Tooran - Thanks for the cucumber question, your photos really pinpoint the problem. The plants appear to suffer from damping off, caused by several fungi or fungus-like microorganisms including phythium ultimum, fusarium and rhizoctonia. They all live in the soil and attack seedlings of many different crops. As seedlings mature, they become less vulnerable tomattack. The disease can be a serious problem in cool wet soils. Although this year, we have enjoyed some rather warm early spring days, the overnight temperatures have remained in the low 50's or colder. Seedlings may be attacked before or after emergence. Young plants suddenly wilt and topple over. Water-soaked lesions are visible on stems at soil line for only a short time before seedling dies. As wet and cool weather is in the immediate forecast, the surviving plants may benefit from mulching to increase soil temp. Grass clippings serve that purpose well. In future: planting after soils warm (55 degrees overnight consistently); mulching to increase soil temp; plant shallow to promote quick emergence and growth; and avoid old seed, which may be slow to emerge.