Not sure what it is. I use a Jiffy mix, which seems to be the best sterile mix out there. My tomato seedlings are doing ok (not great,) but this year the melon seedlings are damping off. I'm not doing anything different, and this is the first time I've had the problem using Jiffy mix. The only thing that I can think of is that it's something in the air this year, a bad batch of Jiffy mix, or possibly when I pre-mix the mix with water it kind of compresses like concrete in the cells. I'm going to try sterilizing the cells, and letting the loose mix absorb water from the bottom up before trying a re-plant. I'm also going to buy a different bag of mix from a different store. Any comments or suggestions are welcome. Thanks.