if you have a white powdery looking substance on the plants just above the cube it is most likely Damping off.
Note: Older plants are rarely killed by damping off primarily because the production of secondary stem tissue forms a protective barrier and limits fungal penetration.
Control: There is no cure for plants that already have damping off, however you can easily prevent the problem by providing good air circulation. A small fan or simply cracking the lid of the germination tray will suffice. The biological fungicide Mycostop may also be used as a seed treatment to prevent seed or soil-borne diseases. Other steps for preventing damping off include the following:
• Make sure your soil mix is light and fast-draining.
• Plant seedlings so that the soil surface is near the top of the container to insure proper air circulation.
• Sow seeds thinly to prevent over crowding which can lead to humid, moist conditions.
• Watering systems that provide seedlings water from below are preferable to overhead watering.
• Never water past noon so that the soil surface and the plants are dry by dark
• Avoid overwatering seedlings
can you tell which part of the post i copy pasted from a website?
google damping off and you can do a little research yourself
