No it won't kill them if you let them dry out. "twice in over a week is hardly overdoing it" can still be overdoing it - easily so - if they don't need it.

It all depends on your soil mix and how well it drains, the size of the container, the air circulation, and the ambient air temps if too warm. It is a fungus and requires very specific conditions to grow. Modify the conditions and it dies off.

And if you added compost to your mix or if you used a non-sterile mix that was already contaminated then it will usually develop no matter what you do.

Move the fan down where it is gently blowing right on the plants and across the soil. Before watering stick you finger down into the soil (if it will fit :) and make sure it really is dry down below. If your finger won't fit use a Q-tip of a piece of tissue on a stick. If it is at all cool and damp when removed then water isn't needed.

If you want to try the peroxide - keeping in mind that you are just adding more water and that peroxide kills bacteria but has little effect on fungi - then a 10:1 dilution is the common recommendation.

Lastly if the containers are large enough that you can carefully mix some dry potting mix into the top 1" of soil that too will help.

Good luck.

Dave

What about adding some sand to the top or some more cinnamon?

I wouldn't add anything. Rather I'd try to subtract ie get them drier. You asked about effective natural remedies. Nothing more natural than a bit of controlled drought.

I think digdirt and I are both advocating a 'less is more' approach to seed raising. Less intervention, including watering, equals more success.

When you think your seedlings need water, before you add some, check the weight of the containers. Do they feel rather light, or do they feel heavy, like they contain water? Don't water if they feel heavy (it does take some practice). I really find this a useful test to avoid over watering.

More seedlings are killed by too much water than by all other issues combined. Control the soil moisture level. It really is that simple.

Millions of perfect seedlings are grown annually without the use of cinnamon or peroxide or sand or compost or egg shells or chamomile or any of the many other things folks come up with to add.

Dave

I am with Dave. We not only over medicate ourselves, we do it to our plants as well. Al

I recently had the white fuzz issue as well with my seedlings in newspaper pots. Dave gave me the advice to allow them to dry out, and I did so. The fungus has decreased drastically since I took his advice, and I really thought I was not over-watering. It's tough to tell sometimes, the surface of my soil looked extremely dry, but the pots were still wet. So definitely can't go by the surface. Ditch the water for a while and you'll see :)

Thanks for your help saving my plants Dave!!

Thanks for all the replies. Here's the main problem: I have other seeds that have not germinated yet in the same trays and the soil was actually looking dry--so much so it was cracking on the surface of most of the cells. It seems like the seedlings that have already germinated don't need as much water as the cells with the seeds that haven't germinated so I do I go about getting enough water to the ungerminated seeds if they're in the same tray as the germinated seedlings that apparently don't need as much water as what I've been giving. I was thinking about cutting up the cells but most of the seeds are not in a perfect number rows as the trays are wide (for instance the trays are 6 cells wide and my cherry tomatoes take up about 2 rows and 2 cells). I watered them today pouring in about 2 inches or so--a little over a quart--of warm water and letting it soak in for 15-20 minutes. Before you all admonish me for watering them after I saw the white fuzz, please realize I did it for the seeds that haven't yet germinated. I put a heater on in the room that the seedlings are in and the temp is up above room temp (in the 80's). Supposedly the rate of mold growth significantly decreases at this temperature. The fan is still on them and I also put a little more cinnamon (I should probably stop with all that cinnamon, huh?). Any solutions to the questions presented in the diatribe would be much appreciated.

Thanks!

If you really feel you must water the cells that haven't germinated (and I'm not convinced you really need to) this is the time to gently spray a little water on just those cells. You're risking all of them by keeping them too wet.

Here's the main problem: I have other seeds that have not germinated yet in the same trays

You are right that is your primary problem and you have learned a valuable lesson about how to plant next year. Never mix different seeds or even different varieties of the same seed in the same pack/row/container. You have to be able to separate and treat each individually.

You'll have to cut the cell packs into individual cells if necessary - commonly done - or prick out and transplant into other posts the ones that have germinated.

And don't overwater the seeds that haven't germinated either. Seeds don't have to be wet to germinate. And if some seeds of the same variety that have already germinated don't pop up within a few days then they aren't going to. You seldom get more than 80-85% germination even with ideal conditions - and your aren't ideal.

6 cells wide and my cherry tomatoes take up about 2 rows and 2 cells)

Not sure why you have rows in cells? Just not seeing that picture. How many cherry tomatoes are you growing? You can germinate 5-6 plants in a 1" square cell, 10-12 in a 2" easily. We germinate 40-50 plants of each variety in 4" cells routinely. Then transplant them to individual cells when the first true leaves pop up.

I put a heater on in the room that the seedlings are in and the temp is up above room temp (in the 80's). Supposedly the rate of mold growth significantly decreases at this temperature.

Not sure what the source of your info is for that. Warming up the air temperature only increases the amount of fungus growth. It doesn't reduce it. Heat is used for germination only, not growing once germinated. And even then it is soil heat that is needed, not air heat.

In the greenhouse as soon as the plants break the soil surface they are immediately removed from any heat source and moved to an area where the temp is maintained at a constant 60-65 degrees max. That and minimal watering is what keeps any molds or fungi from developing.

Young seedlings will tolerate too little water FAR better than they will tolerate too much water.

Dave

I make an anti-mold tea by steeping a chamomile teabag and a couple of cloves of garlic that have been sliced in 2-3 cups of very hot water. Once the solution cools down I put it in a spray bottle and use it to soak the top of the soil after each watering. These two things both have natural fungicidal properties and I find that it gets rid of damp-off and white fuzzies very effectively. I keep the solution for up to 10 days in the refrigerator before brewing new.
I also coat the top of my seed-starting compost with a layer of sand because the sand is inert and doesn't support the fungus as well as the organic matter does. It also dries out completely between waterings and the dryness deters the fungus too.
These two methods combined keep my babies free of fungi.
Note: This is just my way of doing things, but you may find that it works for you too.

BTW: All of the other advice on letting your soil get dry is REALLY good advice. This not only deters the fungus but makes your plants' root systems reach and grow. Just don't let the dirt dry out completely. It's easy enough to tell by color and appearance. I like to look at the soil color through the holes in the bottom of the pot rather than the top because the top has evaporation working with it too and the bottom is a better watering gauge because it doesn't have this issue. Watering from the bottom up by setting your babies in a shallow basin of water or filling the tray or saucer is a good way to water too. Use less water than you think you should and apply more often. Soil that isn't completely drenched is healthier and still has air pockets that the plants need to ensure rapid growth.