The seed starting soil medium MUST be sterile and if its growing white mold on the surface that might indicate its got some bacteria in it. Don't add anything to the soil as it can kill seedlings and prevent germination. Corn meal is not good either as it creates a protein layer that prevents seed germination, just like corn gluten, the more refined product. The watering mat method isn't the best way to do seedlings, unless its allowed to go dry for a day or two. I found that I simply use 11"x22" trays with 2 inch square pots. They are filled with a sterile growing medium that has plenty of airation and helps to drain out excess water, its coir based and thats from shreaded coconut husks. As mentioned in another very recent thread, Gardens Alive Natural Beginnings, mixed with Jiffy mix for seed starting is much healthier way to start seeds. I also use heat mats under my trays and cover each with a clear plastic dome lid. This helps to hold moisture in as the heat mats do dry things out much faster. In any case, once the seedlings are touching the clear cover, I remove the covers and shut off the heat mats. The plant lights they are under are on a timer for about 18 hours per day. In the many years of growing plants from seeds, including some very difficult double and triple blossom impatiens I have not had any problems with any seed starting.

Sorry it took me so long to get back but been working a lot of hours this week. The mold is white and fuzzy looking. I don't think it is algae. Any comments will certainly help. Thanks

Hey there.What you have is called Botrytis or grey mold. Its a fungus and it is everywhere. You may not be able to get rid of all of it every where around you but you can make your seedlings and the soil less attractive by sterilizing the soil with heat or chemicals and then not over watering..
Chemicals cost a lot, they are not friendly to people or pets and sometimes they lay around forever since you don't need a lot. It is lots easier to put some potting soil in a baggie or microwaveable container and nuke it.This kills the fungus quite nicely. The proper instructions are as follows: place 1 to two quarts of potting soil in a microwave proof container. Add at least 1 cup of water to the soil even if it is not particularly dry, more if needs be. This method of sterilization works because of the steam. Loosely close the container and cook on high for 5 minutes. Close it up tightly now and allow it to sit until it cools. Do not attempt to cool it quickly because the steam from the water takes time to work properly. As the steam works it opens the cells of the material in the soil mix and the water is quickly absorbed. This makes the soil able to handle much more water and air so you don't need to water so often.Once the soil has cooled it should be used right away. If you don't use it soon, you should keep the container closed so it stays somewhat sterile. If the soil is too hot, it is going to distort the cells of the plant tray.
I nuke a gallon of soil at a time for 10 minutes, expecting it to take three quarters of an hour to cool enough to open. The longer it stays hot, the fewer problems I am going to have. Sandy

Thanks Sandy...I will give that a try next time I plant.

I have the same problem with some of my trays in cold frame.
The molds basically grow when/where there is no air circulation.
I have the same soil in trays outside, no mold. But I have some in my cold frame with molds. This is because in last 3-4 days it was raining, humidity was over 90% and cold frame was kept closed. Today it is going to be sunny and warm and I am going to put those trays outside in the sun.
That should cure the problem. Molds cannot live when air is moving and humidity is low.
This batch of trays with mold has some rotted wood (cellulose?) from the woods. The previous ones made with other type of soil do not have mold.
I have made a garden grill . today I am going to cook some soil for my seedings today . LOL

Captan can help to reduce the mold. Using sterile seed starting medium is the most important thing though, as it doesn't have any mold in it just waiting to attach tender seedlings.

If you have mold on your dirt of the seedlings can they be saved?

yes, mentzer,

Just scratch the surfaces and blow air on them(cool hair drier might do).
Molds/mildew grow in an environment that is humid and there is no or very littl air circulation. Where do they come from? My experience tell me that tree cellulose and soil near trees in the shade has potential to grow molds.
Sun baked soil hardly can have mold in it.

You want to make your own sterile soil?

In an open area in the garden make a small mound of your seeding soil(picked fromsome place from garde).
Start a very small controlled fire on top of your mound. Keep going,
Do not need to have burn a lot of stuff. Just keep it goinng for about 15 minutes. Burn small twiggs, leaves.
let the fire die(do not spray or pour water on it to kill)
You can cover died fire with alluminum foil and sprinkle some of the warm soil near by , to hold it in place . Let it cool.
Here you have the best sterile soil. The small amount of ash when mixed with it is an added advantage; Potasium is good for stronger roots, disease resistance.

Fortunately, i caught the mold within an hour of forming on the surface. I had a small fan that i placed in the small room where my seedlings were stored and the "fungus" disappeared within a few minutes. I have my grow light plugged into a timer with 2 outlets so i subsequently plugged the fan into the second outlet to come on when the grow light came on. From what i've read, the mold should be kept at bay and not effect the health of the seedlings.

I discovered this same mold on the soil in my greenhouse. Only on the seedlings that I transplanted into potting soil and I think I caught it early. I stupidly didn't sterilize my potting soil. I went through great lengths to sterilize my seed starting mix but not the potting soil. This organic potting soil I have is heavy in bark. So essentially I learned my lesson the hard way. I have since started sterilizing the potting soil for my new seedling transplants. I took the advice I found here. I did a Hydrogen Peroxide mix, sprayed the soil, blow dried the soil on cool, and put the seedlings in the shade outside (it was 80 here yesterday). It seems to have helped. Last night I didn't see any. I was able to keep the humidity in the greenhouse to 70%. This morning there was only a little in the pots of the seedlings that had "the most" originally. Today I am doing the same thing that I did yesterday and hopefully that will get rid of it (it's going to be 84 here today). I would put them in direct sunlight but it's only their second day out of the greenhouse and I don't want to cook them. Thanks for the info!

I dust soil with peat moss and it takes care. I never sterilize soil for seedlings only for starting from seeds. Just water with chamomile tea and put dry peat moss on top. Helps

I was going to recommend watering with a weak chamomile tea too.

I'm unsure where my commentary disappeared too. My garden box has mould only in one of them. We scoured out what we thought was it. Used hydrogen peroxide. Re used the garden bed with expensive soil from garden Center. Our Summer came very early in NZ. Perpetual spinach bolted & went to seed all lettuces planted in expensive soil bed are stunted in growth & bitter of anybody has any ideas to be rid of this fungus type mould. ✨✨

i know the last comment was in 2015 but i just found this in my green house and never seen it before. i am not sure of its the same or not but i am going to try. i have my garden in my green house and they are doing awesome. only now there is alot of mold on the surface of the dirt. it seems to have gone wild on gotten every where, by the looks of this i am going to place the plants that i can in the outside and the rest is going to get the fan treatment of blowing air inside from outside. the question is, Is this the best idea? i don'y want to place any chemicals in there at all if i can help it. this is the best garden i have had in years. please give me some advice. thank you for all the help.