maybe damping off. check this out.
Symptoms
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 (see figures 1 and 2). 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.
Control
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 approaches the point at which plants wilt. This procedure will keep surface soil dry for a maximum time. Avoid frequent sprinkling because this generally keeps surface soil too moist and promotes fungal growth.
Damping-off may not occur at a location for many years and then be devastating in one season. Such situations sometimes can be traced to a change in crops or plant varieties, a change of source of soil, or some change in cultural practice. Prior to initiating any changes, it would be wise to test the intended changes on a small scale before the main planting is done.
Control measures for damping-off fall into several categories depending on the facilities available and whether preventive measures are to be used after the trouble has begun. Preventive measures are much preferred, but make preparations for emergencies. Preventive measures are based on eliminating fungi that cause damping-off or providing chemical barriers to prevent the fungi from growing in the planting medium.
