If the surface of your mix dries out, you can re-wet the mix by misting with a spray bottle. You can also put the pots in water to soak, leaving them until the surface of the mix is damp again. Normally the pots will stay damp without any additional rewetting or spraying, but this does happen occasionally. Sometimes too much bottom heat will make them dry out faster than they would ordinarily. The humidity provided by the box is usually enough to keep them damp till the seedlings are ready for transplanting.
If you see a slimy black or greenish substance on the surface of the soil, you have an algae problem. Either the seed had spores in it or you didn’t sterilize well enough. You can save any seedlings that look like they are in danger of being smothered by transplanting them right away. It is a delicate procedure if they are very small. There is not a proven chemical algaecide that can be used if you have this problem; most options are strong enough chemicals that they will kill the seedlings as well.
This is a disease that kills the small seedlings, and it is usually a problem with too much water, or not enough warmth. There are fungicides you can use, if you already have damping off. You may have to test whatever product you use first to see that it works and doesn’t damage begonia seedlings. Most products that work on mature begonias work well on seedlings too. It’s always best to test first however.
Tall and Leggy Seedlings
If your seedlings appear to be tall and leggy, it means the lights are either not close enough to the small plants, or not turned on long enough each day. Brighten the conditions your seeds are growing under and the stems of emerging seedlings will be short again, and easier to work with.