed by damping-off fungi mainly because the development of secondary stem tissue forms a protective barrier and limits fungal penetration. However, portions of the roots and stems still can be attacked, resulting in poor growth and reduced yields. When seeds are planted in infested soils, damping-off fungi may attack them at any stage. The damping-off fungi may attack the seed prior to germination, or they may attack after the seed has germinated but before the seedling has emerged above the soil line. Infected seed becomes soft and mushy turning a brown to black color, and it eventually disintegrates. Seeds that have germinated and become infected develop water-soaked spots that enlarge and turn brown. The infected tissue collapses, resulting in death of the seedling. Penetration and death of seeds before they emerge is termed preemergence damping-off. Seedlings that have emerged are usually attacked at or below the soil line. The organism can easily penetrate the young soft stem tissue. The infected stem portion becomes discolored and begins to shrink. As this occurs, the supportive strength of the stem's invaded portion is lost, and the seedling topples over. The fungi continue to invade the remaining portion of the seedling, resulting in death. This phase of the disease is termed postemergence damping-off. Older established plants also can be attacked by damping-off fungi. Usually the new developing rootlets are infected, resulting in root rot. Infected plants show symptoms of wilting and poor growth.
Proper conditions for seed germination and seedling emergence also favor vigorous growth of fungi that cause damping-off. Seed and roots must be kept moist and warm until the roots have penetrated the soil and the seedlings have emerged. As the seedlings continue to grow, moisture at the soil surface can be decreased, and the damping-off fungi then will have less of an advantage. When watering, thoroughly saturate the soil and then apply no more water until soil barely approaches the point of dryness at which plants wilt. This procedure will keep surface soil dry for a maximum time.