Answer: Damping off is a general term for a number of problems but most of them can be traced back to the seed starting equipment and procedures. The most common reason, I think, is keeping them too moist and dark. Start out with soil that is just barely moist, like a well wrung out sponge. Plant your seeds and cover them with the plastic wrap to maintain humidity, but open it as soon as they start to sprout and put them immediately into bright light -- for best results use supplemental lighting. Make sure there is also some air circulation where you keep them as stagnant air can encourage fungal growth. Next, make sure the plants are thinned enough to allow for ample light and air to filter through them. Also, do not over fertilize them. They do not need fertilizer until they have several sets of true leaves. You might try watering by dribbling water gently and slowly out of a small pitcher onto the soil rather than spraying them or soaking the pots -- wet foliage or waterlogged soil will also encourage fungal growth. Finally, make sure all of your tools and equipment are clean and that you are using a relatively sterile potting mix such as a soilless seed starter of peat, vermiculite and possibly perlite. Other causes can also be too high or low a temperature or too little light -- many gardeners find it necessary to use supplemental lights for seedlings. Last of all, you might have luck watering them with chamomile tea when you see the fungus appear along with improving the other conditions.
I don't know if your current batch can be saved or not, or if it will be worth saving it. When plants have been stressed they may not recover enough to regain really good quality. There's still time to try again with most things, though.
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