What's it look like?

Damping off is a term given to a fungus or bacteria that affects seedlings. Seedlings may not emerge from the soil, or they may die after a certain stage of development. Sometimes a fungus will attack the stems of young seedlings and it will simply topple over. Other fungus will travel along the leaves and infect other seedlings. You can often see the tiny fungal threads covering the young plants like cobwebs.

What it attacks

All seedlings and seeds.

What to do before it arrives

Make sure that you sow your seeds thinly to avoid overcrowding. Poor air circulation will worsen the problem. It's better to sow to few than too many. A diluted tea made from old chamomile tea bags is great. Allow it to cool and poor it over the seedlings once a week as a preventative. don't overwater the seedlings.

What to do if you already have problems

Use the chamomile tea mentioned above and apply it every couple of days. Make sure that you don't overwater the seedlings. If the problem is severe you may have to destroy the existing seedlings and start again with fresh soil. Do NOT compost or reuse the infected soil.

For further information:

Avoid Damping Off
Avoid Damping Off. A common problem associated with starting seedlings is called damping off. Caused by soil borne fungi, the disease attacks a wide variety of plants, often in the early seedling stage.
http://www.mrs.umn.edu/pyg/tips/soil_planting/tip_1019.shtml

Damping Off
Damping Off. Damping off is most commonly caused by two opportunistic pathogens, pythium and phytophthora. Pythium is normally most active in cool, wet soils when the soybeans are still seedlings. ... having a known history of damping-off should select varieties with resistance ...
http://www.nutechseed.com/management/dampingoff.htm

Tomatoes problems: damping off
Are you having problems with your tomato plants? Tomato seeds not sprouting, tomato seedlings fallin gover right after they emerge? Could be damping off. Learn more. Learn what to do. ... Damping-off is a common problem in tomato plants where the soil is too wet or has ...
http://ks.essortment.com/tomatoesproblem_rckk.htm

Damping Off
... Damping Off. Damping off describes the usually sudden destruction of newly germinated seeds or ... stages, before germination (pre-emergence damping off) and after seedlings grow from ...
http://www.ipm.iastate.edu/ipm/hortnews/2002/2-8-2002/dampoff.html

Damping Off
... fungi which occur naturally in soils. Damping off is most severe on cucumber, cauliflower, cabbage, tomatoes ... Other forms of damping off include wirestem, infection of the tops of ...
http://www.nsac.ns.ca/lib/apascc/acv/pests/99app02.htm